Ladebug Debugger Manual

Information in this page is for Use By All Ladebug Customers

© 2000 Compaq Computer Corporation

Table of Contents

What is the Ladebug debugger?
About this documentation
Getting a Ladebug kit
Reporting problems
Conventions

A Quick Introduction to Using the Ladebug Debugger

A Guide to Using the Ladebug Debugger

What is the Ladebug debugger?

The Compaq Ladebug debugger supports C, C++, and Fortran programming on: It also has limited support for Cobol and Ada.

It supports debugging simple programs, as well as situations involving multiple threads, multiple processes, core files, kernels, and remote systems.

The official Software Product Description is part of the Developers' Toolkit Software Product Description.

About this documentation

This documentation explains how to use the Ladebug debugger.

Getting a Ladebug kit

Ladebug kits, manuals, and the FAQ are available from the following sources:

Reporting problems

Send mail to ladebugsupport@zko.dec.com.

What to report

Please provide the following information when you enter your problem report. Doing so will make it easier for us to reproduce and analyze your problem. If you do not provide this information, we may have to ask you for it. The Ladebug development team can use ftp to fetch sources and executables provided you can place them in an anonymous FTP area. In other cases, there are other less convenient methods we may ask you to use.

Conventions

The following conventions are used in this manual:

Convention Meaning
% A percent sign (%) represents the C shell system prompt.
# A pound sign (#) represents the default superuser prompt.
UPPERCASE
lowercase 		The Tru64 UNIX operating system differentiates between lowercase and uppercase characters. On the operating system level, you must type examples, syntax descriptions, function definitions, and literal strings that appear in text exactly as shown.
Ctrl/C This symbol indicates that you must press the Ctrl key while you simultaneously press another key (in this case, C).
monospaced text This typeface indicates the exact name of a routine, partition, path name, directory, or file. This typeface is also used in interactive examples.
monospaced bold text In interactive examples, this typeface indicates input that you enter. In syntax statements and text, this typeface indicates the exact name of a command or keyword.
monospaced italic text Monospaced italic type indicates variable values, place holders, function argument names, and in syntax definitions, non-terminal names. When a non-terminal name consists of more than one word, the words are joined using the underscore (_), for example, breakpoint_command.
italic text Italic type indicates book names or emphasized terms. 
foo_bar
    : item1
    | item2
    | item3
 A colon starts the syntax definition of a non-terminal name (in this example, foo_bar. Vertical bars separating items that appear in syntax definitions indicate that you choose one item from among those listed.
[] In syntax definitions, brackets indicate items that are optional.
option ;...
option ,...
option  ...		 A set of three horizontal ellipses indicates that you can enter additional parameters, options, or values. A {semicolon, comma, space} preceding the ellipses indicates successive items must be separated by {semicolons, commas, spaces}.
setld(8) Cross-references to online man pages include the appropriate section number in parentheses. For example, setld(8) indicates that you can find the material on the setld command in Section 8 of the reference pages. The man command % man 8 setld shows the man page for this reference.

A Quick Introduction to Using the Ladebug Debugger

This section provides all the information you need to make simple use of the debugger.

You look for a bug by doing the following:

  1. Find a repeatable reproducer of the bug — the simpler the reproducer is, the simpler the following steps will be to do.

  2. Prepare your program for debugging.

  3. Start the debugger.

  4. Give commands to the debugger.

    • Command the debugger to either
      • Prepare to create a process running the program, or
      • Attach to and interrupt a process that you created using normal UNIX methods.

    • Command the debugger to create breakpoints that will pause the process as close as possible to where the bug happened.

    • If you are using the debugger to create the process, tell it to create the process now.

  5. Do whatever it takes to reproduce the bug, so that the breakpoints will stop the process close to where the bug has caused something detectably wrong to happen.

  6. Look around to determine if the bug
    • Is in the code where the debugger has stopped the process, or
    • Hasn't yet happened, or
    • Has happened earlier.

  7. If the bug is in code where the debugger has stopped the process, exit the debugger and fix the bug.

  8. If the bug hasn't happened yet, remove any breakpoints that are triggering too often, create other breakpoints that work better at getting near the problem, and continue the process.

  9. If the bug has already occurred, you would like to take the same steps of creating breakpoints etc., except with the process running backward. Unfortunately, reverse execution is a difficult problem (how do you un-erase that disk?) so the compilers and the debugger don't support it. Instead, you have to rerun from an earlier position (a snapshot if you made one, or else the beginning of the program), first creating breakpoints that stop the process sooner.

Preparing a program for debugging

Compile and link your program using the -g switch.

If the problem only occurs in optimized code, use the -g3 switch.

Starting the debugger

Before you start the debugger, you should make sure that you have correctly set the size information for your terminal, otherwise the debugger's command line editing support may act unpredictably.

For example, if your terminal is 47x80, you need to set the following:

There are four basic alternatives for getting the debugger started on a process:
  1. Have the debugger create the process using the shell command line to identify the executable to run. 
        % ladebug a.out
    Welcome to the Ladebug Debugger Version 4.0-58
    ------------------
    object file name: /usr/users/user1/a.out
    Reading symbolic information ...done
    (ladebug) stop in main
    [#1: stop in int main(void) ]
    (ladebug) run
  2. Have the debugger create the process using the debugger commands to identify the executable to run. 
        % ladebug
    Welcome to the Ladebug Debugger Version 4.0-58
    (ladebug) load a.out
    Reading symbolic information ...done
    (ladebug) stop in main
    [#1: stop in int main(void) ]
    (ladebug) run
  3. Have the debugger attach to a running process using the shell command line to identify which process and executable file that process is running. 
        % a.out &
    [2] 27859
    % jobs	
    [2]  Running
    ...
    % ladebug a.out -pid 27859
    Attached to process id 27859  
    ....
    Type Ctrl/C to interrupt the process.

  4. Have the debugger attach to a running process using the debugger commands to identify which process and executable file that process is running. 
        % a.out &
    [2] 27859
    % jobs
    [2]  Running
    ...
    % ladebug
    (ladebug) attach 27859 a.out
    Attached to process id 27859  
    ....
    Type Ctrl/C to interrupt the process.
NOTE: In the case of FORTRAN, routine main at which your program stops is not your main program unit. Rather it is a main routine supplied by the FORTRAN system that performs some initialization and then calls your code. Just step forward a couple of times (probably twice) and you will soon step into your code.

Entering commands to the debugger

The debugger outputs a prompt when it is ready for the next command from the terminal.

When you enter commands, you use the left and right arrow keys to move within the line and the up and down arrow keys to recall previous commands for editing. When you finish entering a command, press the Enter key to submit the completed line to the debugger for processing.

You can continue a line by ending the line to be continued with a backslash (\) character.

On a blank line, press the Enter key to re-execute the most recent valid command.

Two very useful commands are:

Scripting or repeating previous commands

Another useful command is: The command causes the debugger to read and execute Ladebug commands from filename.

Context for executing commands

Although the debugger supports debugging multiple processes, it only operates on a single process, known as the current process, at a given time.

Processes contain one or more threads of execution. The threads execute functions. Functions are sequences of instructions that come from source lines within source files.

As you enter debugger commands to manipulate your process, it would be very tedious to have to repeatedly specify which thread, source file, etc. you wish the command to be applied to. To prevent this, each time the debugger stops the process, it re-establishes a static context and a dynamic context for your commands. The components of the static context are independent of this run of your program; the components of the dynamic context are dependent on this run.

You can change most of these individually to point to other instances, as described in the relevant portions of this manual, and the debugger will modify the rest of the static and dynamic context to keep the various components consistent.

Running the program under debugger control

As was shown above, you can tell the debugger either how to create the process, or which existing process to attach to.

After you specify the program (either on the shell command line or using the load command), but before you have requested the debugger to create the process, you can still do things that seem to require a running process, for example, you can create breakpoints and examine sources.

Any breakpoints that you create will be inserted into the process as soon as possible after it exec()s your program.

To have the debugger create the process (rather than attaching to an existing process), you request it to run, specifying, if necessary, any input and output redirection and arguments. 

	% ladebug a.out
        Welcome to the Ladebug Debugger Version 4.0-58
	(ladebug) run 
    	    or
	(ladebug) run args
    	    or
	(ladebug) run > output-file
    	    or
	(ladebug) run args > output-file < input-file
The result of using any of the preceding command variations is similar to having attached to a running process. The rerun command repeats the previous run command with the same arguments and files.

Pausing the process at the problem

After your process is running, and the debugger has been attached to it, the four most common ways to pause the process are:
  1. Type Ctrl/C. 
      (ladebug) run
^C
Interrupt (for process)

Stopping process localhost:27903 (a.out).
Thread received signal INT
stopped at [int main(int):5 0x120001138]
      5     while (argc < 2 && i < 10000000)

  2. Wait until the process raises some signal. It will do this when there is an arithmetic exception, an illegal instruction, or an unsatisfiable memory access, such as an attempt to write to memory for which protection is set to read-only. 
      
(ladebug) run
Thread received signal SEGV
stopped at [void buggy(char*, char*):13 0x120001ba4]	
     13         output[k] = input[k];

  3. Create a breakpoint before you run or continue the process. 
      
(ladebug) stop in main 
[#1: stop in int main(void) ]
(ladebug) run
[1] stopped at [int main(void):182 0x1200023f8]	
    182     List<Node> nodeList;

  4. Create a watchpoint before you run or continue the process. 
      
(ladebug) watch variable nodeList._firstNode write 
[#2: watch variable nodeList._firstNode 0x11fffeb48, 0x11fffeb4f write ]
(ladebug) cont 
[2] Address 0x11fffeb48 was accessed at: 
List<Node>::List(void): x_list.cxx
 [line 121, 0x120001d74]	stq	r31, 0(r1)
	0x11fffeb48: Old value = 0xfffffffffffffff8
	0x11fffeb48: New value = 0x0000000000000000
[2] stopped at [List<Node>::List(void):123 0x120001d78]	
    123 }

Examining the paused process

Looking at the sources

You can: Here is an example that shows listing lines and using the / command to search for a string: 
    % ladebug a.out
Aliases are short-hand forms of longer commands. This example shows using the W alias, which lists up to 20 lines around the current line. Note that '>' marks the current line.

Looking at the threads (Tru64 UNIX only)

In a multithreaded application, you can obtain information about the thread that stopped or all the threads and you can then change the context to look more closely at a different thread.

Note that '>' marks the current thread.

You can select any thread to be the focus of commands that show things. For example:

Looking at the call stack

You can examine the call stack of any thread. Even if you aren't using threads explicitly, your process will have one thread running your code. You can move up and down the stack, and examine the source being executed at each call.

Looking at the data

You can look at variables and evaluate expressions involving them.

Looking at the signal state

The debugger shows you the signal that stopped the thread.

Looking at the generated code

You can print memory as instructions or as data. Note that * marks the current instruction. You can examine registers.

Continuing execution of the process

After you are satisfied that you understand what is going on, you can move the process forward and see what happens. The following table shows aliases and commands you can use to do this.

Desired Behavior
Alias
Command
Can Take Repeat Count
Continue until another interesting thing happens
c
cont
No
Single step by line, but step over calls
n
next
Yes
Single step to a new line, stepping into calls
s
step
Yes
Continue until control returns to the caller
return
No
Single step by instruction, over calls
ni
nexti
Yes
Single step by instruction, into calls
si
stepi
Yes

The following example demonstrates stepping at the instruction level:

Snapshots as an undo mechanism

Often when you move the process forward as just described, you will accidentally go too far. For example, you may step over a call that you should have stepped into.

In a program that does not use multiple threads, you can use snapshots to save your state before you step over the call. Then clone that snapshot to get another process positioned just before the call so you can step into it.

The following example shows the stages of a snapshot being used in this way.

  1. The first stage is to build the program and start debugging.
  2. The next stage is to stop the process just before the call and take a snapshot. You can see you are just before the call because the ">" to the left of the source list shows the line about to be executed. 
      
(ladebug) next 2 
stopped at [int main(void):187 0x120002498]	
    187     nodeList.append(newNode);   
(ladebug) list $curline - 10: 20
    177 
    178 //  The driver for this test
    179 //
    180 main()
    181 {
    182     List<Node> nodeList;
    183     
    184     // add entries to list
    185     //
    186     IntNode* newNode = new IntNode(1);
>   187     nodeList.append(newNode);   
    188     
    189     CompoundNode* cNode = new CompoundNode(12.345, 2);
    190     nodeList.append(cNode); 
    191     		    
    192     nodeList.append(new IntNode(3)); 	
    193     	    
    194     IntNode* newNode2 = new IntNode(4);
    195     nodeList.append(newNode2);
    196     
(ladebug) save snapshot 
# 1 saved at 00:19:24 (PID: 23771).
    stopped at [int main(void):187 0x120002498]	
    187     nodeList.append(newNode);
  3. You now step over the call. The execution is now AFTER the call, shown by the ">" being on the following source line. 
      
(ladebug) next 
stopped at [int main(void):189 0x1200024b0]	
    189     CompoundNode* cNode = new CompoundNode(12.345, 2);
(ladebug) list $curline - 10: 20
    179 //
    180 main()
    181 {
    182     List<Node> nodeList;
    183     
    184     // add entries to list
    185     //
    186     IntNode* newNode = new IntNode(1);
    187     nodeList.append(newNode);   
    188     
>   189     CompoundNode* cNode = new CompoundNode(12.345, 2);
    190     nodeList.append(cNode); 
    191     		    
    192     nodeList.append(new IntNode(3)); 	
    193     	    
    194     IntNode* newNode2 = new IntNode(4);
    195     nodeList.append(newNode2);
    196     
    197     CompoundNode* cNode2 = new CompoundNode(10.123, 5);
    198     nodeList.append(cNode2);
  4. Oh, how you wish you hadn't done that! No problem, just clone that snapshot you made. 
      
(ladebug) clone snapshot 
Process has exited
Process 26917 cloned from Snapshot 1.
# 1 saved at 00:19:24 (PID: 23771).
    stopped at [int main(void):187 0x120002498]	
    187     nodeList.append(newNode);
  5. Now you are in a new process before the call is executed.

    NOTE: fork() was used by the debugger both to create the snapshot and to clone it. 

      
(ladebug) list $curline - 10: 20
    177 
    178 //  The driver for this test
    179 //
    180 main()
    181 {
    182     List<Node> nodeList;
    183     
    184     // add entries to list
    185     //
    186     IntNode* newNode = new IntNode(1);
>   187     nodeList.append(newNode);   
    188     
    189     CompoundNode* cNode = new CompoundNode(12.345, 2);
    190     nodeList.append(cNode); 
    191     		    
    192     nodeList.append(new IntNode(3)); 	
    193     	    
    194     IntNode* newNode2 = new IntNode(4);
    195     nodeList.append(newNode2);
    196

A Guide to Using the Ladebug Debugger

This section provides most of the information needed to make expert use of the debugger.

Some additional details have been moved to the parallel portion of the Ladebug Debugger Advanced Topics so they do not hinder the reading of this section.

Preparing a program for debugging

Preparing the source code

No changes are needed to the source code to debug the program.

However, you can do the following to make it easier:

Preparing the compiler and linker environment

Debugging information is put into .o files by compilers. The level of information is controlled by compiler switches. See the man page for your compiler. The switch is probably -g.

