Title and Contents

Part II
A Guide to Using the Ladebug Debugger

Part II provides most of the information needed to make expert use of the debugger.

Some additional details have been moved to Part III: Advanced Topics so they do not hinder the reading of this information.

Chapter 2—Preparing a Program for Debugging

To facilitate debugging, you can prepare your source code and the compiler and linker environment.

2.1 Preparing Your Source Code

You do not need to make changes to the source code to debug the program. However, you can do the following to make debugging easier:

2.2 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 reference page for your compiler. The switch is probably -g.

The debugging information is propagated into the a.out (executable) or .so (shared library) by the ld command. It is removed by the strip command. 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.

If you are debugging C++ applications and you have unused variables in your code, or if opaque classes, structs, or unions keep showing up in Ladebug, you may want to compile particular files with the cxx -gall and -gall_pattern switches. See cxx(1).

If you are debugging optimized code, see the appropriate compiler documentation for information about various -g switches and their relationship to optimization.

Chapter 3—Starting the Debugger

You can start the debugger in the following ways:

This chapter also discusses the following topics:

3.1 Using the ladebug Command to Start 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 you can attach to a running process.

The following is the shell syntax to invoke the debugger using the ladebug command: 

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

Options and Parameters Description
-c Specifies an initialization command file. The default initialization file is .dbxinit. During startup, the debugger searches for this file 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) on Tru64 UNIX systems only.
-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.
-interactive Causes the debugger to act as though stdin is isatty(), regardless of whether or not it is. This flag is sometimes useful when using rsh to run the debugger. Currently, the only effect is to cause the debugger to output the prompt to stdout when it is ready for the next line of input.
-k or -kernel Enables local kernel debugging.
-line Specifies the serial line for remote kernel debugging. You must use this 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.
-parallel In Ladebug Version 67, enables parallel debugging for applications using the Message Passing Interface (MPI) or Unified Parallel C (UPC). Earlier versions of Ladebug do not have this capability.
-pid Specifies the process ID of the process to be debugged. You cannot use this option 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 (" "). You can specify a debugger prompt when you start the debugger from a shell with the -prompt option. The default prompt is (ladebug). 
     % ladebug -prompt ">> " sample
     >> quit
You can also change the prompt by setting the $prompt debugger variable. For example: 
    
(ladebug) set $prompt = "newPrompt>> "
newPrompt>>
-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. You must use this 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 do not 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:

3.2 Starting the Debugger Using Emacs

You can control your debugger process entirely through the Emacs Grand Unified Debugger (GUD) 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 from within Emacs is a 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, see the 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.

3.3 Ending a Debugging Session

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

Alternatively, you can type exit, which is a predefined alias for quit.

3.4 Getting Help

To access the online help about debugger commands, use the help command: 
help_command
        : help [ topic ]
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.

3.5 The Ladebug GUI (Tru64 UNIX Only)

You can start the GUI in either of two ways: When you use the GUI, a Debugger Output window appears, in which you manually enter commands. You can also use the available controls (push buttons, menus, and so on) to execute debugger commands.

You can shut down the GUI and leave the command line session running by choosing 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, choose File/Exit Debugger in the GUI window.

Chapter 4—Giving Commands to the Debugger

The debugger has several different mechanisms you can use to direct its behavior. It receives input from the following:

6.3 The load and unload Commands

Using the load command, you can tell the debugger which executable file 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 file name 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 at the point 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

pid 	
 	: integer_constant
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 an executable file. For example:

6.4 The 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 ... ]
If the rerun command is specified without arguments, the arguments and io_redirection argument of the most recent run command entered with arguments are used. If there was no previous run command, the rerun command defaults to run. If the last modification time 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 argument allows you to change stdin, stdout, and stderr, which are otherwise inherited from the debugger process. For example: 

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, only use the following forms because the grammar may change in a future release of the debugger: 

     > 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:

6.5 The 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:

6.6 The attach Command

If a process already exists, you can have the debugger attach to it: 
attach_command
        : attach pid [ filename ]
The process is specified by its pid: 
pid
        : expression
For 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 pressing Ctrl/C or by 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 ,...
For example:

6.7 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. For example: 
show_environment_variable_command
        : printenv [ environment_variable_name ]
        | export
        | setenv
NOTE: The export and setenv commands 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 you specify an asterisk (*), all environment variables are removed.