The debugging information is propagated into the a.out or *.so by ld(1). It is removed by strip(1). If you strip your programs, keep the unstripped version to use with the debugger.

The debugging information can cause .o files to be very large, causing long link times, but even so it can also be incomplete. C++ users can use the cxx -gall and -gall_pattern switches. See the cxx(1) man page.

Starting the debugger

You can start the debugger in either of two ways:

Starting the debugger from a shell

When you invoke the debugger from a shell, you can bring a program, core file, or local kernel under debugger control, or attach to a running process.

The ladebug command invokes the Ladebug debugger.

The following is the shell syntax to invoke the debugger: 

ladebug 
	[ -c file ] 
     	[ -gui ]
     	[ -i file ]
     	[ -I directory ] 
     	[ -k ]
     	[ -line serial_line ]
     	[ -nosharedobjs ] 
     	[ -pid process_id  ]
     	[ -prompt string ] 
     	[ -remote ]
     	[ -rp remote_debug_protocol ]
     	[ -tty terminal_device ]
	[ -V ] 
		[ executable_file [ core_file ] ]
The following table describes the options and parameters:

Options and Parameters Description
-c Specifies an initialization command file. The default initialization file is .dbxinit. The debugger searches for this file during startup, first in the current directory. If it is not there, the debugger searches your home directory. This file is processed after the target process has been loaded or attached to.
-gui Activates the debugger's graphical user interface (GUI).
-i Specifies a pre-initialization command file. The default pre-initialization file is .ladebugrc. The debugger searches for this file during startup, first in the current directory and then in your home directory. This file is processed before the debugger has connected to the application being debugged, so that commands such as set $stoponattach = 1 will have taken effect when the connection is made.
-I Specifies the directory containing the source code for the target program, in a manner similar to the use command. Use multiple -I options to specify more than one directory. The debugger searches directories in the order in which they were specified on the command line.
-k or -kernel Enables local kernel debugging.
-line Specifies the serial line for remote kernel debugging. This must be used with -rp.
-nosharedobjs Prevents the reading of symbol table information for any shared objects loaded when the process executes. Later in the debug session, you can enter the readsharedobj command to read the symbol table information for a specified object.
-pid Specifies the process ID of the process to be debugged. This option cannot be used with any remote or kernel debugging flags.
-prompt Specifies a debugger prompt. The default debugger prompt is (ladebug). If the prompt argument contains spaces or special characters, enclose the argument in quotes (" ").
-remote Enables remote kernel debugging for use with the kdebug kernel debugger.
-rp Specifies the remote debug protocol. Currently only kdebug is supported; -rp kdebug enables remote kernel debugging.
-tty Specifies the terminal device for remote kernel debugging. This must be used with -rp .
-V Causes the debugger to print its version number and exit without starting a debugging session.
executable_file Specifies the program executable file.
core_file Specifies the core file.

For example, to invoke the debugger on an executable file:

To invoke the debugger on a core file: To invoke the debugger and attach to a running process: To invoke the debugger and attach to a running process when you don't know what file it is executing: To start the Ladebug GUI: To invoke the debugger on the local kernel: To invoke the debugger on the remote kernel:

Starting the debugger from within Emacs

You can control your debugger process entirely through the Emacs GUD (Grand Unified Debugger) buffer mode, which is a variant of shell mode. All the Ladebug commands are available and you can use the shell mode history commands to repeat them.

Ladebug version 4.0-48 and higher supports GNU Emacs Version 19 and higher.
Ladebug version 4.0-58 and higher supports Lucid XEmacs Version 19.14 and higher.

The information in the following sections assumes you are familiar with Emacs and are using the Emacs notation for naming keys and key sequences.

For each Emacs session, before you can invoke the debugger, you must load the Ladebug-specific Emacs LISP code, as follows:

At the Load file: prompt, type:

You can also place a load-file call in your Emacs initialization file (~/.emacs). For example:

To start the debugger with Emacs, type: The following invocation line displays: Edit the invocation line by typing the target program and pressing Return. Emacs remembers the invocation. To debug the same program again, you need only press Return.

Emacs displays the GUD buffer and runs the debugger within it; the debugger starts and displays its (ladebug) prompt, indicating readiness. The GUD buffer saves all of the commands you type and the program output for you to edit. In general, interact with the debugger in the GUD buffer as you would with a debugger started from a shell.

One of the benefits of running the debugger under Emacs is to get closer correlation between program execution and source. When your program stops, for example at a breakpoint, Emacs displays the source of your program in a second buffer (source buffer) and indicates the current execution line with =>.

NOTE: If the source is already loaded into a buffer, Emacs often finds that buffer. However, in some NFS mounting situations, Emacs may use an alternate name for some directories and will create a second buffer for your source (often with <2> appended to the name). Be careful that you do not modify the original buffer or kill it outright.

By default, Emacs sets its current working directory to be the directory containing the target program. Because the debugger does not do this when invoked directly, you may need to change the source code search path when using the debugger from within Emacs. To set an alternate source code search path, use the Ladebug map source directory command.

All Emacs editing functions and GUD key bindings are available. For example:

For more information on Emacs functionality and key bindings, refer to Emacs documentation. For example: Then select the Emacs menu, then the Debuggers menu.

XEmacs will come up with the source buffer displayed. Use C-x 2 and a buffer menu to select the control buffer.

Ending a debugging session

To exit the debugger, use the quit command. 
quit_command
        : quit

Alternative, you can type exit, which is a pre-defined alias for quit.

Getting help

To get help about debugger commands, use the help command. 
help_command
        : help [ topic ]
You can use this command to access the online help for the debugger. Enter help to see a list of help topics. Enter help command to see a list of Ladebug commands. Enter help ladebug to see a list of function-oriented Ladebug commands.

Ladebug GUI (Tru64 UNIX systems only)

The Ladebug GUI is active simultaneously with the terminal-based command-line session so you can enter commands using the terminal at the same time as seeing their effect or giving others commands from the GUI.

You can start the GUI in either of two ways: 

guion_command
        : gui
For example: When you use the GUI, you may use the mouse as well as the original terminal window to enter debugger commands.

You can shut down the GUI and leave the command line session running by selecting File/Close All. In this case, you can restart the GUI any time with the (ladebug) gui command.

To end the command-line session and exit the GUI, select File/Exit Debugger in the GUI window.

Specifying a debugger prompt

You can specify a debugger prompt when you start the debugger from a shell with the -prompt switch. The default prompt is (ladebug). 
	% ladebug -prompt ">> " sample
	>> quit
You can also change the prompt by setting the $prompt debugger variable.

Giving commands to the debugger

The debugger has several different mechanisms you can use to direct its behavior. It gets input from: Some examples of the difference between .ladebugrc and .dbxinit are shown in the following table:

Example Command If used in .ladebugrc If used in .dbxinit
Assume the command, "set $stoponattach = 1", is in one of these files and you invoked the debugger as: 
    % ladebug -pid process_id executable_file
The debugger attaches and stops. The debugger attaches and waits for you to enter Ctrl/C; subsequent attaches will stop.
Assume the command, "stop in main", is in one of these files: The debugger generates a message that there is no main in which to place a breakpoint, because there is no target yet. The debugger sets the breakpoint (assuming there is a main in the target).

Debugger's command processing structure

The debugger processes commands as follows:
  1. Prompts for input
  2. Gets a complete line from the input file and performs:
  3. Parses the entire line according to the parsing rules for the current language
  4. Executes the commands

Interrupting a debugger action

To interrupt program execution or to abort a debugger action, type Ctrl/C. This returns the debugger to the prompt.

Entering and editing command lines

The debugger reads lines from stdin. The debugger supports command line editing when processing stdin, and stdin is a terminal and the debugger variable $editline is non-zero (the default, see the set command to change it). For this to work correctly, you must have the terminal width set to the correct value. After editing, press the Return key to send the line to the debugger. NOTE: While using the up and down arrow keys, the debugger skips duplicate commands. To see a complete list of commands you have entered, use the history command.

Each line from stdin is copied to the record input file, if you have requested that file.

Each line is scanned from the beginning, looking for backslash ('\') characters which 'quote' their immediate following character. If the line ends in a quoted newline, then another line is similarly processed from stdin and appended to the first one, with the quoted newline removed.

Whether or not command line editing is enabled, you can always use your terminal's cut-and-paste function to avoid excessive typing while entering input.

History replacement of the line

Leading spaces and tabs are removed from the assembled line.

For assembled lines that begin with an '!' character, the following rules apply:

For lines that begin with an '^' character, these rules apply:

The assembled line is now appended to the history list.

'!' and '^' cannot be used in command lists built with '{}'; for example {print3; !!3} will not parse. They may be used in scripts.

History in a command list is not limited by '{}', but goes all the way back. For example:

Note: Commands in breakpoint action lists are not entered into the history list.

Alias expansion of the line

The assembled line is now subjected to alias expansion.
  1. This is done by scanning the line, looking for '#', ';' and '{' characters that are not inside strings.

    • Strings are recognized by their opening and closing double or single quotes. Backslash quotation causes a quote character not to terminate the string.

    • Such '#' characters and all that follow to the end of the line are discarded.

      If the '#' is the very first character in the line, the '#' is not discarded because a completely empty line has special meaning.

  2. At the beginning of the line, and immediately after ';' or '{' character not inside strings, the debugger checks for the occurrence of an alias identifier.

  3. If it finds an alias identifier, it associates the formal parameters of the alias with the specified actual parameters.

    If there are no formal parameters, this match consumes no more of the input.

    1. If there are formal parameters, white space is skipped, and then a '(' character is checked for and skipped. The characters following the '(' up to the first non-nested ',' or ')' character are associated with the formal parameter.

      Again, the characters within strings are not tested. Nesting is caused by '(' and ')' characters outside of strings.

    2. If there are more formal parameters, the ',' character is treated as the terminator of the actual parameter. It is skipped and processing continues as for the first parameter.

  4. After the alias and the correct number of actuals have been identified, all the characters from the start of the alias identifier to its end (no parameters) or the trailing ')' (one or more parameters) are replaced by the expansion.

  5. Within the definition of the alias, all occurrences of the formal parameter are replaced by the actual parameter, regardless of whether or not it is in a string.

Environment variable expansion

The debugger expands environment variables and the leading tilde (~) in the following cases: As in any shell, you can group an environment variable name using a pair of curly braces ({...}), and quote a dollar sign ($) by preceding it with a backslash (\).

The following table shows how various environment variables expand and assumes that the home directory is /usr/users/hercules and the environment variable BIN is /usr/users/hercules/bin.

Command with environment variable
Expands into:
load ~/a.out load /usr/users/hercules/a.out
load $BIN/a.out load /usr/users/hercules/bin/a.out
load ${BIN}2/a\$b load /usr/users/hercules/bin2/a$b
map source directory $BIN ${BIN}2 
map source directory \
/usr/users/hercules/bin/usr/users/\
hercules/bin  
/usr/users/hercules/bin/usr/users/\
hercules/bin2
stop at "$BIN/a.out":20 stop at "/usr/users/hercules/\ bin/a.out":20
run $BIN/a.out ~/core 
run /usr/users/hercules/bin/a.out \
/usr/users/hercules/core

Syntax of commands

The debugger has different parsing rules for each of the different languages it supports. A line is processed according to the current language, even if executing the line will change the current language.

Lexical elements of commands

Lexical parsing depends on:

As the debugger starts tokenizing a line into a command, it starts processing the characters using the lexical state LKEYWORDS. It uses the rules for lexical tokens in this state, recognizing the longest sequence of characters that forms a lexical token.

After the lexical token is recognized, the debugger appends it to the tokenized form of the line, perhaps changes to another lexical state, and starts on the next token.

Grammar of commands

Each command line must parse as one of: 
input
        : command_list
        | comment
A command list is a sequence of commands that are executed one after the other. 
command_list
        : command ;... 
        | command ;
        | command
A comment is a line that begins with a '#' character. 
comment
        : #
Any text after an unquoted '#' is ignored by the debugger. If the first non-whitespace character on a line is a '#' character, the whole line is ignored.

Note: The difference between a command line which is blank and a command line which is a comment: The blank line causes the debugger to repeat the previous command while the comment line does not.

General categories of commands

Commands usually start with, and often contain, keywords. These keywords must be lowercase.

A command is one of: 

command
        : alias_command
	| attach_command
	| braced_command_list
	| breakpoint_command
	| browse_source_command
	| call_stack_command
	| command_repetition_command
	| continue_command
	| detach_command
	| dbgvar_command
	| edit_file_command
	| environment_variable_command
	| execute_commands_from_file_command
	| execute_shell_command
	| guion_command
	| help_command
        | history_command
	| kernel_debugging_command
	| kill_command
	| load_command
	| look_around_command
	| machinecode_level_command
	| modifying_command
	| multiprocess_command
	| quit_command
        | record_command
        | run_command
        | snapshot_command
	| shared_library_command
	| thread_command
	| unload_command

Names and expressions within commands

Debugger keywords

In debugger commands, the identifiers in the following list are treated as keywords unless they are somewhere within parentheses:

For example:

Using braces to make a composite command

It is possible to surround a command_list with braces to make it work like a single command. There are places in the grammar that require a braced_command_list just for readability, or to assist the debugger in understanding your input. 

braced_command_list
        : { command_list }

Debugger variables

Debugger variables are pseudo-variables that exist within the debugger instead of within your program. They are used to:

There are three different varieties of debugger variables:

User-defined variables Created by you and can be set to a value of any type.
Preference variables Can be modified by you to change debugger behavior. You can only set the variable to a value that is valid for that particular variable.
Display/state variables Displays the parts of the current debugger state and cannot be modified by you.

For more information about debugger variables, see Appendix 1—Debugger variables.

The commands that specifically deal with debugger variables are: 

dbgvar_command
        : set dbgvar_name = expression
        | set dbgvar_name
        | set 
        | unset dbgvar_name

The dbgvar_name should not exist anywhere in your program, or you may confuse yourself about which of the occurrences you are actually dealing with. The predefined debugger variables all start with a dollar sign ($), to help avoid this confusion. It is strongly recommended that you follow the same practice; in a future release of Ladebug, Ladebug will require that all debugger variables start with a dollar sign.

NOTE: If there is a debugger variable which shares a name with a program variable, and you print an expression involving that name, which of the two Ladebug finds is undefined.

The first form creates the debugger variable if it doesn't already exist. It then sets the value of the debugger variable to the result of evaluating the expression.

The second form is equivalent to the command set dbgvar_name = 1.

The set form shows all the debugger variables and their values.

To see the value of just one debugger variable, print it. For example:

The unset form deletes the debugger variable. Some predefined debugger variables either can't be deleted, or are automatically recreated in the future when needed.

Scripting or repeating previous commands

command_repetition_command
        : !! 
        | ! integer
	| !- integer
	| ! string
To repeat the last command line, enter two exclamation points or press the Return key. You can also enter !-1.

To repeat a command line entered during the current debugging session, enter an exclamation point followed by the integer associated with the command line. (Use the history command to see a list of commands used.) For example, to repeat the seventh command used in the current debugging session, enter !7. Enter !-3 to repeat the third-to-the-last command. See also History replacement of the line.