NOTE: There is no command to simply return to the initial state the environment variables had when the debugger started. You must use set_environment_variable commands and unset_environment_variable commands appropriately.

6.8 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, you can control only one of the processes that the debugger controls. 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
        | *
For 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 the debugger exits or until you switch to it and continue it.

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

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

6.9 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.

In the preceding example, note the following:

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.

In the preceding example, note the following:

NOTE: If you catch the child but not the parent, and the parent code tries to execute a wait on the child, the target will get stuck if you do not let the child run to completion. This happens because the parent is running but making no progress, and the child is stopped by the debugger. For example:

6.10 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 executed process. The debugger keeps a history of the progression of the executed files.

In the following scenario, you set the predefined variables $catchforks and $catchexecs to 1. The debugger will notify you when an execution 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 run-time loader entry point:

Note the following:

6.11 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 Part III.

6.12 Kernel Debugging

The debugger supports kernel debugging, which is a task normally performed by systems engineers or system administrators. Debugging the kernel is a specialized process and can take several forms. This debugger supports local and remote kernel debugging, and crash dump analysis. For more information on kernel debugging, see Kernel Debugging in Part III.

Chapter 7—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 at which you observe the first evidence of the problem. 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 you will not see 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 actions you want the debugger to take when that event is detected.

You can define breakpoints based on the following actions:

You can also enable, disable, or delete breakpoints.

Breakpoint commands include the following: 

breakpoint_command
        : breakpoint_definition_command
        | simple_stop_command
        | signal_command
        | obsolete_breakpoint_definition_command
        | breakpoint_table_command
This chapter discusses the following topics:

7.1 Breakpoint Definitions

The following is a particularly common breakpoint:

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. For example:

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 (if specified) and breakpoint_actions (if specified) collectively specify the action part.

NOTE: Additional obsolete forms of breakpoint definition are retained only for backward compatibility with earlier versions of the debugger. These forms are explained in Obsolete Breakpoint Commands. 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:

7.1.1 Disposition

disposition
        : stop
        | when
The stop command 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.

The when command 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.

7.1.2 The quiet Specifier

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 specifier tells the debugger to omit this information, as in the following example:

7.1.3 Detectors

The debugger uses several kinds of detectors, 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 locations 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 access detector specifies any kind of memory access using an unaligned access.

This section describes each type of detector.

7.1.3.1 Place Detectors
You can use place detectors 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

The in function_name detector specifies the event at which execution reaches the entry of the named function.

If the function name is ambiguous (more than one function can match 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.

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

The pc address_expression detector specifies the event at which execution reaches the given machine address:

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

The every procedure entry detector 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 breakpoints 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.

The every instruction detector 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. Be aware 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.

7.1.3.2 Watch Detectors
You can use watch detectors 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 expression
        | 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, or can include all accesses.

If you specify a variable, the memory to be watched includes all of the memory for that variable, as determined by the variable's type. The following example watches for write access to variable _nextNode, which is allocated in the 8 bytes at the address shown in the last line of the example:

The specified variable is watched. If "p" is a pointer, watch variable p will watch the content of the pointer, not the memory pointed to by "p". Use watch memory *p to watch the memory pointed to by "p", as well as watch variable *p.

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

If you specify the within modifier, then only those accesses that occur within the given function (but not any function it calls) are watched. For example:

7.1.3.3 Signal Detectors
You can use signal detectors to determine when a particular UNIX signal is raised: 
signal_detector
        : signal signal_id ,...

signal_id
        : integer_constant
        | signal_name
You can specify signals by numeric value or by their conventional UNIX names, without or without the leading "SIG": If the debugger catches a signal event, then a subsequent simple continue will resume execution without raising the signal again in your process. However, you can specify a signal as part of the continue command to send the signal to your process when it resumes.
7.1.3.4 Unaligned Access Detectors (Tru64 UNIX Only)
You can use an unaligned access detector 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 administrator can choose a different default. See uac(1) for more information.) 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:

7.1.3.5 Unaligned Access Detector (Linux Only)
Unaligned accesses are automatically handled and quietly corrected on Linux. The debugger cannot detect these events.

7.1.4 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 only if the thread in which the event occurs matches one of the given threads. If not, the detection is quietly ignored.

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

7.1.5 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 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 rechecked 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.

An error in the expression does not change the disposition. If continuation was specified, then that is still what occurs. For example:

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.

7.1.6 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 ;...
7.1.6.1 Special Commands
The following debugger commands behave differently in some fashion when used within a breakpoint action list:
7.1.6.2 Commands to Use with Caution
You must be 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 or where you really are in your program. Such confusion may mislead or misdirect your debugging effort. For further discussion, see the section Recursive Breakpoints.

7.1.6.3 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 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.

7.1.7 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 only if all of the breakpoints so specify in their disposition. If any one of them specifies a break, the debugger prompts you for further commands.

7.1.8 Recursive Breakpoints

The following 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 or where you really are in your program. Such confusion may mislead or misdirect your debugging effort. See the call command example, which shows suspended execution in nested function calls.

7.1.9 Breakpoints and C++

This section describes how to use breakpoints when debugging C++ programs.
7.1.9.1 Member Functions
The following program illustrates setting breakpoints in C++ member functions:

You must name member functions in a way that makes them visible at the current position, according to the normal C++ visibility rules. For example:

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. For example:

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

7.1.9.2 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.

The following source text illustrates debugging of template instantiations:

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

7.1.9.3 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 the following functions, which 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.

The following source code illustrates these special library functions:

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

7.1.10 Special Signal Breakpoints

UNIX signals are operating-system-defined events that can be handled by the debugger. This section discusses breakpoint commands for UNIX signal events. It also discusses unaligned accesses and using Ctrl/C to interrupt your program.
7.1.10.1 The catch and ignore Commands
You can use two special breakpoint commands, catch and ignore, 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 debugger should catch and handle the given UNIX signal. You can specify the signal 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.
7.1.10.2 Unaligned Accesses (Tru64 UNIX Only)
You can request the debugger to catch unaligned accesses: This command is very much like the stop unaligned command.

Although this looks like a normal catch command, it differs in several respects:

Like other catch commands, the following rules apply:

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) by using the following command:

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

7.1.11 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 reprocesses 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 reprocesses the breakpoint and de-activates any events that apply to the unloaded image.

7.1.12 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
7.1.12.1 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 ]
An obsolete watchpoint and a stop command differ in the following respects: These differences are purely syntactic; the semantics are the same.
7.1.12.2 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_specifier [ logical_filter ] [ breakpoint_actions ]

where_modifier
        : in function_name
        | at line_specifier
	
line_specifier
        : filename:line_number 
        | line_number
Following are the differences between an obsolete tracepoint and a when command: 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. For example:

This is equivalent to the when every proc entry command (with equivalent performance degradation).
7.1.12.3 Instruction-Related Breakpoint Commands
The following commands control obsolete instruction-related breakpoints: 
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 the following differences:

7.2 Breakpoint Tables

As breakpoints are defined, they are recorded in a breakpoint table associated with the current program. You can display and modify this table 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. You can recall this value for later use if desired. For example:

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

7.2.1 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. For example:

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.

7.2.2 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. You can disable a breakpoint 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:

Chapter 8—Looking at the Code, the Data, and Other Process Information

This chapter describes how to look at the following components of a running process:

8.1 Looking at the Source Files

The debugger supports commands to perform the following operations with source files: 
browse_source_command
        : source_directory_mapping_command
        | source_searchlist_command
        | select_source_file_command
        | list_source_file_command
        | search_source_file_command
Special debugging information that the compiler puts in the .o files correlates the machine instructions and data back to the source files and the positions they came from.

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. All the information the debugger needs comes from the executable files or shared libraries, not from the source files.

8.1.1 How the Debugger Finds Source Files

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.
  2. If step 1 fails to find a readable file:
    Case 1: If dir_name is absolute, look for dir_name/base_name.
    Case 2: If dir_name is relative, for each entry use_dir in use_list, look for use_dir/dir_name/base_name. 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. Just as in step 2, the use_list entries are tried in the order they appear in the use_list.
  4. If step 3 fails, the debugger cannot find any source file.
The debugger uses the first-found readable file as the source file.

The debugger has source directory mapping commands that do the following:

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.