To repeat the most recent command starting with a string, use the last form of the command. For example, to repeat a command that started with bp, enter !bp.

There are other ways to reuse old commands and save typing effort:

If you place commands in a file, you can execute them directly from the file rather than cutting and pasting them to the terminal. 

execute_commands_from_file_command
        : source filename
        | playback input filename
Use the source command to read and execute commands from a file. (Note that you can also execute debugger commands when you invoke the debugger by creating an initialization file named .dbxinit.) These commands can be nested, and as each comes to an end, reading resumes from where it left off in the previous file.

Be aware, however, that blank lines in these files repeat the last command, just as they do when entered from the terminal. Format the commands as if they were entered at the debugger prompt.

Use '#' to create comments to format your scripts.

The following example shows how to execute a debugger script. Given the following script:

The following example shows how to execute it. When a command file is executed, the value of the $pimode debugger variable determines whether the commands are echoed. If the $pimode variable is set to 1, commands are echoed; if $pimode is set to 0 (the default), commands are not echoed. The debugger output resulting from the commands is always echoed.

Recording input and output

To help you make command files, as well as help you see what has happened before, the debugger can write both its input and its output to files. 
record_command
        : record io  [ filename ]
        | record input [ filename ]
        | record output [ filename ]

Use record input to save Ladebug commands to a file. The commands in the file can be executed using the source command or the playback input command.

If no file name is specified, the debugger creates a file with a random file name in /tmp as the record file. The debugger issues a message giving the name of that file.

To stop recording debugger input or output, redirect as shown in the following example or exit the debugger:

The following example shows how to use the record input command to record a series of debugger commands in a file named myscript. The recorded input in myscript : The record output command saves the debugger output to a file. The output is simultaneously written to stdout (normal output) or stderr (error messages). For example: After the above commands are executed, myscript contains:

The record io command saves both input to and output from the debugger. For example:

History

You can see all the commands you have already entered by using the history command. Use history_number to indicate how many commands to show, starting with the most recent. If you don't specify $historylines, the 20 previous commands are shown. See also History replacement of the line. 
history_command
        : history [ integer_constant ]
For example:

User-defined commands—aliases

You can extend the set of debugger commands by defining aliases.

When the debugger is tokenizing a command line, it expands aliases and then retokenizes the expansion. 

alias_command
        : alias [ alias_name ] 
        | alias alias_name [ (argument_name ,...) ] string
        | unalias alias_name

The following example shows how to define and use an alias.

The following example further modifies the cs alias to specify the breakpoint's line number when you enter the cs command.

Use the unalias command followed by an alias name to delete the specified alias.

Executing shell commands

You can have the debugger execute a call to the UNIX system function. This function is documented in the man pages, see system(3). The call results in the sh shell executing the string you specify. 
execute_shell_command
        : sh string
For example, you can spawn a shell from the debugger by issuing: To execute more than one command at the specified shell, spawn a shell as follows, for example:

Invoking your editor

You can use the edit command to invoke the editor defined by the EDITOR environment variable. 
edit_file_command
        : edit [ string ]
The editor is given the string as the file name to edit. If no file name is specified, the editor is given the current file. If there is no current file, the editor is started without a file.

If the EDITOR environment variable is undefined, the debugger invokes the vi editor.

The following example shows invoking the Emacs editor on file chars.c:

The following example shows invoking the nedit editor on file ~/foo/bar.f:

Context for executing commands

Multiple processes

The debugger supports debugging multiple processes at a time, but at any given time is only operating on a single process, known as the current process. The debugger variable $curprocess contains the process id for this process. Naming and switching the debugger between processes is described here.

Creating processes

The debugger can find and control:

Specifying an executable file on the shell command line or executing the load command causes the debugger to gain control of a process that you may request it to create later.

NOTE: In the background, the debugger immediately creates a process executing the program, stalls it, and uses it to answer questions about which shared libraries are mapped, etc. This process never continues, and is killed when:

Using the run command on such a potential process causes the debugger to create a process which is identified as currently running and recreatable.

Specifying a pid on the shell command line or executing the attach command causes the debugger to know about the process as currently running and not recreatable.

Catching a fork() causes the new child process to be identified as currently running and not recreatable.

Multiple call frames, threads, and sources

Processes contain one or more threads of execution. The threads execute functions. Functions are sequences of instructions that are generated by compilers from source lines within source files.

As you enter the debugger commands to manipulate your process, it would be very tedious to have to repeatedly specify which thread, source file, etc., you wish the command to be applied to. To prevent this, each time the debugger stops the process, it re-establishes a static context and a dynamic context for your commands. The components of the static context are independent of this run of your program; the components of the dynamic context are dependent on this run.

Some pieces of these contexts are available as debugger variables.

You can switch most of these individually to point to other instances, as described in the relevant portions of this manual, and the debugger will modify the rest of the static and dynamic context to keep the various components consistent.

Running the program under debugger control

Often running the program in a process just requires fork()ing a process and exec()ing the program within it with the right environment variables, argc/argv, file descriptors, etc. This is what usually happens when you run your program from a shell command line.

However, sometimes the program requires more context, or a process may already have been created. Perhaps it is part of a pipe, perhaps it is a long-running process, or perhaps it is created from a shell script or makefile.

Hence, the following situations are possible:

Running your program as a child process

If your program has a simple command line, and only requires stdin, stdout, and stderr connected, you can run it as a child process of the debugger process.

For example: 

	% ladebug a.out
or 
	% ladebug
        (ladebug) load a.out

Attaching to a process

If your program is any of the following, you can use the debugger's ability to attach to any process it has access to:

For example: 

	% ladebug -pid process_id a.out
or
	% ladebug
	(ladebug) attach process_id a.out

When you do this, the process continues execution until it raises a signal that the debugger intercepts, for example, $stoponattach.

One method you can use to make the attach work in a predictable way is to modify your program to loop in a known function until the debugger interrupts it, for example, when you use Ctrl/C:

load and unload commands

Using the load command, you can tell the debugger which executable file that you intend to execute in some process. The load command reads the symbol table information of an executable file and, optionally, a core file. (This is done automatically when you give the debugger a filename on the shell command line.) 
load_command
	: load filename [ filename ]
For example: or: The second file name is used to specify a core file. If you specify a core file, the debugger acts as though it is attached to the process just before it died, except that you cannot execute commands that require a runnable process, such as commands that try to continue the process or evaluate function calls.

Creating a process both creates the debugger's knowledge of it, and makes it the current process that the debugger is controlling.

The opposite of loading an executable file is unloading an executable file. 

unload_command
	: unload pid ,...
	| unload filename
The unload command removes all related symbol table information that the debugger associated with the process being debugged, specified by either a process id or executable file. For example:

run and rerun commands

After you have loaded a program, you can create a process executing this program using either of the following forms of the run command. 
run_command 
        : run   [ argument_string ] [ io_redirection ... ]
        | rerun [ argument_string ] [ io_redirection ... ]
rerun is a synonym for run.

If the rerun command is specified without arguments, the arguments and io_redirection entered with the most recent run command entered with arguments are used. If the last modification time and/or size of the binary file or any of the shared objects used by the binary file has changed since the last run or rerun command was issued, the debugger automatically rereads the symbol table information. If this happens, the old breakpoint settings may no longer be valid after the new symbol table information is read.

The argument_string provides both the argc and argv for the created process in the same way a shell does.

The debugger breaks up the argument_string into words, and supports several shell features, including tilde (~) and environment variable expansion, wildcard substitution, single quote ('), double quote ("), and single character quote (\).

The io_redirection allows you to change stdin, stdout, and stderr, which are otherwise inherited from the debugger process. 

io_redirection
        : <  filename
        | >  filename
        | 1> filename
        | 2> filename
        | >& filename
The various forms have the same effect as in the csh(1) shell. NOTE: Although the grammar currently allows more than the following forms of redirection, you should only use the following because we may change the grammar in the future. 
     > filename               Redirect stdout
    1> filename               Redirect stdout
    2> filename               Redirect stderr
    >& filename               Redirect stdout and stderr
    1> filename 2> filename   Redirect stdout and stderr to different files
For example:

kill command

You can kill the current process: 

kill_command
        : kill
Killing a process leaves the debugger running. Any breakpoints previously set are retained. You can later rerun the program. For example:

attach command

If a process already exists, you can have the debugger attach to it by: 
attach_command
        : attach pid [ filename ]
The process is specified by its pid. 
pid
	: expression
Example: The file name must be an executable file that the process is executing, or a copy of it, or an unstripped copy of it. If file name is not specified, the current executable is used.

Attaching to a process both creates the debugger's knowledge of it, and makes it the current process that the debugger is controlling. When you do this, the process continues execution until it raises a signal that the debugger intercepts. Usually you do this by typing Ctrl/C or using the shell command kill in another window. Any other mechanism for raising a signal within the process will also do. You can set the debugger variable $stoponattach to 1 to direct the debugger to immediately stop any process that it attaches to.

The opposite of attaching to a process is detaching from a process. When you detach the debugger from a process, all breakpoints are removed and the process continues to run but the debugger can no longer identify or control it. 

detach_command
        : detach pid ,...
Example:

Controlling the process environment

You can set and unset environment variables for processes created in the future to set up an environment different from the environment of the debugger and from the shell from which the debugger was invoked. When set, the environment variables apply to all new processes you debug.

NOTE: The environment commands have no effect on the environment of any currently running process. The environment commands do NOT change or show the environment variables of the debugger or of the current process. They only affect the environment variables that will be used when a new process is created. 

environment_variable_command
        : show_environment_variable_command
        | set_environment_variable_command
        | unset_environment_variable_command
To print either all the environment variables that are currently set, or a specific one, use a show_environment_variable_command. 
show_environment_variable_command
        : printenv [ environment_variable_name ]
        | export
        | setenv
NOTE: export and setenv without any arguments are equivalent.

To add or change an environment variable, use a set_environment_variable_command. If the environment_variable_value is not specified, the environment variable value is set to "". 

set_environment_variable_command
        : export environment_variable_name  = environment_variable_value 
        | setenv environment_variable_name  environment_variable_value 

environment_variable_value
	: string
For example:

To remove an environment variable, use the unsetenv command. 

unset_environment_variable_command
        : unsetenv environment_variable_name
        | unsetenv *

If * is specified, all environment variables are removed.

NOTE: There is no way of simply getting back to the initial state of the environment variables after the debugger starts.

Multiprocess debugging

The debugger can find and control more than one process at a time. The debugger can find and control a process for one of three reasons:

At any one time, only one of the processes the debugger controls can be controlled by the user. The rest are stalled. You must explicitly switch the debugger to the process you want to work with, stalling the one it was controlling. 

multiprocess_command
        : show_process_command
        | switch_process_command

You can show the processes the debugger controls. 

show_process_command
        : show process [ all ]
        | process
	
all
	: all
	| *
Example:

You can explicitly command the debugger to control a different process. 

switch_process_command
        : process pid
        | process filename
The process you are switching away from remains stalled until either the debugger exits, or you switch to it and continue it.

The following example creates two processes and switches from one to the other.

Both the run command and the attach command switch the debugger to the process they operate on.

Processes that use fork()

The debugger has the following predefined variables that you can set for debugging a program that forks:

When a fork occurs, the debugger sets the debugger variables $childprocess and $parentprocess to the child and parent process IDs, respectively.

In the following example, the debugger notifies you that the child process has stopped. The parent process continues to run.

Note:

Continuing the previous example, the following shows how to switch the debugger to the child process. Listing the source code shows the source for the child process.

NOTE:

Processes that use exec()

Set $catchexecs to 1 to instruct the debugger to stop the process and notify you when an exec occurs. The process stops before executing any user program code or static initializations. You can debug the newly exec'd process. The debugger keeps a history of the progression of the exec'd files.

In the following scenario, you set the predefined variable $catchforks and $catchexecs to 1. The debugger will notify you when an exec occurs. Because $catchforks is set, you will also be tracking the child process and, therefore, you will be notified of any exec in the child process.

The following example shows an exec occurring on the current context and the child process stopped on the runtime-loader entry point.

Note:

Core file debugging

When the operating system encounters an unrecoverable error, for example a segmentation violation (SEGV), the system creates a file named core and places it in the current directory. The core file is not an executable file; it is a snapshot of the state of your process at the time the error occurred. It allows you to analyze the process at the point it crashed. For more information on core file debugging, see Core file debugging in Ladebug Debugger Advanced Topics.

Kernel debugging

The debugger supports kernel debugging, which is a task normally performed by systems engineers or system administrators. A systems engineer might debug a kernel space program, which is built as part of the kernel and refers to kernel data structures. A systems administrator might debug a kernel when a process is hung, when kernel parameters need to be examined or modified, or when the operating system hangs, panics, or crashes. Kernel debugging aids in analyzing crash dumps.

Security

You may need to be the superuser (root login) to examine either the running system or crash dumps. Whether or not you need to be the superuser depends on the directory and file protections for the files you attempt to examine.

Compiling a kernel for debugging

The kernel should be compiled without full optimization and without stripping the kernel of its symbol table information. Otherwise, your ability to debug the kernel is greatly reduced.

By default, compilation does not strip the symbol table information and optimization is only partial. If you do not change these defaults, there should not be a problem.

Patching a disk file

From within the debugger, you can use the patch command to correct bad data or instructions in an executable disk file. The text, initialized data, or read-only data areas can be patched. The bss segment cannot be patched because it does not exist in disk files.

For example:

Setting the thread context

The debugger variable $tid contains the thread identifier of the current thread. When there is no process or program, $tid is set to 0. The $tid value is updated implicitly by the debugger when program execution stops or completes.

You can modify the current thread context by setting $tid to a valid thread identifier.

The debugger variable $tid is the same as $curthread except that $tid is used for kernel debugging.

Local kernel debugging

When you have a problem with a process, you can debug the running kernel or examine the values assigned to system parameters. (It is generally recommended that you avoid modifying the value of the parameters, which can cause problems with the kernel.)

Invoke the debugger with the following command:

The -k flag maps virtual to physical addresses to enable local kernel debugging. The /vmunix and /dev/mem parameters cause the debugger to operate on the running kernel.

Now you can use Ladebug commands to display the current process identification numbers (pids) and trace the execution of processes. The following example shows the use of the kps command to display the process IDs: 

kernel_debugging_command
        : kps
For example: The Ladebug commands cont, next, rerun, run, step, and stop are not available nor can you change values in registers when you do local kernel debugging. (Stopping the kernel would also stop the debugger.)

If you want to examine the stack of, for example, the kloadsrv daemon, you set the $pid symbol to its pid(3) and enter the where command, as in the following example:

Examining the stack trace may reveal the problem. Then you can modify parameters, restart daemons, or take other corrective actions.

Crash dump analysis

If your system panics or crashes, you can often find the cause by using the debugger to analyze a crash dump.

The operating system can crash in the following ways:

If the system crashes because of a hardware fault or an unrecoverable software state, a dump function is invoked. The dump function copies the core memory into the primary default swap disk area as specified by the /etc/fstab file structure table and the /sbin/swapdefault file. At system reboot time, the information is copied into a file, called a crash dump file.

You can analyze the crash dump file to determine what caused the crash. For example, if a hardware trap occurred, you can examine variables, such as savedefp, the program counter ($pc), and the stack pointer ($sp), to help you determine why the crash occurred. If a software panic caused the crash, you can use the debugger to examine the crash dump and the uerf utility to examine the error log. Using these tools, you can determine which function called the panic() routine.