The following command displays a summary of the source directories in a.out. The ellipsis (...) here means that solarSystemSrc contains one or more source directories.

The following command directs the debugger to look for source files originally in solarSystemSrc in movedSolarSystemSrc instead. This time, the debugger finds the source file. The following 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.

The show all source directory command is identical to the show source directory command except that the mapping information of all the descendants of directory_name is displayed:

When you further expand ellipsis points (...), where directory is the directory on the line above the ellipsis points:

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. For example:

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

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 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 in which 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 nonzero (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.

8.1.2 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. The 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 ]
        : fileexpr [ expression ]

Use 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.

If the file name is not a literal, use the fileexpr command. For example, if you have a script that calculates a file name in a debugger variable or in a routine that returns a file name as a string, you can use fileexpr to set the file.

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. This example also shows the fileexpr command setting the current scope back to the original file, which is solarSystem.cxx.

8.1.3 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 one exists.

However, some primitive inspection capabilities are 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 (the line number of the first line to display within the current source file) or a function (the first line of the function).

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

8.1.4 Searching the Content 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 string is actually just the rest of the line, not a string literal. The rest of the line is still having alias expansion performed on it.

Use a slash (/) to search forward from the most recently listed line; use a question mark (?) to search 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. That line becomes the starting point for any further searches, or for a list command. For example:

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

8.2 Looking at the Threads (Tru64 UNIX Only)

A thread is a single, sequential flow of control within a process. Each thread contains 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 the POSIX Threads Library.

8.2.1 Thread Levels

The debugger supports two levels of threads:

To specify the thread level, set the $threadlevel debugger variable to one of the following strings:

For example:

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

8.2.2 Thread Manipulation Commands

You can use 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

8.2.3 Thread Display Commands

You can use the following commands to display threads: 
show_thread_command
        : show thread [ thread_id_list ] [ thread-state-filter ]

thread_id_list
        : thread_id ,...
        | *
	
thread_id
        : expression 

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:

NOTE: In the output, the right bracket indicator (>) marks the current thread, whereas the asterisk (*) indicator marks the thread with the event that stopped the application.

You can switch to a different thread as the current thread. The debugger variable $curthread contains the thread identifier of the current thread: 

switch_thread_command
          : thread [ thread_id ]
The $curthread value is updated when program execution stops or completes. You can modify the current thread 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.

8.2.4 Mutex Queries

A mutex (mutual exclusion) semaphore is a programming flag that allows multiple pthreads to synchronize access to shared resources, to ensure the following: 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

eq
        : ==               (for Ada, C, and C++)
        | .eq.             (for Fortran)
        | =                (for Cobol)
        | equal [ to ]     (for Cobol)
	
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 if the $threadlevel is set to native, an appropriate message is issued.

8.2.5 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 must 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_id
        : integer_constant 
        
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 if the $threadlevel is set to native, an appropriate message is issued.

8.2.6 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.

8.2.7 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

8.3 Looking at the Call Stack

Most programming languages have some concept of functions, routines, or subroutines, capturing the notion of code that 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 the information needed to provide 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. See Call Frames and Optimized Code and Call Frames and Machine Code Correlation for more information.

The debugger controls the call stacks of all the threads; you can use it to examine and manipulate call stacks, 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 caused the stoppage, or the call stack of any other thread.

The following commands show the most recent call frames on the call stack of the current or specified threads: 

show_stack_command
        : where [ expression ] [ thread_specifier ]

thread_specifier
        : thread thread_id ,...
        | thread all
	
thread_id
        : expression

If specified, the expression must evaluate to a nonnegative integer. You can specify 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 command can be voluminous. You can set the control variable $stackargs to 0 to suppress the output of argument values in the where command.

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. Its caller is at level 1.
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.

If your call stack seems to be missing routines, you may be seeing the result of a compiler optimization known as "tail calls."

If your call stack is corrupted, you may see random numbers without any routine names. In this case, it is likely that your application has gotten lost. Typically, this type of call stack display means that your application has lost track of the real stack and real code location, and is now executing random bits of memory, interpreting them as instructions.

If you are coding in C++, one of the most common ways to get a corrupt stack is for your code to try to execute a method on an invalid object. If the object has already been deleted, has not yet been initialized, is not there, or is of a completely different type, then the virtual function table will not be correct, and the application will be treating random memory as the virtual function table and calling a random place. In this case, you may find the history tool useful to locate the problem. See the Ladebug Web page FAQ for more information about the history tool.

8.3.1 Navigating the Call Stack

You can select one of the call frames as the starting point for examining variables. This call frame provides the current scope in the program for which variables exist, and tells the debugger which instance of those variables whose values you want to see: 
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 in the specified direction, the debugger issues a warning message and the call frame does not change.

When the current call frame changes, 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 up and down commands can be voluminous. You can set the control variable $stackargs to 0 to suppress the output of argument values in the up and down 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 either as the name of the function or as an integer expression evaluating to the call level. If you specify the name, the most-recently entered call frame for that function becomes the current call frame.

If no frames are available 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 at the level 0 function.

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.) The final stack trace in this example lists a call frame for function Planet::print as the current call frame (denoted by the > character).

8.3.2 The pop Command

The pop command removes one or more call frames from the call stack: 

pop_stack_frame_command
        : pop [ expression ]
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.

NOTE: Because it is extremely unlikely this will fix 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.

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

8.3.4 Call Frames and Machine Code Correlation

On a RISC processor, such as a Alpha processor, the following is the machine code typically generated for a call to a function:

When the thread is partway 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.

8.3.5 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:

8.4 Looking at the Data

After you have seen the call stack (show_stack_command), 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. You can use the print command and the call command to do this. You can also use the following 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
	| printi_command
        | print_registers_command
	| printt_command
        | dump_command

8.4.1 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
        | printb [ expression ,... ]
	| printd [ expression ,... ]
	| printo [ expression ,... ]
	| printx [ expression ,... ]
	    
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. For arrays or lists, you can use debugger variables and aliases to do a traversal of the data structure (see the array navigation example).

Use the $hexints, $decints, or $octints variables to select a radix for the output of the print command. If you do not want to change the radix permanently, use the printx, printd, printo, and printb commands to print expressions in hexadecimal, decimal, octal, or binary base format, respectively.

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:
8.4.1.1 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:
8.4.1.2 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.
8.4.1.3 Printing Floating Point Numbers
Floating point numbers are represented inside the computer in binary floating point. They are converted to decimal floating point when printed. The two formats are not the same, and some numbers are easily represented in decimal but not in binary (for example the number 1.1). The internal binary form for such numbers is an approximation, the closest that can be made given the number of bits available.

Normally, when a binary floating point number is printed, the shortest decimal number that would be represented by that binary number is used as the number to print, as it is a legitimate representation of the internal binary number. However, to see a more exact (extended form) representation of a binary floating point number, you can set the $floatshrinking debugger variable to 0 (zero).

The following example shows the result of converting 1.1 (shortened form) to the closest long double binary floating point number (extended form): 

	(ladebug) p $floatshrinking
    	1
    	(ladebug) p 1.1
    	1.1
    	(ladebug) set $floatshrinking = 0
    	(ladebug) p 1.1
    	1.10000000000000000000000000000000008
Currently, the extended forms are only available for long double variables and expressions.

For more detail on floating point representation, see ANSI IEEE Standard 754-1985.

8.4.1.4 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.

8.4.2 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. The printf command requires a running target program because it uses libc. 
printf_command
        : printf [ format_string [ , expression  ,... ] ]
For example:

8.4.3 The printi Command

The printi command takes one or more numerical expressions and interprets each one as an assembly instruction, printing out the instruction, and its arguments when applicable. This command is typically used by engineers performing machine-level debugging. 
printi_command
	: printi [ expression  ,... ]
For example:

8.4.4 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 hexadecimal radix, set the $hexints variable to 1. 
print_registers_command
        : printregs
For example:

8.4.5 The printt Command

The printt command takes one or more numerical expressions and interprets each one as the number of seconds since the Epoch (00:00:00 UTC 1 Jan 1970; see ctime(3) for more information). 
printt_command
        : printt [ expression  ,... ]
For example:

8.4.6 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 dump command can be voluminous. You can set the control variable $stackargs to 0 (zero) to suppress the output of argument values in the dump command.

8.4.7 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++ applications, 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 the 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 changed permanently.