Crash dump files, such as vmunix.n and vmcore.n, usually reside in the /var/adm/crash directory. The version number (n in vmunix.n and vmcore.n ) must match for the two files.

For example, you might use the following command to examine dump files:

Examining the exception frame
When you debug your code by working with a crash dump file, you can examine the exception frame using the debugger. The variable savedefp contains the location of the exception frame. (No exception frames are created when you force a system to dump.) Refer to the header file /usr/include/machine/reg.h to determine where registers are stored in the exception frame. The following example shows an exception frame:
Extracting the character message buffer
You can use the debugger to extract the preserved message buffer from a running system or dump files to display system messages logged by the kernel. For example:
The crashdc utility
The crashdc utility collects critical data from operating system crash dump files or from a running kernel. You can use the data it collects to analyze the cause of a system crash. The crashdc utility uses existing system tools and utilities to extract information from crash dumps. The information garnered from crash dump files or from the running kernel includes the hardware and software configuration, current processes, the panic string (if any), and swap information.

See Compaq TRU64 UNIX Kernel Debugging and the crashdc man page for more information.

Managing crash dump file creation
To ensure that you are able to analyze crash dump files following a system crash, you must understand the crash dump file creation process. This process requires that you reserve space on the system for crash dump files. The amount of space you save depends on your system configuration and the type of crash dump file you want the system to create.
Saving dumps to a file system
See Compaq TRU64 UNIX Kernel Debugging for more information.
Crash dump files
Crash dump files are either partial (the default) or full. See Compaq TRU64 UNIX Kernel Debugging for more information.
Selecting a crash dump type
The default is to use partial crash dumps. If you want to use full dumps, you can modify the default behavior in the following ways:
Determining crash dump partition size
If you intend to save full crash dumps, you need to reserve disk space equal to the size of memory, plus one additional block for the dump header. For example, if your system has 128 MB of memory, you need a crash dump partition of at least 128 MB, plus one block (512 bytes).

See Compaq TRU64 UNIX Kernel Debugging for more information.

Procedures for creating dumps of a hung system
If necessary, you can force the system to create dump files when the system hangs. The method for forcing crash dumps varies according to the hardware platform. The methods are described in Compaq TRU64 UNIX Kernel Debugging.
Guidelines for examining crash dump files
In examining crash dump files, there is no one way to determine the cause of a system crash. However, the following guidelines should help you identify the events that led to the crash: For more information and for examples, see Compaq TRU64 UNIX Kernel Debugging. This manual contains detailed information on the following topics related to crash dump analysis:

NOTE: Crash dump analysis is possible only with local, not remote, kernel debugging.

Locating the site of a problem

To determine why a problem is happening, you usually want to execute your program up to or just before the point that the first evidence of the problem is observed. Then you can examine the internal state of your program and try to identify something that explains the visible problem. Possibly you will see right away how the problem occurs, in which case you are finished debugging. You then correct your program, recompile, relink, and confirm that the correction works as intended.

Often, you will see something about the program state that is wrong but not how it got that way. In that case, you need to make a guess at where the mistake might have occurred. Then, repeat this whole process, trying to stop at or just before the possible trouble point.

For simple problems, it may be easy to describe the conditions under which you want to stop the program; for example, "the first time traverse is called" or "when division_by_zero occurs". Other situations may require either more complex descriptions or repeated trial-and-error attempts to discover the critical information needed to solve your problem. Breakpoints provide the means by which you specify to the debugger an event (or condition) under which you want to intervene in the execution of your program and what action(s) you want the debugger to take when that event is detected.

You can define breakpoints based on:

These are collectively referred to as breakpoints.

Breakpoint commands include the following: 

breakpoint_command
        : breakpoint_definition_command
        | simple_stop_command
        | signal_command
        | obsolete_breakpoint_definition_command
        | breakpoint_table_command

Breakpoint definitions

A particularly common breakpoint is:

This command tells the debugger that when execution enters the function main, you want the debugger to suspend execution and return control to you.

The debugger responds to a breakpoint command by displaying how it recorded the request internally. The debugger assigns a number to the breakpoint (in this case, it is 1), which it uses later to refer to that breakpoint. The debugger does not just repeat the command as you entered it; it provides a more complete description of the function main to help you confirm that it has correctly identified the function you meant.

Later, after you cause the program to execute, if that event occurs, the debugger reports the event and then prompts you for what to do next.

Both the event part and the action part of a breakpoint definition command consist of several subparts: 

breakpoint_definition_command
	: disposition
            [ quiet ]
              detector
            [ thread_filter ]
            [ logical_filter ]
            [ breakpoint_actions ]
where the detector, thread_filter (if specified), and logical_filter (if specified) collectively specify the event part, and the disposition, quiet keyword (if specified) and breakpoint_actions (if specified) collectively specify the action part.

NOTE: There are additional obsolete forms of breakpoint definition that are retained only for backward compatibility with earlier versions of the debugger. These forms are explained later. The obsolete forms may be eliminated in a future release.

There are three distinct points in time at which a breakpoint definition has an effect:

  1. When the command is entered

    The command is parsed, names and expressions that occur in any of the event parts are evaluated, and the breakpoint actions are parsed and checked for correctness (but not evaluated).

  2. When the debugger initiates program execution

    For each breakpoint (that is not disabled), appropriate modifications are made to the program to enable detection of the specified event.

  3. When a detector triggers during program execution

    The thread filter specification (if present) and logical filter (if present) are evaluated to determine whether the breakpoint as a whole has triggered. If not, then execution is resumed (silently). If so, the breakpoint actions are performed, after which execution stops or resumes according to the specified disposition.

Disposition

disposition
        : stop
        | when
stop specifies that when the event specified by the breakpoint occurs and all processing for that breakpoint has been completed, the debugger should prompt for further commands.

when specifies that when the event specified by the breakpoint occurs and all processing for that breakpoint has been completed, the debugger may resume execution of the program. See the section When multiple breakpoints trigger at once for an explanation of how the debugger determines when to resume execution.

quiet option

By default, when an event is detected and the debugger determines that the breakpoint actions should be performed, the debugger prints a line that identifies the breakpoint, for example:

The optional quiet keyword tells the debugger to omit this information.

Detectors

There are several kinds of detector, each corresponding to a particular kind of event. 

detector
        : place_detector
        | watch_detector
	| signal_detector
	| unaligned_detector

A place detector specifies a place or location in your program. It can refer to the beginning of a function, a particular line in one of your source files, a specific value of the PC (program counter), or certain sets of these.

A watch detector specifies a variable or other memory location(s) that should be monitored to detect certain kinds of access (read, write, and so on).

A signal detector specifies a set of UNIX signals to be monitored.

An unaligned detector specifies any kind of memory access using an unaligned access.

Place detectors
A place detector is used to determine when execution reaches a particular place or location in your program. 
place_detector
        : in function_name
        | in all function_name
	| pc address_expression
        | at line_specifier
	| every proc entry
	| every procedure entry
	| every instruction
        | expression

in function_name specifies the event where execution reaches the entry of the named function.

If the function name is ambiguous (there can be more than one function that matches the name in some languages, including C++), the debugger prompts you with a list of alternatives from which to choose.

If you choose the last option ("None of the above") then no function is selected and no breakpoint is defined.

in all function_name is the same as in function_name except that it specifies all of the functions that match the given name, whether one or more.

pc address_expression specifies the event where execution reaches the given machine address.

at line_specifier specifies the event where code associated with a particular line of the source is reached. If there is no code associated with the given line number, the debugger finds and substitutes the closest higher line number that has associated code.

every procedure entry specifies that a breakpoint should be established for every function entry point in the program.

NOTE: This command can be very time-consuming because it searches your entire program — including all shared libraries that it references — and establishes breakpoints for every entry point in every executable image! This can also considerably slow execution of your program as it runs.

A disadvantage of this command is that it establishes breakpoint for hundreds or even thousands of entry points about which you have little or no information. For example, if you use stop every proc entry immediately after loading a program and then run it, the debugger will stop or trace over 100 entry points before reaching your main entry point! About the only thing that you can do if execution stops at most such unknown places is continue until some function relevant to your debugging is reached.

every instruction specifies a breakpoint for every instruction in your entire program.

When used with the stop disposition, a subsequent continue behaves essentially the same as a step by instruction command (see stepi).

When used with the when disposition, subsequent next and step commands allow you to trace all of the instructions that are executed as a result of those stepping commands. Beware that even when next is used to step over a called routine, the trace output includes all of the instructions that are executed within the called routine (and any routines that it calls). NOTE: This command will slow execution of your program considerably.

The detector expression (that is, an expression not preceded by one of the keywords in, at, or pc) specifies either a function name or line number depending on how the expression is parsed and evaluated. An expression that evaluates to the name of a function is handled just like the equivalent command that uses in in the detector; otherwise, it is handled like the equivalent command that uses at in the detector.

Watch detectors
A watch detector is used to determine when a variable or other memory location is read or written and/or changed. Breakpoints with watch detectors are also known as watchpoints. 
watch_detector
        : basic_watch_detector watch_detector_modifiers

basic_watch_detector
        : variable variable_name
	| memory start_address_expression
        | memory start_address_expression , end_address_expression
        | memory start_address_expression : byte_count_expression

watch_detector_modifiers
        : [ access_modifier ] [ within_modifier ]

access_modifier
        : write
        | read
        | changed
	| any

within_modifier
        : within function_name

You can specify a variable whose memory is to be watched, or specify the memory directly. The accesses that are considered can be limited to those that write (the default), read, write and actually change the value, as well as including all accesses.

If a variable is specified, the memory to be watched includes all of the memory for that variable, as determined by the variable's type.

This watches for write access to variable _nextNode which is allocated in the 8 bytes at the address shown in the last line of the above example.

If memory is specified directly in terms of its address, the memory to be watched is defined as follows:

If a within_modifier is specified, then only those accesses that occur within the given function (but not any function it calls) are watched.

Signal detectors
A signal detector is used to determine when a particular UNIX signal is raised. 
signal_detector
        : signal signal_id ,...

signal_id
        : integer
        | signal_name
Signals may be specified by numeric value or by their conventional UNIX names, without or without the leading "SIG". Note that if the debugger catches a signal event, then a subsequent simple continue will resume execution without raising the signal again in your process. However, a signal can be specified as part of the continue command to send the signal to your process when it resumes.
Unaligned access detectors (Tru64 UNIX only)
An unaligned access detector is used to determine when an unaligned memory access occurs. 
unaligned_detector
        : unaligned

Unaligned accesses are automatically handled by the Tru64 UNIX operating system. By default, an unaligned access results in an information message and then is corrected so that your program can continue. (You or your system manager can choose a different default. See the uac(1) man page for details.) This message looks like this: 

Unaligned access pid=30231  va=0x11ffff791 pc=0x120001af4 ra=0x120001b84 inst=0xa0220000

You can request the debugger to detect unaligned accesses:

Unaligned access detector (Linux only)
Unaligned accesses are automatically handled and quietly corrected on Linux. The debugger cannot detect these events.

Thread filter

A thread filter determines whether a detected event should be further considered for breakpoint processing. 
thread_filter
        : thread thread_id ,...
The thread_id expressions are evaluated at the time the breakpoint command is entered and each must yield an integer value.

A detected event is retained for further consideration if and only if the thread in which the event occurs matches one of the given thread ids. If not, the detection is quietly ignored.

Note that if the thread_filter does not indicate a match, then any related logical filter is not evaluated.

Logical filter

A logical filter determines whether a detected event should be further considered for breakpoint processing. 
logical_filter
        : if expression
A detected event is retained for further consideration if and only if the given expression evaluates to true. If not, the detection is quietly ignored.

The expression is checked syntactically in the context of the place where the breakpoint command is given: it must be syntactically valid according to the language rules that apply there. However, the expression is not evaluated and names that occur in the expression need not be visible. After the syntax check, the expression is remembered in an internal form and is not re-checked later when it is evaluated.

If an error occurs when the expression is evaluated, for example, because a name in the expression is not defined, then the error is reported and the value of the expression is assumed to be true.

Note that an error in the expression does not change the disposition. If continuation was specified, then that is still what occurs.

It is valid for a logical filter expression to contain a call to another routine in your program. Such a call is evaluated in the same way as if it occurred in a call or print command. However, execution of the called routine might result in triggering a breakpoint; this is called a recursive breakpoint and is discussed later.

Breakpoint actions

The action part of a breakpoint command specifies actions to be performed when the event part has triggered (including passing any thread and/or logical filters). 
breakpoint_actions
        : { action_list }

action_list
        : command 
        | command ;
        | command ;...
Special commands
A few the debugger commands behave differently in some fashion when used within a breakpoint action list.
Commands to use with caution
You should be very careful when using some commands in breakpoint action lists. The following commands cause the debugger to resume execution of your program in the midst of action list processing:

It is easy in such cases to lose track of just what state breakpoint processing is really in and/or where you really are in your program. Such confusion may mislead or misdirect your debugging effort. For further discussion, see the section on Recursive breakpoints.

Commands to avoid
You should avoid altogether some commands in breakpoint action lists. The following are commands that directly or indirectly change the process that the debugger is controlling:

The debugger does not explicitly prohibit these commands, but their behavior within action lists is implementation-defined and subject to change from release to release. In very specialized cases, you may be able to obtain useful results by using them in action lists, but do not expect the same behavior over the long term.

When multiple breakpoints trigger at once

It is possible for multiple breakpoints to specify the same event, or possibly overlapping events. Thus, more that one breakpoint detector may trigger at the same time.

When more than one breakpoint detector triggers, the thread filters and logical filters of all the breakpoints involved are processed before the action part of any breakpoint is performed.

After the set of breakpoints that trigger is determined, the action parts of each of them are performed in an undefined order.

After all action parts are performed, execution of the program is resumed if and only if all of the breakpoints so specify in their disposition. If any one of them specifies break, the debugger prompts you for further commands.

Recursive breakpoints

Various commands cause the debugger to resume execution of your program while in the midst of action list processing:

In all of these cases, the debugger temporarily "suspends" processing of the current breakpoint to start your program executing again and then waits for that execution to complete. As long as no new breakpoint is triggered during that execution, all will be fine. However, if a new breakpoint triggers, in particular one with the stop disposition, then you may be prompted for new command input for the recursive breakpoint even before the initial breakpoint has completed. Further, continuing execution may ultimately allow the original breakpoint to complete, at which time its disposition will come into play.

It is easy in such cases to lose track of just what state breakpoint processing is really in and/or where you really are in your program. Such confusion may mislead or misdirect your debugging effort. See example which locates suspended execution in nested function calls.

Breakpoints and C++

Member functions
Setting breakpoints in C++ member functions is illustrated using the following program:

Member functions must be named in a way that makes them visible at the current position according to the normal C++ visibility rules.

If not positioned within a member function of a class, it is generally necessary to name the desired member function using type qualification, an object of the class type, or a pointer to an object of the class type.

You can avoid the ambiguity associated with an overloaded function by specifying a complete signature for the function name.

Templates and instantiations
The debugger has no knowledge of templates that may occur in your program. However, you can usually debug template instantiations the same way as the equivalent non-instantiated class or function.