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 a called function is executing. 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:

8.4.8 The whatis Command

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

8.4.9 The whereis Command

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

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 whereis_string

whereis_name
        : identifier_or_typedef_name
        | ( identifier_or_typedef_name )
	
whereis_string
	: string
You can use the whereis command with the whereis_name to obtain 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: See also the which command for another example of the whereis command.

If you are not sure how to spell a symbol, you can use the whereis command with the whereis_string to search the symbol table for the regular expression represented by the quoted string. All symbols that match the rules of the regular expression are displayed in ascending order. For example:

You can use the $symbolsearchlimit debugger variable to specify the maximum number of symbols that are returned by the whereis command for a regular expression search. The default value for the $symbolsearchlimit variable is 100; a value of 0 indicates no limit.

8.4.10 The which Command

Use the which command to determine 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:

8.4.11 Notes on C++ Debugging

The following sections describe the debugger commands specific to debugging C++ programs.
8.4.11.1 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 return to the default (current function) scope, use the command func 0.

Explicitly setting the debugger's current context to a class enables you to view 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<Node> in order to make member function append visible so a breakpoint can be set in append:

8.4.11.2 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 the following:

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, because 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:

8.4.11.3 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 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:

8.4.11.4 Displaying Static and Dynamic Type Information
When displaying object information for C++ class pointers or references, you have the option of viewing either static type information or 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 is displayed. The default value for this variable is true (1), which indicates that the dynamic type information is displayed. Setting this variable to false (0) instructs the debugger to display static type information. The output of the print, trace, tracei, and whatis commands are affected.

The display of dynamic type information is supported for C++ class pointers and references. All other types display static type information. In addition, if the dynamic type of an object cannot be determined, the debugger defaults to the use of 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 (by Margaret E. Ellis and Bjarne Stroustrup, 1990, Addison-Wesley Publishing Company).

In order for 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 is 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.
8.4.11.5 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 two members in an object have 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 in which the expanded syntax can be used:

8.4.11.6 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.

8.4.11.7 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. Following 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):
8.4.11.8 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. The compiler generates several other class members 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:

8.5 Looking at the Generated Code

This section discusses the following topics:

8.5.1 Memory Display and Search Commands

You can use the following commands to read arbitrary memory locations in your program: 
machinecode_level_command
     : examine_command
     : search_command

examine_command
     : address_expression / [ count ] [ mode ]
     | address_expression ? [ count ] [ mode ]
     | address_expression , address_expression / [ mode ]

search_command
     : address_expression / [ count ] search_mode   value  mask 
     | address_expression ? [ count ]  search_mode  value  mask 
     | address_expression , address_expression /  search_mode  value  mask 

count
     : integer_constant	

mode
     : d 	Print a short (2 byte) word in decimal
     | dd      	Print a 32-bit (4 byte) decimal display
     | D      	Print a long (8 byte) word in decimal
     | u	Print a short (2 byte) word in unsigned decimal
     | uu	Print a 32-bit (4 byte) unsigned decimal display
     | U	Print a long (8 byte) word in unsigned decimal
     | o	Print a short (2 byte) word in octal
     | oo	Print a 32-bit (4 byte) octal display
     | O	Print a long (8 byte) word in octal
     | x	Print a short (2 byte) word in hexadecimal
     | xx	Print a 32-bit (4 byte) hexadecimal display
     | X       	Print a long (8 byte) 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

search_mode
     : m   	32-bit search mode
     | M       	64-bit search mode

value
     : integer_constant
     
mask
     : integer_constant
The first examine_command displays the count number of memory values in the requested format, starting at address_expression. If count is not specified, 1 is assumed. The count value must be a positive value.

If you want to see memory values leading up to the address_expression, use the second examine_command. The second examine_command displays count number of memory values in the requested format ending at the address_expression. If count is not specified, 1 is assumed. The count value must be a positive value.

The third examine_command displays memory values in the requested format starting at the smaller of the two address_expressions and ending at the larger address_expression.

You can display stored values in the following formats by specifying mode. If mode is not specified, the mode used in the previous / command is assumed. If no previous / command exists, X is assumed.

When disassembling machine instructions, use the $regstyle variable to customize how the registers are displayed.