Debugging of template instantiations is illustrated using this source text.

Normal debugging commands then apply to the instantiation (not the template as such).

Exception handlers
When working with exception handlers, you can set a breakpoint at the appropriate line to determine if an exception is thrown. In addition, you can set breakpoints in these functions that are part of the C++ library support for exceptions:

terminate Gains control when any unhandled exception occurs, which will result in program termination.
unexpected Gains control when a function containing an exception specification tries to throw an exception that is not included in that specification.

These special library functions are illustrated using the following source:

You can trace the flow of execution as in the following:

Special signal breakpoints

UNIX signals are operating-system-defined events that can be handled by the debugger.
catch and ignore
Two special breakpoint commands, catch and ignore, can be used to handle UNIX signal events. 
signal_command
        : catch_command
        | ignore_command

catch_command
        : catch [ signal_id ]

ignore_command
        : ignore [ signal_id ]

A catch command with an operand specifies that the given UNIX signal should be caught and handled by the debugger. The signal can be specified by integer number or by standard signal name, with or without the leading "SIG". The catch command is equivalent to the breakpoint command:

or with these exceptions:

An ignore command with an operand specifies that the given UNIX signal should not be caught or handled by the debugger; rather such a signal is passed to your program. The ignore command is equivalent to deleting the breakpoint created by a catch command for that signal.:

A catch command without an operand lists all signals that are currently being handled. Similarly, an ignore command without an operand lists the signals that are currently being ignored. Together the two lists show all signals known to the debugger.

You can issue these commands immediately after the debugger starts to show which signals are caught and which are ignored by default:

Note: Signals RTMIN, RTMIN1,...,RTMIN7, RTMAX, and RTMAX7,...,RTMAX1 apply only on Tru64 UNIX.
Unaligned accesses (Tru64 UNIX only)
You can request the debugger to catch unaligned accesses: This command is very much like the command: 
    stop unaligned
Although this looks like a normal catch command, it differs in several respects: Like other catch commands:

NOTE: You cannot specify unaligned in a signal detector of a normal breakpoint definition.

You can request the debugger to ignore unaligned accesses when catch unaligned is in effect (the default) using

However, if a breakpoint was defined using an unaligned access detector, then it must be disabled using a disable or delete breakpoint command.
Unaligned accesses (Linux only)
Unaligned accesses are automatically handled and quietly corrected on Linux. The debugger cannot catch these events.
Ctrl/C
If your program seems to be caught in a loop, you can type Ctrl/C. The debugger interprets this as a command to send signal interrupt (SIGINT) to your program. Because the debugger itself catches signal SIGINT by default, this interrupts your program and returns control to the Ladebug prompt.

If you give the command ignore SIGINT, then it is no longer possible to regain control of your program using Ctrl/C. In that case, signal SIGINT is delivered directly to your program. Unless your program has explicitly arranged otherwise, SIGINT will result in program termination.

Breakpoint interactions with exec(), fork(), dlopen() and dlclose() system calls

A process starts with a copy of its parent's memory as the result of a fork() system call; after running for a while within that memory, the process will often make an exec() system call to start a new executable file within that process.

The debugger keeps track of the exec() calls that occur so that it can keep track of various properties associated with each executable file. In particular, the breakpoint table is one of those properties. Thus, if you run or rerun your program, the same breakpoints can be re-established, even though a new process is initiated. Similarly, if you work with more than one process, each process has a distinct breakpoint table associated with it.

When a dlopen() system call occurs, the debugger re-processes the current breakpoint table and automatically sets up the means to detect any events that apply to the newly loaded image.

When a dlclose() system call occurs, the debugger also re-processes the breakpoint and de-activates any events that apply to the unloaded image.

Obsolete breakpoint commands

The following forms of breakpoint commands are obsolete, but are still supported for backward compatibilty with earlier versions of the debugger. 
obsolete_breakpoint_definition_command
        : obsolete_watch_breakpoint_definition_command
        | obsolete_trace_breakpoint_definition_command
	| obsolete_stopi_breakpoint_definition_command
	| obsolete_wheni_breakpoint_definition_command
	| obsolete_tracei_breakpoint_definition_command
Obsolete watchpoint definition
An obsolete watchpoint definition is similar to a stop variable or stop memory breakpoint. 
obsolete_watch_breakpoint_definition_command
        : watch obsolete_watch_detector
            [ obsolete_watch_modifiers ]
            [ breakpoint_actions ]

obsolete_watch_detector
        : variable variable_name
	| [ memory ] start_address_expression
        | [ memory ] start_address_expression , end_address_expression
        | [ memory ] start_address_expression : byte_count_expression

obsolete_watch_modifiers
        : [ access_modifier ]
          [ thread_filter ]
          [ within_modifier ]
          [ logical_filter ]
The differences between an obsolete watchpoint and a stop command are: These differences are purely syntactic; the semantics are the same.
Obsolete tracepoint definition
An obsolete tracepoint definition is similar to a when in or when at breakpoint, possibly combined with watching for a change of a variable's value. 
obsolete_trace_breakpoint_definition_command
        : trace [ variable_name ]
            [ thread_filter ]
            [ where_modifier ]
            [ logical_filter ]
            [ breakpoint_actions ]
        | trace function_name [ logical_filter ] [ breakpoint_actions ]
        | trace line_number [ logical_filter ] [ breakpoint_actions ]

where_modifier
        : in function_name
        | at line_number
The differences between an obsolete tracepoint and a when command are: For example:

If the trace command is given with no arguments, the debugger prints a trace identification line when each function in your program is entered.

This is equivalent to the when every proc entry command (with equivalent performance degradation).
Instruction-related breakpoint commands
obsolete_stopi_breakpoint_definition_command
        : stopi [ expression ]
            [ thread_filter ] [ match_address ] [ logical_filter ]

obsolete_tracei_breakpoint_definition_command
        : tracei [ expression ]
            [ thread_filter ] [ match_address ] [ logical_filter ]

obsolete_wheni_breakpoint_definition_command
        : wheni [ expression ]
            [ thread_filter ] [ match_address ] [ logical_filter ]
            breakpoint_actions

match_address
        : at address_expression

The stopi, tracei, and wheni forms of breakpoint definition are similar to the corresponding stop, trace, and when forms, with these differences:

Breakpoint tables

As breakpoints are defined, they are recorded in a breakpoint table, associated with the current program. This table can be displayed and modified in certain limited ways. 
breakpoint_table_command
        : show_all_breakpoints_command
        | delete_breakpoint_command
        | enable_breakpoint_command
        | disable_breakpoint_command
Each entry in the breakpoint table has the following properties:

In addition to the main effects of a breakpoint definition as discussed in Breakpoint definitions, a breakpoint definition also sets the debugger variable $lasteventmade to the breakpoint number of the breakpoint just defined. This value can be recalled for later use if desired.

If an error occurs in a breakpoint command, the variable $lasteventmade is not changed.

Showing breakpoint status

Use the status command to display the current breakpoint table: 
show_all_breakpoints_command
        : status
Each entry in the current breakpoint table is displayed showing all of its properties.

When large or complex values are passed by value to the routine in the status line, the output can be voluminous. You can set the control variable $statusargs to 0 to suppress the output of argument type information in the status line.

Enabling, disabling, and deleting breakpoints

When a breakpoint is defined, it is enabled by default. When the debugger starts or resumes process execution, it first adapts the process so that it can detect when the given events occur. A breakpoint can be disabled so it is not involved in determining when the process should next stop. A breakpoint that is no longer required can be deleted entirely. 
disable_breakpoint_command
        : disable all
        | disable breakpoint_number_expression ,...

enable_breakpoint_command
	: enable all
        | enable breakpoint_number_expression ,...

delete_breakpoint_command
        : delete all
	| delete breakpoint_number_expression ,...
For example:

Support for 'looking around'—at the code, the data, and previously obtained information

There are many interesting aspects of a running process that you may wish to look at:
  1. The source files
  2. The threads, their mutexes, and their condition variables
  3. The call stack of one or more threads
  4. The actual machine instructions and data
  5. The shared libraries that are loaded

Looking at the sources

Special debugging information the compiler puts in the .o files correlates the machine instructions and data back to the source files and the positions they came from.

The debugger has to find and read any source files that you wish it to display or search. The debugger itself has no need to read the source files. All the information for the debugger comes from the executable files or shared libraries.

The debugger supports commands to: 

browse_source_command
        : source_directory_mapping_command
        | source_searchlist_command
	| select_source_file_command
        | list_source_file_command
        | search_source_file_command

Source directory mapping

Source files are compiled and linked into executable files. During debugging, the debugger tries to find these source files to display them for you. If the source files have moved, or if the paths to them are relative, the debugger may not be able to locate them.

The debugger has "source directory mapping" commands that:

The following example shows how to use source directory mapping. Suppose you compile x_solarSystem as follows: 
	% pwd
        /usr/users/ladebug/sandbox/test/src/common/Examples
	% ls -R
        bin/ src/

	./bin:
        x_solarSystem*                 
   
	./src:
	solarSystemSrc/

        ./src/solarSystemSrc:
        base_class_includes/    main/                   star.cxx
        derived_class_includes/ orbit.cxx                        
        heavenlyBody.cxx        planet.cxx
        
        ./src/solarSystemSrc/base_class_includes:
        heavenlyBody.h  orbit.h

        ./src/solarSystemSrc/derived_class_includes:
        planet.h  star.h

        ./src/solarSystemSrc/main:
        solarSystem.cxx
        % cd src
	% cc -g -o ../bin/x_solarSystem \
          -IsolarSystemSrc/base_class_includes \
          -IsolarSystemSrc/derived_class_includes \
          main/solarSystem.cxx heavenlyBody.cxx orbit.cxx planet.cxx star.cxx
Then you move the directory solarSystemSrc elsewhere: 
	% mv solarSystemSrc movedSolarSystemSrc
Now debug x_solarSystem in /usr/users/ladebug/sandbox/test/src/common/Examples/bin: The debugger cannot find the file because it has been moved to another directory. This command displays a summary of the source directories in a.out. The “...” here means that there is one or more source directories under src. This command directs the debugger to look for source files originally in solarSystemSrc in movedSolarSystemSrc instead. This time, the debugger finds the source file. This command gives a complete list of source directories. As you can see, solarSystemSrc is mapped to movedSolarSystemSrc. As a side effect of mapping solarSystemSrc to movedSolarSystemSrc, the subdirectories in solarSystemSrc are mapped to their counterparts under movedSolarSystemSrc.

To summarize, the debugger provides the following four commands for checking and setting source directory mappings: 

source_directory_mapping_command
        : show source directory [ directory_name ]
	| show all source directory [ directory_name ]
	| map source directory from_directory_name to_directory_name
	| unmap source directory  from_directory_name
Use the show source directory command to display the directory mapping information of directory_name and its child directories (or immediate subdirectory). If directory_name is not specified, the mapping information of all the source directories whose parent is not a source directory is displayed.

This command is identical to the previous command except that the mapping information of all the descendants of directory_name is displayed.

When you further expand a '...' where directory is the directory on the line above the '...':

Use the map source directory command to tell the debugger that the source files in the directory from_directory_name can now be found in to_directory_name.

The unmap source directory command maps from_directory_name back to itself; in other words,if from_directory_name has been mapped to some other directory, this command will restore its default mapping.

NOTE: *=> means that you are setting the mapping explicitly using the map source directory command, whereas => means that the mapping is derived from an explicit set.

How the debugger finds the source files

Usually the compiler copies the source file names from the shell command line into the .o files, without further prepending the current working directory, so that you can use links or other mechanisms to move the files around without invalidating the information.

The debugger looks in a use_list of directories to find the first occurrence of a source file. The use_list is specified as one or more directory names, without trailing slashes.

By default, the use_list is: (1) the current directory and (2) the directory containing the executable file. Each process has its own use_list. You can also use the ladebug command -I option to specify search directories.

The debugger searches for a source file (dir_name/base_name) using the following algorithm:

  1. If dir_name is mapped to another source directory (mapped_dir_name), look for mapped_dir_name/base_name. Otherwise, look for the original file dir_name/base_name.
  2. If Step 1 fails to find a readable file, for each entry use_dir in use_list, look for use_dir/dir_name/base_name. Note that the use_list entries are tried in the order they appear in the use_list.
  3. If Step 2 fails, for each entry use_dir in use_list, look for use_dir/base_name. Identical to Step 2, the use_list entries are tried in the order they appear in the use_list.
  4. If Step 3 fails, no source file can be found.
The debugger uses the first found readable file as the source file.

The following commands let you view and modify the use_list. 

source_searchlist_command
        : use_command
        | unuse_command
Enter the use command without an argument to list the directories the debugger searches for source code files. Specify a directory argument to make source code files in that directory available to the debugger. You can also use the ladebug command -I option to specify search directories, which puts those directories in the use_list.

You can customize your debugger environment source-code search paths by adding commands to your .dbxinit file that use the use command. 

use_command
	: use [directory_name ...]

If the directory_name is specified, it is either appended to or replaces the use_list, depending on whether the value of the $dbxuse debugger variable is zero (append) or non-zero (replace).

The unuse command removes entries from the use_list. 

unuse_command
        : unuse [directory_name ...]
        | unuse *

Enter the unuse command without the directory_name to set the search list to the default (the home directory, the current directory, and the directory containing the executable file). Include the directory names to remove them from the search list. The asterisk (*) argument removes all directories from the search list.

How the debugger chooses which source file to list

The debugger has a concept of current source file so you do not have to explicitly specify a source file in many commands.

Whenever the process stops, the current source file is set to the source file for the code currently executing.

Other commands up, down, class, and file also set the current source file.

You can see and modify the current source file selection. 

select_source_file_command
        : file [ filename ]

Enter the file command without a file name to display the name of the current file scope. Include the file name to change the file scope. Change the file scope to set a breakpoint in a function not in the file currently being executed.

To see source code for or set a breakpoint in a function not in the file currently being executed, use the file command to set the file scope.

The following example uses the file command to set the debugger file scope to a file different from the main program, and then stops at line number 26 in that file.

Listing source files

The simplest way to see a source file is to use a text editor. The edit command will display an editor on the current file, using the current definition of the EDITOR environment variable, if there is one.

However, there are some primitive inspection capabilities built into the debugger. The list command displays source lines beginning with the source code line corresponding to one of the following: 

list_source_file_command
        : list [ line_expression ]
        | list line_expression , line_expression
        | list line_expression : line_expression

line_expression
	: expression

If specified, the first expression must evaluate to either an integer (in which case it is the line number within the current source file of the first line to display) or a function (in which case the first line of the function is the first line to display).

Specify the exact range of source lines by including either a comma followed by the expression for the last line, or a colon followed by the expression for the the number of lines.

Such a second expression must evaluate to an integer value.

If a second expression is not given, the debugger shows 20 lines, fewer if the end of source file is reached.

For example, to list lines 16 through 20:

For example, to list 6 lines beginning with line 16:

Searching the contents of source files

The following search commands search through the current source file to help you find the lines to list: 
search_source_file_command
        : / [ string ]
        | ? [ string ]

NOTE:

The / form searches forward from the most recently listed line; the ? form searches backward. Like most searches, it will stop at the end (or beginning) of the file being searched, and will wrap if the command is repeated at that point.

When the string is omitted, the previous search continues from where it found the string.

When the string is present, the search starts from either the start (/) or the end (?) of the current line.

When a match is found, the debugger lists the line number and the line, but that line does not become the current line.

For example:

  1. To locate _firstNode:
  2. Then to locate append before line 69:
  3. Then to locate append after line 65:

Looking at the threads (Tru64 UNIX only)

A thread is a single, sequential flow of control within a process. Within each thread there is a single point of execution. Threads execute within (and share) a single address space; therefore, a process's threads can read and write the same memory locations. For further information, see the Tru64 UNIX Guide to DECthreads.

Thread levels

The debugger supports two levels of threads:

To specify the thread level, set the $threadlevel debugger variable to:

For example:

For core file debugging, the $threadlevel is always set to native.

Thread manipulation commands

There are a variety of commands to manipulate the threads. 

thread_command
        : show_thread_command
        | switch_thread_command
        | show_condition_variable_command
        | show_mutex_variable_command
        | pthread_command

Thread display commands

show_thread_command
	: show thread [ thread_id_list ] [ thread-state-filter ]
 
thread_id_list
	: thread_id ,...
        | *
	
thread_state_filter
	: with state eq thread_state

eq
	: ==               (for Ada, C, and C++)
	| .eq.             (for Fortran)  
	| =                (for Cobol)
	| equal [ to ]     (for Cobol)

thread_state
	: ready
	| running
	| terminated
	| blocked

Use the show thread command without parameters to list all the threads known to the debugger.

If you specify one or more thread identifiers, the debugger displays information about the threads you specify, if the thread matches what you specified in the list. If you omit a thread specification, the debugger displays information for all threads.

Use the show thread commands to list threads that have specific characteristics, such as threads that are currently blocked.

For example:

You can switch to a different thread as the current thread. The debugger variable $curthread contains the thread identifier of the current thread. The $curthread value is updated when program execution stops or completes. 

switch_thread_command
  	  : thread [ thread_id ]

The current thread can be modified by assigning $curthread a valid thread identifier. This is equivalent to issuing the thread thread_id command.

When there is no process or program, $curthread is set to 0.

Use the thread command without a thread identifier to identify the current thread. Supply a thread identifier to make another thread the current thread.

Mutex queries

A mutex (mutual exclusion) semaphore is a programming flag that allows multiple pthreads to synchronize access to shared resources to ensure that: Use the show mutex command to list information about currently available pthread mutexes. 
show_mutex_variable_command
        : show mutex  [ mutex_id_list ] [ mutex_state_filter ]

mutex_id_list
	: mutex_id  ,...
        | (mutex_id ,...)

mutex_state_filter
	: with state eq mutex_state
	
mutex_state
	: locked
If you specify one or more mutex identifiers, the debugger displays information about only those mutexes specified, provided that the list matches the identifiers of currently available mutexes. If you omit the mutex identifier specification, the debugger displays information about all mutexes currently available.

Use the show mutex with state == locked command to display information exclusively for locked mutexes.

If $verbose is set to 1, the sequence numbers of the threads locking the mutexes are displayed.

The following example shows the output from a simple show mutex command.

If the application being debugged has no pthreads or the $threadlevel is set to native, an appropriate message is issued.

Condition variable queries

A condition variable is a pthread synchronization object used in conjunction with a mutex. A condition variable is used when a thread has "locked a mutex" to gain access to data, and then finds it needs to wait for some other thread to change some aspect of the data before it can continue. 
show_condition_variable_command
        : show condition [ condition_id_list ] [ condition_state_filter ]

condition_id_list
        : condition_id ,...
	| (condition_id ,...)

condition_state_filter
	: with state eq condition_state	   
	
condition_state
	: wait

Use the show condition command to list information about currently available condition variables. If you supply one or more condition identifiers, the debugger displays information about the condition variables you specify, provided that the list matches the identities of currently available condition variables. If you omit the condition variable specification, the debugger displays information about all the condition variables currently available.

Use the show condition with state == wait command to display information only for condition variables that have one or more threads waiting. If $verbose is set to 1, the sequence numbers of the threads waiting on the condition are displayed.

The following example shows output from a simple show condition command.

If the application being debugged has no pthreads or the $threadlevel is set to native, an appropriate message is issued.

Other thread commmands

You can use the where command to display the stack trace of current threads. You can specify one or more threads or all threads.

The print command evaluates an optional expression in the context of the current thread and displays the result.

The call command evalutes an expression in the context of the current thread and makes the call in the context of the current thread.

The printregs command prints the registers for the current thread.

Undocumented pthread support

You can pass an undocumented string directly into the undocumented pthread debugging support. This is an internal debugging aid, not intended for general use. 

pthread_command
        : pthread string

Looking at the call stack

Most programming languages have some concept of functions, routines, or subroutines capturing the notion of code which is invoked from many places. A running program needs a call stack of call frames for the called functions. Each call frame contains both the information needed to return to its caller, and to contain the local variables of the function.

The machine-code generated for these functions maintains this call stack. Some of this maintenance is done before the call, some at the start of the called function, some at the end of the called function, and some after the call.

Non-optimized machine-code is usually very easy to correlate with the source code, but optimized machine-code can be tricky. Details of this are given later in this section.

The debugger controls the call stacks of all the threads, and lets you examine and manipulate them, and use them as a basis for further queries. 

call_stack_command
        : show_stack_command
        | change_stack_frame_command
        | pop_stack_frame_command

When your process is stopped by the debugger, you can show the call stack of the thread that was the cause of the stoppage, or of any other thread. 

show_stack_command
	: where [ expression ] [ thread_specifier ]

thread_specifier
	: thread thread_id ,...	
        | thread all
This shows the most recent call frames on the call stack of the current or specified threads.

If specified, the expression must evaluate to a non-negative integer, and specifies the number of call frames to show. If not specified, all the call frames for the thread are shown.

If specified, the thread_specifier specifies the threads whose call stacks are to be shown. If not specified, just the current thread is used.

When large and complex values are passed by value to a routine on the stack, the output of the where, up, down, and dump command can be voluminous. You can set the control variable $stackargs to 0 to suppress the output of argument values in the where, up, down, and dump commands.

The stack trace provides the following information for each call level:
Call level The number used to refer to a call level on the stack. The function entered most recently is at level 0.
Memory address The address of the next instruction to be executed at this level.
Function name The name of the function for the memory address.
File name The source file for the memory address.
Line number The number of the next source line of the memory address.

Special C++ issues

For nonstatic member functions, the implicit this pointer is displayed as the address on the stack trace along with the class type of the object, as shown in the following example:

You can select one of the call frames as the starting point for examining variables. This call frame provides both the current scope within the program for which variables exist, and tells the debugger which instance of those variables you want to see the values for. 

change_stack_frame_command
        : up   [ expression ]
        | down [ expression ]
        | func [ loc ]

Use the up command or the down command without the expression to change to the call frame located one level up or down the stack.

Specify an expression that evaluates to an integer to change the call frame up or down the specified number of levels. If the number of levels exceeds the number of active calls on the stack, the debugger issues a warning message and the call frame does not change.

When the current call frame is changed, the debugger displays the source line corresponding to the last instruction executed in the function executing the selected call frame.

When large and complex values are passed by value to a routine on the stack, the output of the where, up, down, and dump command can be voluminous. You can set the control variable $stackargs to 0 to suppress the output of argument values in the where, up, down, and dump commands.

Use the func command without the loc to display the current function.

To change the function scope to a function that has a call frame in the call stack, specify the loc as either the name of the function or as an integer expression evaluating to the call level. If the name is specified, the most recently entered call frame for that function becomes the current call frame.

If there are no frames to select from, the debugger context is set to the static context of the named function. The current scope and current language are set based on that function. Types and static variables local to that function are now visible and can be evaluated.

If you enter an integer expression, the debugger moves to the frame at level n, just as if you had entered up n.

In the following example, the current call frame is changed to one for method Planet::print so that a variable in that instance of print() can be displayed.

In the previous example, instead of entering func Planet::print you can enter down 2. (You would use down in this case because the current call frame at the start of the example was not the bottommost frame.) Note that the final stack trace in this example lists a call frame for function Planet::print as the current call frame (denoted by the > character).

The pop command

pop_stack_frame_command
        : pop [ expression ]
The pop command removes one or more call frames from the call stack. The default is one call frame. The pop command undoes the work already done by the removed execution frames. It does not, however, reverse side effects such as changes to global variables. Because it is extremely unlikely this will fix up all the effects of a half-executed call, this command is not recommended for general use. Furthermore, the pop command does not provide a way to specify a return value when the frame being discarded corresponds to a function that should return a value. You may need to use the assign command to restore the values of global variables.

Instead of the pop command, you may want to use the return command which finishes the call corresponding to the selected frame.

Call frames and optimized code

When optimized machine-code is generated by the compilers, the compiler generates code that maintains the call stack, but sometimes the function boundaries are changed in one of two ways:

Depending on the information the compiler makes available to the debugger, inlined calls may or may not show up in the call stack display.

Outlined calls will show up, and be correlated to the code they came from. The compiler will probably have supplied the debugger with some invented name for the function.

Call frames and machine code correlation

On a RISC processor such as an Alpha processor, the following is the machine-code typically generated for a call to a function: When the thread is part of the way through the call frame creation or tear down, the debugger will still show the call frame, but will not be able to show correct values for the variables or parameters.

Looking at the data

Once you have seen the call stack (show_stack_command) and selected the call frame containing the variables you wish to examine (change_stack_frame_command), and looked at the source this function is executing (looking at the source) , you usually want to examine some of the variables or even evaluate some expressions. There is a print command and a call command to do this. There are other commands to help you determine what to look at and what you are seeing. 
look_around_command
        : various_print_command
        | c++_look_around_command
	| call_command
        | whatis_command
        | whereis_command
        | which_command

various_print_command
        : print_command
        | printf_command
        | print_registers_command
        | dump_command

The print command

You can print the values of one or more expressions or all local variables. You can also use the print command to evaluate complex expressions involving typecasts, pointer dereferences, multiple variables, constants, and any legal operators allowed by the language of the program you are debugging. 
print_command
	: print [ expression ,...  ]
        | print rescoped_expression
	| print printable_type
	 
rescoped_expression
	: filename ` qual_symbol
	| ` qual_symbol
	    
qual_symbol
	: expression
	| qual_symbol ` expression
For an array, the debugger prints every cell in the array if you do not specify a specific cell.

Consider the following declarations in a C++ program:

The following example uses the print command to display a nonstring array. The following example shows how to print individual values of an array.
Dereferencing pointers
Pointers are variables that contain addresses. By dereferencing a pointer in the command interface, you can print the value at the address pointed to by the pointer. In C and C++ programs, variables containing a pointer are dereferenced using the * operator. The following example shows how to dereference a pointer in C++ programs.
Printing C strings
The debugger does not print more than the first $maxstrlen characters of a null-terminated string. Change this debugger variable if it is showing either more or less than you wish to see.
Restrictions on the print command
Expressions containing labels are not supported. Variables involving static anonymous unions and enumerated types may not be able to be printed. Printing a structure that is declared but not defined in a compilation unit may generate an error message indicating that the structure is opaque.

The printf command

Use the printf command to format and display a complex structure. The first argument is a string expression of characters and conversion specifications using the same format specifiers as the printf C function. 
printf_command	
	: printf [format_string [, expression  ,... ]]
For example:

The printregs command

Use the printregs command to display the values of all the hardware registers. The list of registers displayed by the debugger is machine-dependent. By default, most values are displayed in decimal radix. To display the register values in hexidecimal radix, set the $hexints variable to 1. 
print_registers_command
	: printregs
For example:

The dump command

Use the dump command without an argument to list the parameters and local variables in the current function. To list the parameters and local variables in an active function, specify it as an argument.

Use the dump . command (include the dot) to list the parameters and local variables for all functions active on the stack. 

dump_command	
	: dump qual_symbol
        | dump .
For example:

When large and complex values are passed by value to a routine on the stack, the output of the where, up, down, and dump command can be voluminous. You can set the control variable $stackargs to 0 to suppress the output of argument values in the where, up, down, and dump commands.

The call command

After a breakpoint or a signal suspends program execution, you can execute a single function in your program by using the call command, or by including a function call in the expression argument of a debugger command. Calling a function lets you test the function's operation with a specific set of parameters. 
call_command
	: call call_expression
Specify the function as if you were calling it from within the program. If the function has no parameters, specify empty parentheses.

For multithreaded applications, the call is made in the context of the current thread.

For C++: When you set the $overloadmenu debugger variable to 1 and call an overloaded function, the debugger lists the overloaded functions and calls the function you specify.

When the function you call completes normally, the debugger restores the stack and current context that existed before the function was called.

While the program counter is saved and restored, calling a function does not shield the program state from alteration if the function you call allocates memory or alters global variables. If the function affects global program variables, for instance, those variables will be permanently changed.

Functions compiled without the debugger option to include debugging information may lack important parameter information and are less likely to yield consistent results when called.

The call command executes the specified function with the parameters you supply and then returns control to you (at the Ladebug prompt) when the function returns. The call command discards the return value of the function. If you embed the function call in the expression argument of a print command, the debugger prints the return value after the function returns.

The following example shows both methods of calling a function.

In the previous example, the call command results in the return value being discarded while the embedded call passes the return value of the function to the print command, which in turn prints the value. You can also embed the call within a more involved expression, as shown in the following example: All breakpoints or tracepoints defined and enabled during the session are active when executing a called function. When program execution halts during function execution, you can examine program information, execute one line or instruction, continue execution of the function, or call another function.

When you call a function when execution is suspended in a called function, you are nesting function calls, as shown in the following example:

Restrictions on the call command
The debugger supports function calls and expression evaluations that call functions, with the following limitations:

The whatis command

You can print information about the basic nature of a whatis_expression. The expression can be a normal language expression, name of type, function, and other language entities. The debugger will show you information about the entity rather than evaluating it. It will evaluate any contained expressions, such as pointers, needed to determine which entity you are referring to. 
whatis_command
	: whatis whatis_expression
The following example uses the whatis command to determine the storage representation for the data member _classification.

The whereis command

The whereis command lists all declarations of a variable together with each declaration's scope information fully qualified.

The scope information of a variable usually consists of the name of the source file that contains the function in which the variable is declared, the name of that function, and the name of the variable. The components of the scope information are separated by back-quotes (`). 

whereis_command
	: whereis whereis_name
        
whereis_name
	: identifier_or_typedef_name
	| ( identifier_or_typedef_name )
The whereis command is useful for obtaining information needed to differentiate overloaded identifiers that are in different units, or within different routines in the same unit. The following example shows how to set breakpoints in two C++ methods, both named print.

The which command

Use the which command to show you which declaration an identifier resolves to. The which command shows the fully qualified scope information for the instance of the specified expression in the current scope.

The scope information of a variable usually consists of the name of the source file that contains the function in which the variable is declared, the name of that function, and the name of the variable. The components of the scope information are separated by back-quotes (`). 

which_command
	: which which_name
        
which_name
	: identifier_or_typedef_name
	| ( identifier_or_typedef_name )
The following example shows how to use the whereis and which commands to determine a variable's scope.

Notes on C++ debugging

The following sections describe the debugger commands specific to debugging C++ programs.
Setting the class scope using the class command
The debugger maintains the concept of a current context in which to perform lookup of program variable names. The current context includes a file scope and either a function scope or a class scope. The debugger automatically updates the current context when program execution suspends.

The class command lets you set the scope to a class in the program you are debugging. 

c++_look_around_command
        : class [ class_name ]
If class_name is not specified, the class command displays the current class context.

Setting the class scope nullifies the function scope and vice versa. To get back to the default (current function) scope, use the command func 0

Explicitly setting the debugger's current context to a class allows for visibility into a class to:

After the class scope is set, you can set breakpoints in the class's member functions and examine data without explicitly mentioning the class name. If you do not want to affect the current context, you can use the scope resolution operator (::) to access a class whose members are not currently visible. Use the class command without an argument to display the current class scope. Specify an argument to change the class scope. After the class scope is set, refer to members of the class by omitting the classname:: prefix.

The following example shows the use of the class command to set the class scope to List in order to make member function append visible so a breakpoint can be set in append.

Displaying class information
The whatis and print commands display information on a class. Use the whatis command to display static information about the classes. Use the print command to view dynamic information about class objects.

The whatis command displays the class type declaration, including:

For classes that are derived from other classes, the data members and member functions inherited from the base class are not displayed. Any member functions that are redefined from the base class are displayed.

The print command lets you display the value of data members and static members. Information regarding the public, private, or protected status of class members is not provided, since the debugger relaxes the related access rules to be more helpful to users.

The type signatures of member functions, constructors, and destructors are displayed in a form that is appropriate for later use in resolving references to overloaded functions.

The following example shows the whatis and print commands in conjunction with a class.

Displaying object information
The whatis and print commands display information on instances of classes (objects). Use the whatis command to display the class type of an object. Use the print command to display the current value of an object.

You can also display individual object members using the member access operators, period (.) and right arrow (->), in a print command.

You can use the scope resolution operator (::) to refer to global variables, to refer to hidden members in base classes, to explicitly refer to a member that is inherited, or otherwise to name a member hidden by the current context.

When you are in the context of a nested class, you must use the scope resolution operator to access members of the enclosing class.

The following example shows how to use the whatis and print commands to display object information.

Displaying static and dynamic type information
When displaying object information for C++ class pointers or references, you have the option of viewing either the static type information or the dynamic type information.

The static type of a class pointer or reference is its type as defined in the source code and thus cannot change. The dynamic type is the type of the object being referenced, before any casts were made to that object and thus may change during program execution.

The debugger provides a debugger variable, $usedynamictypes, which allows you to control which form of the type information will be displayed. The default value for this variable is true (1), which indicates that the dynamic type information will be displayed. Setting this variable to false (0) instructs the debugger to display static type information. The output of the following commands will be affected: print, trace, tracei, and whatis.

The display of dynamic type information is supported for C++ class pointers and references. All other types display their static type information. In addition, if the dynamic type of an object cannot be determined, the debugger defaults to the use of the static type information.

This debugger functionality does not relax the C++ visibility rules regarding object member access through a pointer/reference (only members of the static type are accessible). For more information about the C++ visibility rules, see The Annotated C++ Reference Manual.

In order for the dynamic type information to be displayed, the object's static type must have at least one virtual function defined as part of its interface (either one it introduced or one it inherited from a base class). If no virtual functions are present for an object, only the static type information for that object will be available for display.

The following example shows debugger output with $usedynamictypes set to 0 (false).

The following example displays debugger output with $usedynamictypes set to 1 (true). The output is for the same object as the previous example, at the same point in program execution.
Displaying virtual and inherited class information
When you use the print command to display information on an instance of a derived class, the debugger displays both the new class members as well as the members inherited from a base class.

Pointers to members of a class are not supported.

When you use the print command to display the format of C++ classes, the class name (or structure/union name) is displayed at the top of the output. Data members of a class that are inherited from another class are commented using a double slash (//). Only those data members that are inherited within the current class being printed are commented.

The following example shows how the debugger uses C++ style comments to identify inherited class members. In the example, class CompoundNode inherits from class IntNode, which inherits from class Node. When printing a class CompoundNode object, the data member _data is commented with "// class IntNode", signifying that it is inherited from class IntNode. The member _nextNode is commented with "// class IntNode::Node" showing that it is inherited from class IntNode which inherits it from class Node. This commenting is also provided for C++ structs.

If you have two members in an object with the same name but different base class types (multiple inheritance), you can refer to the members using the following syntax: 

	object.class::member
or 

	object->class::member
This syntax is more effective than using the object.member and object->member syntaxes, which can be ambiguous. In all cases, the debugger uses the C++ language rules as defined in The Annotated C++ Reference Manual to determine which member you are specifying.

The following example shows a case where the expanded syntax can be used.

Member functions on the stack trace
The implicit this pointer, which is a part of all nonstatic member functions, is displayed as the address on the stack trace. The class type of the object is also given.

Sometimes the debugger does not see class type names with internal linkage. When this happens, the debugger issues the following error message: 

	Name is overloaded.
Trying to examine an inlined member function that is not called results in the following error: 
	Member function has been inlined.
The debugger will report this error regardless of the setting of the -noinline_auto compilation flag. As a workaround, include a call to the given member function somewhere in your program. (The call does not need to be executed.)

If a program is not compiled with the -g flag, a breakpoint set on an inlined member function may confuse the debugger.

Resolving ambiguous references to overloaded functions
In most cases, the debugger works with one specific function at a time. In the case of overloaded function names, you must specify the desired overloaded function. There are two ways to resolve references to overloaded function names, both under the control of the $overloadmenu debugger variable (the default setting of this debugger variable is 1):
Advanced program information - verbose mode
By default, the debugger gives no information on virtual base class pointers for the following: By setting the $verbose debugger variable to 1, you can request that this information be printed in subsequent debugger responses. When the $verbose debugger variable is set to 1 and you display the contents of a class using the whatis command, several of the class members listed are not in the source code of the original class definition. The following line shows specific output from the whatis command for one of the additional members: The __vptr variable contains the addresses of all virtual functions associated with the class. Several other class members are generated by the compiler for internal use.

The compiler generates additional parameters for nonstatic member functions. When the $verbose debugger variable is set to 1, these extra parameters are displayed as part of each member function's type signature. If you specify a version of an overloaded function by entering its type signature and the variable is set to 1, you must include these parameters. Do not include these parameters if the variable is set to 0.

When the $verbose variable is set to 1, the output of the dump command includes not only standard program variables but also compiler-generated temporary variables.

The following example prints class information using the whatis command under different settings of the $verbose variable.

Looking at the generated code

Memory display command - '/'

The debugger gives you a way to read and write arbitrary memory locations in your program. 
machinecode_level_command
     : address_expression /[ count ] [ mode ]
     | address_expression , address_expression / [ mode ]
The first command displays the values stored in each of the memory chunks specified by the count, whose size depend on mode (see below), starting at address_expression. If count is not specified, 1 is assumed. The second command displays the values stored in the memory block starting at address_expression and ending at address_expression.

You can display stored values in the following formats by specifying mode: 

mode
	: d       Print a short word in decimal
        | dd      Print a 32-bit (4-byte) decimal display
        | D       Print a long word in decimal
	| u       Print a short word in unsigned decimal
	| uu      Print a 32-bit (4-byte) unsigned decimal display
	| U       Print a long word in unsigned decimal
	| o       Print a short word in octal
	| oo      Print a 32-bit (4-byte) octal display
	| O       Print a long word in octal
	| x       Print a short word in hexadecimal
	| xx      Print a 32-bit (4-byte) hexadecimal display
	| X       Print a long word in hexadecimal
	| b	  Print a byte in hex
	| c	  Print a byte as a character
	| s	  Print a string of characters (a C-style string ending in null)
       	| C	  Print a wide character as a character
        | S	  Print a null terminated string of wide characters
        | f	  Print a single precision real number
        | g	  Print a double precision real number
        | L	  Print a long double precision real number
        | i	  Disassemble machine instructions
If mode is not specified, the mode used in the previous / command is assumed. In the case that there is no previous / command, 'X' is assumed.

Machine-level debugging

The debugger lets you debug your programs at the machine-code level as well as at the source-code level. Using debugger commands, you can examine and edit values in memory, print the values of all machine registers, and step through program execution one machine instruction at a time.

Only those users familiar with machine-language programming and executable-file-code structure will find low-level debugging useful.

For more information on machine-level debugging, see Machine-level debugging in Ladebug Debugger Advanced Topics.

Looking at shared libraries

shared_library_command
        : listobj
        | readsharedobj filename
        | delsharedobj  filename
Use the listobj command to list all loaded objects, including the main image and the shared libraries.

For each object, the information listed consists of the full object name (with pathname) and the starting and ending addresses for the .text, .data, and .bss sections.

Use the readsharedobj command to read in the symbol table information for the specified shared object. This object must be a shared library or loadable kernel module. The command can be used only when a debuggee program is specified; that is, either the debugger has been invoked with it, or the debuggee was loaded by the load command.

Conversely, use the delsharedobj command to remove the symbol table information for the shared object from the debugger.

Modifying the process

In addition to the normal side-effects of evaluating expressions, including calls, you can explicitly modify the memory of the current process, and also modify the actual loadable file (either executable file or shared library) that has been mapped into memory. 
modifying_command
        : assign target = expression
        | patch  target = expression
	
target
	: unary_expression

The assign command

Use the assign command to change the value associated with a variable, memory address, or expression that is accessible according to the scope and visibility rules of the language. The expression can be any expression that is valid in the current context.

The following example shows how to deposit the value 5 into the data member _data of a C++ object.

The following example shows how to change the value associated with a variable and the value associated with an expression.
For C++: 
	assign [classname::]member = ["filename"] `expression 
	assign [object.]member = ["filename"] `expression
For C++, use the assign command to modify static and object data members in a class, and variables declared as reference types, type const, or type static. The address referred to by a reference type cannot be changed, but the value at that address can be changed.

The assign command in machine-level debugging

You can use the assign command to alter the contents of memory specified by an address. For more information about machine-level debugging, see Machine-level debugging.

The patch command

Use the patch command to correct bad data or instructions in executable disk files. The text, initialized data, or read-only data areas can be patched. The bss segment, as well as stack and register locations, cannot be patched because they do not exist on disk files.

Use this command exclusively when you need to change the on-disk binary. Use the assign command when you need only to modify debuggee memory. If the image is executing when you issue the patch command, the corresponding location in the debuggee address space is updated as well. (The debuggee is updated regardless of whether the patch to disk succeeded, as long as the source and destination expressions can be processed by the assign command.) If your program is loaded but not yet started, the patch to disk is performed without the corresponding assign to memory.

Continuing execution of the process

Before continuing, you should decide whether or not to make a snapshot, in case you want to revert to that snapshot state and try a different set of steps.

After this is done, use the following commands to continue executing the program. 

continue_command
        : step_into_command
        | step_over_command
        | step_out_of_command
        | cont_command
        | cont_from_place_command

The step and stepi commands

Use the step command to execute a line of source code. When the line being stepped contains a function call, the step command steps into the function and stops at the first executable statement.

Use the stepi command to step into the next machine instruction. When the instruction contains a function call, the stepi command steps into the function being called.

For multithreaded applications, use these commands to step the current thread while putting all other threads on hold.

If the optional expression argument is supplied, the debugger evaluates the expression as a positive integer that specifies the number of times to execute the command. The expression can be any expression that is valid in the current context. 

step_into_command
        : step  [ step_number ]
        | stepi [ step_number ]

step_number
        :  expression
In the following example, two step commands continue executing a C++ program. The following example shows stepping by instruction (stepi). To see stepping into calls, see the next example.

The next and nexti commands

Use the next command to execute a line of source code. When the next line to be executed contains a function call, the next command executes the function being called and stops the process at the line immediately after the function call.

Use the nexti command to execute a machine instruction. When the instruction contains a function call, the nexti command executes the function being called and stops the process at the instruction immediately after the call instruction.

For multithreaded applications, use these commands to step the current thread while putting all other threads on hold.

If the optional expression argument is supplied, the debugger evaluates the expression as a positive integer that specifies the number of times to execute the command. The expression can be any expression that is valid in the current context. 

step_over_command
        : next  [ step_number ]
        | nexti [ step_number ]
For example: The following example shows the difference between stepi and nexti over the same call:

The return command

Use the return command without an argument to continue execution of the current function until it returns to its caller. If you include a function name, execution continues until control is returned to the specified function. The function must be active on the call stack. 
step_out_of_command
        : return
        | return qual_symbol
In the following example, the next command is used to step through process execution in the append method. The return command is used to finish the append method and return control to the caller.

The cont command

Use the cont command without a parameter value to resume process execution until a breakpoint, a signal, an error, or normal process termination is encountered. Specify a signal parameter value to send an operating system signal to the process. 
cont_command
        : cont [ in loc ]  
        | cont [ signal ] [ to_source_line ]
        | number_expression cont [ signal ]  
        | conti to address_expression

to_source_line
	: to [filename_string :] line_number  
        
number_expression
	: address_expression
When you use the cont command, the debugger resumes execution of the entire process.

In the following example, a cont command resumes process execution after it was suspended by a breakpoint.

The signal parameter value can be either a signal number or a string name (for example, SIGSEGV). The default is 0, which allows the process to continue execution without specifying a signal. If you specify a signal parameter value, the process continues execution with that signal.

The in argument is used to continue until the named function is reached. The function name must be valid. If the function name is overloaded and you do not resolve the scope of the function in the command line, the debugger prompts you with the list of overloaded functions bearing that name from which to choose.

The to parameter value is used to resume execution and then halt when the specified source line is reached.

The form of the optional to parameter must be either:

You can repeat the cont command (n +1) times by entering n cont.

You can set a one-time breakpoint on an instruction address before continuing by entering conti to address_expression.

You can modify the PC before continuing using the goto command, however this is extremely dangerous and can yield unpredictable results. We don't recommend using this command. 

cont_from_place_command	
        : goto line_expression

Snapshots as an undo mechanism

You can save the current state of the debuggee process in a snapshot, and later "revert" to that state and try a different set of steps. Conceptually speaking, this feature is similar to the "undo" function in text editors, except that with snapshots you have control of the granularity of each undo. See the Introduction for a quick overview. 
snapshot_command
        : save_snapshot_command
        | clone_snapshot_command
	| show_snapshot_command
        | delete_snapshot_command

The save snapshot command

Use the save snapshot command to save the state of the current process in a snapshot. Snapshots are numbered sequentially starting from 1. 
save_snapshot_command
        : save snapshot
In the following example, the first line of the save snapshot message shows the snapshot_number (1), the time it is saved, and the ID number of the process that implements the snapshot. The next two lines show the status of the snapshot.

The clone snapshot command

Use the clone snapshot command to revert the state of the debuggee process to that of a previously saved snapshot. By doing this, you can conveniently return to the state saved in the snapshot as opposed to re-running the process and re-entering the debugger command sequence that brought you to that state.

Note that rerun and clone snapshot are different in that rerun always executes the process from the beginning, whereas clone snapshot doesn't execute the process at all; it simply duplicates the saved snapshot (using a mechanism similar to the fork system call) and "pretends" that the process execution has stopped at the point when the snapshot was saved. 

clone_snapshot_command
        : clone snapshot [ snapshot_id ]
The clone snapshot command clones the snapshot specified by snapshot_id. If no snapshot_id is specified, the most recently saved existing snapshot is cloned.

There are two side-effects to cloning a snapshot:

For example:

The show snapshot command

Use the show snapshot * and show snapshot all commands to display all the snapshots that have been saved from the current process. Use show snapshot snapshot_id_list to display the snapshots specified. If no snapshots are specified, the most recently saved existing snapshot is displayed. 
show_snapshot_command
        : show snapshot [ snapshot_id_list ]

snapshot_id_list
	: snapshot_id ,...
	| all
	| *
For example:

The delete snapshot command

Use the delete snapshot * and delete snapshot all commands to delete all the snapshots that have been saved from the current process. Use delete snapshot snapshot_id_list to delete the specified snapshots. If no snapshots are specified, the most recently saved existing snapshot is deleted. 
delete_snapshot_command
        : delete snapshot [ snapshot_id_list ]
For example:

Snapshot limitations

Debugging optimized code (Linux only)

The debugger for the Alpha Linux platform provides several features that make it easier to debug a program that has been compiled with optimization. These features are described in this section.

Support for these features is limited to programs compiled using compilers from Compaq Computer Corp. These include Compaq Fortran 90, Compaq C and Compaq C++.

Taking advantage of these features only requires compilation of your program as described below. The debugger automatically takes advantage of specialized information that is included in the resulting executable program.

There are no Ladebug commands that control the debugger behavior when debugging optimized code. Rather, existing commands sometimes behave somewhat differently than you would see in the absence of optimization.

The sections below provide some insight into how Compaq compilers and the debugger deal with the consequences of key optimizations and how that may influence debugging of your program.

Why debug optimized code?

Highly optimizing compilers such as supplied by Compaq generally transform a program in many ways to make your program run as quickly as possible. Expressions may be combined, rearranged or even eliminated if determined not to be needed; expressions and even complete statements may be moved out of loops; the order of statements may be changed; calls to functions may be replaced with a copy of the called function (inlining), the instructions for a single statement may be interspersed with instructions for other statements both before and after, and so on. All of these transformations greatly complicate the ability of a debugger to display information about the state and progress of your program as well as to control its execution. Compaq compilers and the debugger try to avoid presenting erroneous, misleading or incomplete information when debugging optimized code, but this not always possible. For these reasons, it is almost always easier to debug an unoptimized version of your program.

Why would you ever try to debug an optimized version? The most likely reason is that the program appears to work correctly when unoptimized but somehow fails when optimized. As a result, you may have little choice but to try to isolate the problem using the optimized program.

The most common reason that a program apparently works correctly when unoptimized but fails when optimized is this:

Your program performs some action whose behavior is undefined or implementation dependent, and the optimized version is different from the unoptimized version when performing this action.

For example, your program might read and depend on the value of a variable that did not get assigned a value. When executed in unoptimized form, the value that happens to be in that variable might accidently result in the desired behavior. But when optimized, the variable might have some other value that leads to different behavior. As another example, sometimes your program may be subtly dependent on the exact order in which operations are performed—and optimization can result in a different order. There are many other examples that are beyond the scope of this discussion.

It is also possible that there is a bug in the compiler. While it does happen, experience with Compaq compilers indicates that this is rare.

In any case, to determine the cause or nature of the problem requires debugging using the optimized version. Then you can determine how best to resolve the problem. (Of course, you could also choose to reduce the level of optimization, possibly to none, to obtain the desired behavior but that may not result in acceptable performance.)

Program preparation

For Compaq compilers, preparing a program for debugging with optimization involves compiling that program using the -g3 option. All other preparation is unchanged.

Note that the -g3 option differs from the -g option in that it does not affect the optimization level. That is, -g (equivalently -g2) sets the optimization level to zero (that is, -O0), even overriding an explicit optimization setting; -g3 leaves the optimization level alone. See the manual pages for the respective compilers for further details.

Split lifetime variables

A variable is said to have split lifetimes if the set of fetches and stores of the variable can be partitioned such that none of the values stored in one subset are ever fetched in another subset. When such a partition exists, the variable can be "split" into several independent "child" variables, each corresponding to a partition. As independent variables, the child variables can be allocated independently. The effect is that the original variable can be thought to reside in different locations at different points in time--sometimes in a register, sometimes in memory, and sometimes nowhere at all. Indeed, it is even possible for the multiple child variables to be active simultaneously.

Split lifetime information in the debugging symbol table describes each of the child variables associated with the main variable, where it is allocated, and the exact range of addresses over which each child is valid.

Because assignments may not occur in the same order as in the source code, the split lifetime information also includes a list of all of the places that the current value may have been assigned. In general this is a list of possibilities because several execution paths may converge bringing together multiple assignment possibilities; the debugger does not trace the exact execution path that reaches a stopping point so it can only report the set of relevant alternatives.

When a variable does not have a value at the current location, the debugger cannot print a value for it and reports an error as follows:

The first error message line indicates that there is a symbol L, but that it does not (currently) have a value. The preceding informational line distinguishes between two cases:

If a variable is not declared at all, then the error report looks like:

When a variable has a value, there may also be information concerning where that variable was assigned: The value may be assigned from several places: It is possible, though unusual, for the same line to be listed more than once; this means that there is more than one instruction from the same line that assigns a value.

The following limitations apply:

Semantic stepping

A major problem with stepping by line using the next and step commands through optimized code is that the apparent source program location "bounces" forward and back, forward and back, with the same line often appearing again and again. In large part this bouncing is due to a compiler optimization called code scheduling, in which instructions that arise from the same source line are scheduled, that is, reordered and intermixed with other instructions, for better execution performance. For example, in sample programs from a prominent benchmark suite, the average number of instructions in sequence that share the same line number is typically between 2 and 3—and typically 50 to 70 percent of those sequences consist of just 1 instruction!

Semantic stepping causes the program to execute up to, but not including, an instruction that causes a semantic effect, as well as being in a different line. Instructions that cause semantic effects are instructions that:

The visible effect of this support is that the step and next commands will generally make more or less normal progress through your program. The effects of optimization cannot be hidden entirely, however there will be some occasional stepping backwards as well as forwards due to code reordering, and some lines/statements will not appear because they were optimized away, but the result will be generally more usable.

Discontiguous scopes

In addition to the blurring of line number boundaries as described in the discussion of semantic stepping, a similar blurring occurs at the beginning and end of an inner scope boundary. That is, one instruction may be related to the beginning of a new scope (and line) where a new variable is declared and becomes visble, while the next instruction may revert to a source location (line) before the scope begins.

Debugging optimized code information in the symbol table includes a detailed description of the possibly multiple disjoint instruction ranges that belong to or make up a scope. This helps assure that variable lookups find the right symbols at the current location.

You are not likely to directly perceive the effects and benefits of this support; just know that it is part of "getting the right answer" in the presence of optimization.

General cautions

When debugging optimized code, there are some Ladebug commands that should be used with caution or even avoided completely.

Use with caution

down, dump, func, return, where, pop, up

These commands generally depend on there being a distinct call frame on the execution stack for each called function. However, inlining can merge a called routine into the caller with the result that there is one frame instead of the two (or more) that might be expected. These commands can be used, but be careful to make sure that you end up in the frame you intend after each use and/or don't get misled.

next, nexti

For a call that is inlined, the next and nexti commands will appear to step into the called function instead of stepping over it.

Avoid completely

assign

It is generally not possible to reliably assign to a variable before the value of the variable has been used.

goto

It is generally not possible to reliably determine where the first instruction of a line begins or to avoid repeating instructions from the destination line that may have already been executed.
if logical filter in breakpoint commands
The condition expression may not have a value at all in some places where the expression needs to be evaluated. Worse yet, the debugger sometimes attempts to cache the address of a variable, which does not correctly support split variables.
watch variable
The debugger does not support watching a split variable. This command will most likely fail because the debugger cannot watch a variable (child or otherwise) that is allocated in a register; even if it does appear to succeed, the debugger will be watching the location of just one child even when that location is not relevant.
operator &
It is generally not possible to reliably determine whether a variable has only one lifetime and thus a unique address.

Limitations on support for Compaq C++

The debugger interprets C++ names and expressions using the language rules described in The Annotated C++ Reference Manual (Ellis and Stroustrup, 1990, Addison-Wesley). C++ is a distinct language, rather than a superset of C. Where the semantics of C and C++ differ, the debugger provides the interpretation appropriate for the language of the program being debugged.

To make the debugger more useful, it relaxes some standard C++ name visibility rules. For example, you can reference public, protected, and private class members.

The following limitations apply when you debug a C++ program:

Limitations for debugging templates include:

Limitations on support for Compaq Fortran

The debugger and the Tru64 UNIX operating system support the Compaq Fortran language with certain limitations, that are described in the following sections.

Be aware of the following data-type limitations when you debug a Fortran program:

The following limitations apply only to Compaq Fortran 90:

Limitations on procedure invocations

Limitations on support for Compaq Ada

The debugger and the Tru64 UNIX operating system support the Compaq Ada language with certain limitations, which are described in the following sections.

Limitations for expressions in Ladebug commands

Expressions in Ladebug commands use C source language syntax for operators and expressions. Data is printed as the equivalent C data type. The following table shows Ada expressions and Ladebug equivalents.

Ada Expression
Debugger Equivalent
Binary operations and unary operations Only integer, floating, and Boolean expressions are easily expressed.
a+b,-,*a+b,-,*
a/ba/b
a = b /= < <= > >= a = = b != < <= > >=
a and ba&&b
a or ba | | b
a rem ba%b
not(a=b)!(a==b)
-a-a
Qualified expressionsNone. There is no easy way of evaluating subtype bounds.
Type conversions Only simple numeric conversions are supported, and the bounds checking cannot be done. Furthermore, float -> integer truncates rather than rounds.

integer -> float

(ladebug) print (float) (2147483647)
2147483648.0
(ladebug) print (double) (2147483647)
2147483647.0

AttributesNone, but if E is an enumeration type with default representations for the values then
E'PRED(X) is the same as x-1.
E'SUCC(X) is the same as x+1
p.all *p (pointer reference)
p.m p -> m (member of an "access record" type)

Limitations in data types

This section lists the limitation notes by data type. For more information on these types, with examples, see the Developing Ada Programs on Tru64 UNIX Systems manual. Also see the Compaq Ada release notes for detailed information on debugging.

All types

The debugger, unlike the Ada language, allows out-of-bounds assignments to be performed.

Integer types

If integer types of different sizes are mixed (for example, byte-integer and word-integer), the one with the smaller size is converted to the larger size.

Floating-point types

If integer and floating-point types are mixed in an expression, the debugger converts the integer type to a floating-point type.

The debugger displays floating-point values that are exact integers in integer literal format.

Fixed-point types

The debugger displays fixed-point values as real-type literals or as structures. The structure contains values for the sign and the mantissa. To display the structure's value, multiply the sign and mantissa values. For example:

Enumeration types

The debugger displays enumeration values as the actual enumeral or its position.

Enumeration values must be manually converted to 'pos values before you can use them as array indices.

Array types

The debugger displays string array values in horizontal ASCII format, enclosed in quotation ("x") marks. A single component (character) is displayed within single quotation ('x') marks.

The debugger allows you to assign a component value to a single component; you cannot assign using an entire array or array aggregate.

Arrays whose components are neither a single bit nor a multiple of bytes are described to the debugger as structures; a print command displays only the first component of such arrays.

Records

The debugger cannot display record components whose offsets from the start of the record are not known at compile time.

For variant records, however, the debugger can display the entire record object that has been declared with the default variant value. The debugger allows you to print or assign a value to a component of a record variant that is not active.

Access types

The debugger does not support allocators, so you cannot create new access objects with the debugger. When you specify the name of an access object, the debugger displays the memory location of the object it designates. You can examine the memory location value.

Limitations for tasking programs

When you debug Ada tasking programs, you use the debugger and the DEC Ada ada_debug routine.

Limitations on support for Compaq Cobol

Limitations on assignment

The following limitations apply to assignment in COBOL debugging:

Other limitations

Other limitations when you debug COBOL programs include:

Appendix 1—Debugger Variables

The debugger has the following predefined variables:

Variable
Default Setting
Description
$ascii
1
 Prints ASCII or all ISO Latin-1.
$beep
1
 Beeps on illegal command line editing.
$catchexecs
0
 Stops execution on program exec.
$catchforkinfork
0
 Notifies you as soon as the forked process is created (otherwise you are notified when the call finishes).
$catchforks
0
 Notifies you on program fork.
$childprocess
None
 When the debugger detects a fork, it assigns the child process ID to $childprocess.
$curevent
0
 Current breakpoint number.
$curfile
(null)
 Current source file.
$curfilepath
(null)
 Current source file access path.
$curline
0
 Current source line.
$curpc
0
 Current point of program execution.
$curprocess
0
 Current process ID.
$cursrcline
0
 Last source line plus one.
$curthread
0
 Current thread ID.
$dbxoutputformat
0
 Displays various data structures in dbx format.
$dbxuse
0
 Replaces current use paths.
$decints
0
 Displays integers in decimal radix.
$doverbosehelp
1
 Displays the help menu front page.
$editline
1
 Enables command line editing.
$eventecho
1
 Echoes events with event numbers.
$funcsig
1
 Displays function signature at breakpoint.
$giveladebughints
1
 Displays hints on Ladebug features.
$hasmeta
0
 Interprets multibyte characters.
$hexints
0
 Displays integers in hex radix.
$historylines
20
 Number of commands to show for history.
$indent
1
 Prints structures with indentation.
$lang
"None"
 Programming language of current routine.
$lasteventmade
0
 Number of last (successful) breakpoint definition.
$lc_ctype
"C"
 Current locale information.
$listwindow
20
 Number of lines to show for list.
$main
"main"
 Name of first routine in program.
$maxstrlen
128
 Largest string to print fully.
$octints
0
 Displays integers in octal radix.
$overloadmenu
1
 Prompts for choice of overloaded C++ name.
$page
1
 Paginates debugger terminal output.
$pagewindow
0
 Number of lines per output page.
$parentprocess
None
 When the debugger detects a fork, it assigns the parent process ID to $parentprocess.
$pimode
0
 Echoes input to log file on playback input.
$prompt
"ladebug"
 Specifies debugger prompt.
$repeatmode
1
 Repeats previous command on .
$showlineonstartup
0
 Displays the first executable line in main.
$showwelcomemsg
1
 Displays welcome message at startup time.
$stackargs
1 (in dbx mode, 0)
 Shows arguments in the call stack.
$statusargs
1 (in dbx mode, 0)
 Prints breakpoints without parameters.
$stepg0
0
 Steps over routines with minimal symbols.
$stoponattach
0
 Stops the running process on attach.
$stopparentonfork
0
 Stops parent process execution on fork. When set to a non-zero value, this variable instructs the debugger to stop the parent process after it forks a child process. The child process continues to run if $catchforks is set, otherwise it does not. The default is 0.
$threadlevel
"decthreads"
 DECthreads or native threads.
$usedynamictypes
1
 Evaluates using C++ static or dynamic type.
$verbose
0
 Produces even more output.



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