The search_commands allow you to search memory. Use the address_expression and count to determine the range of memory to search. Use the search_mode to specify whether you want to search 32-bit or 64-bit chunks. The debugger will start at the specified starting address and read a chunk of memory (either 32 or 64 bits in size) and apply the mask and comparison on that chunk of memory. For example, if you want to search memory for a particular instruction or search an array of either integer or floating-point values, the 32-bit search would be efficient because machine instructions and integer and floating-point data types are 32 bits in length.

Use the value to specify the memory value to seek. Use the mask to specify those bits that must match the same bits in the specified value. To ensure that a possible match will be found, the debugger applies the mask to the input value prior to starting the search, to remove any bits that could prevent a match from occurring. Then, for each memory location searched, the debugger applies the mask to the memory value and then compares it with this new input value. If a match is found, then the address and memory value are displayed.

For example, suppose the user wants to check an array of 100 integers in memory to see if any values are NULL (0): 

	(ladebug) array,&(array[99])/m 0x0 0xfffffff
	0x1400005d0:  0x00000000
Suppose the user wants to find the first occurrence of the particular value 0x55 in the thousand-element array data: Use the debugger variable $memorymatchall to cause the debugger to output all matches in the specified range. For example, suppose you want to find all occurrences of numbers ending in the value 0x55 in the same array:

8.5.2 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 Part III.

8.6 Looking at Shared Libraries

Most programs consist of a main image and shared libraries. Use the listobj command to show them. Most programmers will not need to use the readsharedobj or delsharedobj commands unless they use the -nosharedobjs option on the command line. 
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. You can use the command only when you specify the debuggee; 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.

Chapter 9—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 following sections discuss these commands.

9.1 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++, 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. You cannot change the address referred to by a reference type, but you can change the value at that address. 
     assign [classname::]member = ["filename"] `expression
        assign [object.]member = ["filename"] `expression

NOTE: Do not use the assign command to change the PC. When you change the PC, no adjustment to the contents of registers or memory is made. This means that if you adjust the PC forward, the skipped instructions are not executed and any changes they would have made do not occur. If you adjust the PC backward, the instructions you backed up over are not undone, and any changes they made will be in effect when execution continues again.

Because most instructions change registers or memory in ways that can impact the meaning of the application, changing the PC is very likely to cause your application to do incorrect calculations and arrive at the wrong answer. Access violations and other errors and signals may result from changing the value in the PC.

The assign Command in Machine-Level Debugging

You can use the assign command to alter the contents of memory specified by an address as shown in the following example:

See Machine-Level Debugging for more information.

9.2 The patch Command

Use the patch command to correct bad data or instructions in executable disk files. You can patch the text, initialized data, or read-only data areas. You cannot patch the bss segment, or stack and register locations, 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 could have been processed by an assign command.) If your program is loaded but not yet started, the patch to disk is performed without the corresponding assign to memory.

NOTE: When you use the patch command, the original binary is not overwritten, but is saved with the string ~backup appended to the file name. This allows you to revert to the original binary if necessary. A file with the string ~temp appended to the file name may also be created. You can delete it after the debugging session is over.

Chapter 10—Continuing Execution of the Process

Before continuing the process, 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 creating the snapshot, 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

10.1 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.

NOTE: If the instruction is a load locked instruction, special rules apply for stepi.

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

If you supply the optional expression argument, 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.

10.2 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 move the current thread while putting all other threads on hold.

If you supply the optional expression argument, 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 ]

step_number
        :  expression
For example: The following example shows the difference between stepi and nexti over the same call:

10.3 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_opt]

qual_symbol_opt
	: expression
	| qual_symbol_opt ` expression
In the following example, the next command is stepping through process execution in the append method. The return command finishes the append method and returns control to the caller. The return command is sensitive to the user's location in the call stack. Suppose function A calls function B, which calls function C. Execution has stopped in function C, and you entered the up command, so you were now in function B, at the point where it called function C. Using the return command here would return you to function A, at the point where function A called function B. Functions B and C will have completed execution.

10.4 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
        : expression
	
signal
        : integer_constant
	| signal_name
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.

Use the in argument 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.

Use the to parameter value to resume execution and then halt when the specified source line is reached. Use one of the following forms of the optional to parameter:

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.

10.5 The goto Command

The goto command is intended for advanced users who want to skip over the execution of a portion of source code. In general, its usage is not recommended. 
cont_from_place_command
        : goto line_expression

line_expression
	: expression

Chapter 11—Using 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 following sections discuss these commands and address the limitations of snapshots.

11.1 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.

11.2 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 rerunning 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 does not execute the process at all; it simply duplicates the saved snapshot (using a mechanism similar to the fork system call) and behaves as though the process execution has stopped at the point when the snapshot was saved.

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. 

clone_snapshot_command
        : clone snapshot  [ snapshot_id ]

snapshot_id
	: expression

Cloning a snapshot has the following two side effects:

11.3 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
        | *
	
snapshot_id
	: expression
For example:

11.4 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 ]

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

11.5 Snapshot Limitations

Snapshots have the following limitations:

Chapter 12—Debugging Optimized Code

This chapter provides insight into how highly optimizing compilers and the debugger deal with the consequences of key optimizations and how that may influence debugging of your program.

This chapter contains the following sections:

12.1 Why Debug Optimized Code?

Highly optimizing compilers, such as those supplied with the Tru64 UNIX operating system, 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 it is determined they are not 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 or to control its execution. The 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 was not 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 the compilers supplied with the operating system 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.)

12.2 Program Preparation

Preparing a program for debugging with optimization involves compiling that program using the -g3 option. All other preparation is unchanged.

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

12.3 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 where 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 the following:

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:

12.4 Semantic Stepping

A major problem with stepping through optimized code by line using the next and step commands in some debuggers 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 Ladebug's support of semantic stepping is that the step and next commands generally make normal progress through your program. The effects of optimization cannot be hidden entirely, however; there will be some occasional stepping backward as well as forward due to code reordering, and some lines and statements will not appear because they were optimized away, but the result will be generally more usable.

12.5 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) in which a new variable is declared and becomes visible, 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 obtaining correct information in the presence of optimization.

12.6 Tail Calls

If you enter the where command and the resulting stack seems to be missing routines, you may be seeing the result of a compiler optimization called tail calls. That optimization works as follows:

If a procedure MIDDLE calls a procedure INNER just before returning and certain conditions are met, then MIDDLE might simply jump to INNER, instead of saving its own context on the stack first. In this case, INNER will eventually return directly to MIDDLE's caller, OUTER, and there will be no record of MIDDLE's existence on the stack.

Then if you stop the application in INNER and enter the where command, you will see INNER and OUTER, but not MIDDLE.

Because this transformation can occur more than once, it is possible for several intermediate calls to appear missing from the context stack. The following example shows one instance of this transformation:

The conditions that permit this optimization include but are not restricted to the following:

See your compiler documentation for more information.

12.7 Alpha Linux Features

If you are debugging optimized code under the Alpha Linux operating system, the following information applies:

12.8 General Cautions

When debugging optimized code, you must use some Ladebug commands with caution or avoid them completely.

Use with Caution

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

These commands generally depend on a distinct call frame being on the execution stack for each called function. However, inlining can merge a called routine into the caller, resulting in one frame instead of the two (or more) that might be expected. You can use these commands, but be careful to make sure that you end up in the frame you intend after each use and do not be 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 been executed already.
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.

Chapter 13—Support Limitations

This chapter contains sections describing the limitations on support for the following languages:

13.1 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 be more useful, the debugger relaxes some standard C++ name visibility rules. For example, you can reference public, protected, and private class members as well as public ones.

The following limitations apply to debugging a C++ program:

Limitations for debugging templates include the following:

13.2 Limitations on Support for Compaq Fortran

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

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

The following limitations apply only to Compaq Fortran 90:

13.2.1 Limitations on Procedure Invocations

Following are the limitations on Compaq Fortran procedure invocations:

13.3 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.

13.3.1 Limitations on Expressions

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)

13.3.2 Limitations on 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.

You must manually convert enumeration values 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.

13.3.3 Limitations on Tasking Programs

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

13.4 Limitations on Support for Compaq COBOL

This section describes the limitations on support for Compaq COBOL.

13.4.1 Limitations on Assignment

The following limitations apply to assignment in COBOL debugging:

13.4.2 Other Limitations

Other limitations when you debug COBOL programs include the following: