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INFO-DIR-SECTION Programming & development tools.

START-INFO-DIR-ENTRY

* Gdb: (gdb).                     The GNU debugger.

END-INFO-DIR-ENTRY

   This file documents the GNU debugger GDB.

   This is the Ninth Edition, December 2001, of `Debugging with GDB:

the GNU Source-Level Debugger' for GDB Version 5.3.

   Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996,

1998,

1999, 2000, 2001, 2002 Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document

under the terms of the GNU Free Documentation License, Version 1.1 or

any later version published by the Free Software Foundation; with the

Invariant Sections being "Free Software" and "Free Software Needs Free

Documentation", with the Front-Cover Texts being "A GNU Manual," and

with the Back-Cover Texts as in (a) below.

   (a) The Free Software Foundation's Back-Cover Text is: "You have

freedom to copy and modify this GNU Manual, like GNU software.  Copies

published by the Free Software Foundation raise funds for GNU

development."

File: gdb.info,  Node: Disabling,  Next: Conditions,  Prev: Delete Breaks,  Up: Breakpoints

Disabling breakpoints

---------------------

   Rather than deleting a breakpoint, watchpoint, or catchpoint, you

might prefer to "disable" it.  This makes the breakpoint inoperative as

if it had been deleted, but remembers the information on the breakpoint

so that you can "enable" it again later.

   You disable and enable breakpoints, watchpoints, and catchpoints with

the `enable' and `disable' commands, optionally specifying one or more

breakpoint numbers as arguments.  Use `info break' or `info watch' to

print a list of breakpoints, watchpoints, and catchpoints if you do not

know which numbers to use.

   A breakpoint, watchpoint, or catchpoint can have any of four

different states of enablement:

   * Enabled.  The breakpoint stops your program.  A breakpoint set

     with the `break' command starts out in this state.

   * Disabled.  The breakpoint has no effect on your program.

   * Enabled once.  The breakpoint stops your program, but then becomes

     disabled.

   * Enabled for deletion.  The breakpoint stops your program, but

     immediately after it does so it is deleted permanently.  A

     breakpoint set with the `tbreak' command starts out in this state.

   You can use the following commands to enable or disable breakpoints,

watchpoints, and catchpoints:

`disable [breakpoints] [RANGE...]'

     Disable the specified breakpoints--or all breakpoints, if none are

     listed.  A disabled breakpoint has no effect but is not forgotten.

     All options such as ignore-counts, conditions and commands are

     remembered in case the breakpoint is enabled again later.  You may

     abbreviate `disable' as `dis'.

`enable [breakpoints] [RANGE...]'

     Enable the specified breakpoints (or all defined breakpoints).

     They become effective once again in stopping your program.

`enable [breakpoints] once RANGE...'

     Enable the specified breakpoints temporarily.  GDB disables any of

     these breakpoints immediately after stopping your program.

`enable [breakpoints] delete RANGE...'

     Enable the specified breakpoints to work once, then die.  GDB

     deletes any of these breakpoints as soon as your program stops

     there.

   Except for a breakpoint set with `tbreak' (*note Setting

breakpoints: Set Breaks.), breakpoints that you set are initially

enabled; subsequently, they become disabled or enabled only when you

use one of the commands above.  (The command `until' can set and delete

a breakpoint of its own, but it does not change the state of your other

breakpoints; see *Note Continuing and stepping: Continuing and

Stepping.)

File: gdb.info,  Node: Conditions,  Next: Break Commands,  Prev: Disabling,  Up: Breakpoints

Break conditions

----------------

   The simplest sort of breakpoint breaks every time your program

reaches a specified place.  You can also specify a "condition" for a

breakpoint.  A condition is just a Boolean expression in your

programming language (*note Expressions: Expressions.).  A breakpoint

with a condition evaluates the expression each time your program

reaches it, and your program stops only if the condition is _true_.

   This is the converse of using assertions for program validation; in

that situation, you want to stop when the assertion is violated--that

is, when the condition is false.  In C, if you want to test an

assertion expressed by the condition ASSERT, you should set the

condition `! ASSERT' on the appropriate breakpoint.

   Conditions are also accepted for watchpoints; you may not need them,

since a watchpoint is inspecting the value of an expression anyhow--but

it might be simpler, say, to just set a watchpoint on a variable name,

and specify a condition that tests whether the new value is an

interesting one.

   Break conditions can have side effects, and may even call functions

in your program.  This can be useful, for example, to activate functions

that log program progress, or to use your own print functions to format

special data structures. The effects are completely predictable unless

there is another enabled breakpoint at the same address.  (In that

case, GDB might see the other breakpoint first and stop your program

without checking the condition of this one.)  Note that breakpoint

commands are usually more convenient and flexible than break conditions

for the purpose of performing side effects when a breakpoint is reached

(*note Breakpoint command lists: Break Commands.).

   Break conditions can be specified when a breakpoint is set, by using

`if' in the arguments to the `break' command.  *Note Setting

breakpoints: Set Breaks.  They can also be changed at any time with the

`condition' command.

   You can also use the `if' keyword with the `watch' command.  The

`catch' command does not recognize the `if' keyword; `condition' is the

only way to impose a further condition on a catchpoint.

`condition BNUM EXPRESSION'

     Specify EXPRESSION as the break condition for breakpoint,

     watchpoint, or catchpoint number BNUM.  After you set a condition,

     breakpoint BNUM stops your program only if the value of EXPRESSION

     is true (nonzero, in C).  When you use `condition', GDB checks

     EXPRESSION immediately for syntactic correctness, and to determine

     whether symbols in it have referents in the context of your

     breakpoint.  If EXPRESSION uses symbols not referenced in the

     context of the breakpoint, GDB prints an error message:

          No symbol "foo" in current context.

     GDB does not actually evaluate EXPRESSION at the time the

     `condition' command (or a command that sets a breakpoint with a

     condition, like `break if ...') is given, however.  *Note

     Expressions: Expressions.

`condition BNUM'

     Remove the condition from breakpoint number BNUM.  It becomes an

     ordinary unconditional breakpoint.

   A special case of a breakpoint condition is to stop only when the

breakpoint has been reached a certain number of times.  This is so

useful that there is a special way to do it, using the "ignore count"

of the breakpoint.  Every breakpoint has an ignore count, which is an

integer.  Most of the time, the ignore count is zero, and therefore has

no effect.  But if your program reaches a breakpoint whose ignore count

is positive, then instead of stopping, it just decrements the ignore

count by one and continues.  As a result, if the ignore count value is

N, the breakpoint does not stop the next N times your program reaches

it.

`ignore BNUM COUNT'

     Set the ignore count of breakpoint number BNUM to COUNT.  The next

     COUNT times the breakpoint is reached, your program's execution

     does not stop; other than to decrement the ignore count, GDB takes

     no action.

     To make the breakpoint stop the next time it is reached, specify a

     count of zero.

     When you use `continue' to resume execution of your program from a

     breakpoint, you can specify an ignore count directly as an

     argument to `continue', rather than using `ignore'.  *Note

     Continuing and stepping: Continuing and Stepping.

     If a breakpoint has a positive ignore count and a condition, the

     condition is not checked.  Once the ignore count reaches zero, GDB

     resumes checking the condition.

     You could achieve the effect of the ignore count with a condition

     such as `$foo-- <= 0' using a debugger convenience variable that

     is decremented each time.  *Note Convenience variables:

     Convenience Vars.

   Ignore counts apply to breakpoints, watchpoints, and catchpoints.

File: gdb.info,  Node: Break Commands,  Next: Breakpoint Menus,  Prev: Conditions,  Up: Breakpoints

Breakpoint command lists

------------------------

   You can give any breakpoint (or watchpoint or catchpoint) a series of

commands to execute when your program stops due to that breakpoint.  For

example, you might want to print the values of certain expressions, or

enable other breakpoints.

`commands [BNUM]'

`... COMMAND-LIST ...'

`end'

     Specify a list of commands for breakpoint number BNUM.  The

     commands themselves appear on the following lines.  Type a line

     containing just `end' to terminate the commands.

     To remove all commands from a breakpoint, type `commands' and

     follow it immediately with `end'; that is, give no commands.

     With no BNUM argument, `commands' refers to the last breakpoint,

     watchpoint, or catchpoint set (not to the breakpoint most recently

     encountered).

   Pressing  as a means of repeating the last GDB command is

disabled within a COMMAND-LIST.

   You can use breakpoint commands to start your program up again.

Simply use the `continue' command, or `step', or any other command that

resumes execution.

   Any other commands in the command list, after a command that resumes

execution, are ignored.  This is because any time you resume execution

(even with a simple `next' or `step'), you may encounter another

breakpoint--which could have its own command list, leading to

ambiguities about which list to execute.

   If the first command you specify in a command list is `silent', the

usual message about stopping at a breakpoint is not printed.  This may

be desirable for breakpoints that are to print a specific message and

then continue.  If none of the remaining commands print anything, you

see no sign that the breakpoint was reached.  `silent' is meaningful

only at the beginning of a breakpoint command list.

   The commands `echo', `output', and `printf' allow you to print

precisely controlled output, and are often useful in silent

breakpoints.  *Note Commands for controlled output: Output.

   For example, here is how you could use breakpoint commands to print

the value of `x' at entry to `foo' whenever `x' is positive.

     break foo if x>0

     commands

     silent

     printf "x is %d\n",x

     cont

     end

   One application for breakpoint commands is to compensate for one bug

so you can test for another.  Put a breakpoint just after the erroneous

line of code, give it a condition to detect the case in which something

erroneous has been done, and give it commands to assign correct values

to any variables that need them.  End with the `continue' command so

that your program does not stop, and start with the `silent' command so

that no output is produced.  Here is an example:

     break 403

     commands

     silent

     set x = y + 4

     cont

     end

File: gdb.info,  Node: Breakpoint Menus,  Next: Error in Breakpoints,  Prev: Break Commands,  Up: Breakpoints

Breakpoint menus

----------------

   Some programming languages (notably C++) permit a single function

name to be defined several times, for application in different contexts.

This is called "overloading".  When a function name is overloaded,

`break FUNCTION' is not enough to tell GDB where you want a breakpoint.

If you realize this is a problem, you can use something like `break

FUNCTION(TYPES)' to specify which particular version of the function

you want.  Otherwise, GDB offers you a menu of numbered choices for

different possible breakpoints, and waits for your selection with the

prompt `>'.  The first two options are always `[0] cancel' and `[1]

all'.  Typing `1' sets a breakpoint at each definition of FUNCTION, and

typing `0' aborts the `break' command without setting any new

breakpoints.

   For example, the following session excerpt shows an attempt to set a

breakpoint at the overloaded symbol `String::after'.  We choose three

particular definitions of that function name:

     (gdb) b String::after

     [0] cancel

     [1] all

     [2] file:String.cc; line number:867

     [3] file:String.cc; line number:860

     [4] file:String.cc; line number:875

     [5] file:String.cc; line number:853

     [6] file:String.cc; line number:846

     [7] file:String.cc; line number:735

     > 2 4 6

     Breakpoint 1 at 0xb26c: file String.cc, line 867.

     Breakpoint 2 at 0xb344: file String.cc, line 875.

     Breakpoint 3 at 0xafcc: file String.cc, line 846.

     Multiple breakpoints were set.

     Use the "delete" command to delete unwanted

      breakpoints.

     (gdb)

File: gdb.info,  Node: Error in Breakpoints,  Prev: Breakpoint Menus,  Up: Breakpoints

"Cannot insert breakpoints"

---------------------------

   Under some operating systems, breakpoints cannot be used in a

program if any other process is running that program.  In this

situation, attempting to run or continue a program with a breakpoint

causes GDB to print an error message:

     Cannot insert breakpoints.

     The same program may be running in another process.

   When this happens, you have three ways to proceed:

  1. Remove or disable the breakpoints, then continue.

  2. Suspend GDB, and copy the file containing your program to a new

     name.  Resume GDB and use the `exec-file' command to specify that

     GDB should run your program under that name.  Then start your

     program again.

  3. Relink your program so that the text segment is nonsharable, using

     the linker option `-N'.  The operating system limitation may not

     apply to nonsharable executables.

   A similar message can be printed if you request too many active

hardware-assisted breakpoints and watchpoints:

     Stopped; cannot insert breakpoints.

     You may have requested too many hardware breakpoints and watchpoints.

This message is printed when you attempt to resume the program, since

only then GDB knows exactly how many hardware breakpoints and

watchpoints it needs to insert.

   When this message is printed, you need to disable or remove some of

the hardware-assisted breakpoints and watchpoints, and then continue.

File: gdb.info,  Node: Continuing and Stepping,  Next: Signals,  Prev: Breakpoints,  Up: Stopping

Continuing and stepping

=======================

   "Continuing" means resuming program execution until your program

completes normally.  In contrast, "stepping" means executing just one

more "step" of your program, where "step" may mean either one line of

source code, or one machine instruction (depending on what particular

command you use).  Either when continuing or when stepping, your

program may stop even sooner, due to a breakpoint or a signal.  (If it

stops due to a signal, you may want to use `handle', or use `signal 0'

to resume execution.  *Note Signals: Signals.)

`continue [IGNORE-COUNT]'

`c [IGNORE-COUNT]'

`fg [IGNORE-COUNT]'

     Resume program execution, at the address where your program last

     stopped; any breakpoints set at that address are bypassed.  The

     optional argument IGNORE-COUNT allows you to specify a further

     number of times to ignore a breakpoint at this location; its

     effect is like that of `ignore' (*note Break conditions:

     Conditions.).

     The argument IGNORE-COUNT is meaningful only when your program

     stopped due to a breakpoint.  At other times, the argument to

     `continue' is ignored.

     The synonyms `c' and `fg' (for "foreground", as the debugged

     program is deemed to be the foreground program) are provided

     purely for convenience, and have exactly the same behavior as

     `continue'.

   To resume execution at a different place, you can use `return'

(*note Returning from a function: Returning.) to go back to the calling

function; or `jump' (*note Continuing at a different address: Jumping.)

to go to an arbitrary location in your program.

   A typical technique for using stepping is to set a breakpoint (*note

Breakpoints; watchpoints; and catchpoints: Breakpoints.) at the

beginning of the function or the section of your program where a problem

is believed to lie, run your program until it stops at that breakpoint,

and then step through the suspect area, examining the variables that are

interesting, until you see the problem happen.

`step'

     Continue running your program until control reaches a different

     source line, then stop it and return control to GDB.  This command

     is abbreviated `s'.

          _Warning:_ If you use the `step' command while control is

          within a function that was compiled without debugging

          information, execution proceeds until control reaches a

          function that does have debugging information.  Likewise, it

          will not step into a function which is compiled without

          debugging information.  To step through functions without

          debugging information, use the `stepi' command, described

          below.

     The `step' command only stops at the first instruction of a source

     line.  This prevents the multiple stops that could otherwise occur

     in `switch' statements, `for' loops, etc.  `step' continues to

     stop if a function that has debugging information is called within

     the line.  In other words, `step' _steps inside_ any functions

     called within the line.

     Also, the `step' command only enters a function if there is line

     number information for the function.  Otherwise it acts like the

     `next' command.  This avoids problems when using `cc -gl' on MIPS

     machines.  Previously, `step' entered subroutines if there was any

     debugging information about the routine.

`step COUNT'

     Continue running as in `step', but do so COUNT times.  If a

     breakpoint is reached, or a signal not related to stepping occurs

     before COUNT steps, stepping stops right away.

`next [COUNT]'

     Continue to the next source line in the current (innermost) stack

     frame.  This is similar to `step', but function calls that appear

     within the line of code are executed without stopping.  Execution

     stops when control reaches a different line of code at the

     original stack level that was executing when you gave the `next'

     command.  This command is abbreviated `n'.

     An argument COUNT is a repeat count, as for `step'.

     The `next' command only stops at the first instruction of a source

     line.  This prevents multiple stops that could otherwise occur in

     `switch' statements, `for' loops, etc.

`set step-mode'

`set step-mode on'

     The `set step-mode on' command causes the `step' command to stop

     at the first instruction of a function which contains no debug line

     information rather than stepping over it.

     This is useful in cases where you may be interested in inspecting

     the machine instructions of a function which has no symbolic info

     and do not want GDB to automatically skip over this function.

`set step-mode off'

     Causes the `step' command to step over any functions which

     contains no debug information.  This is the default.

`finish'

     Continue running until just after function in the selected stack

     frame returns.  Print the returned value (if any).

     Contrast this with the `return' command (*note Returning from a

     function: Returning.).

`until'

`u'

     Continue running until a source line past the current line, in the

     current stack frame, is reached.  This command is used to avoid

     single stepping through a loop more than once.  It is like the

     `next' command, except that when `until' encounters a jump, it

     automatically continues execution until the program counter is

     greater than the address of the jump.

     This means that when you reach the end of a loop after single

     stepping though it, `until' makes your program continue execution

     until it exits the loop.  In contrast, a `next' command at the end

     of a loop simply steps back to the beginning of the loop, which

     forces you to step through the next iteration.

     `until' always stops your program if it attempts to exit the

     current stack frame.

     `until' may produce somewhat counterintuitive results if the order

     of machine code does not match the order of the source lines.  For

     example, in the following excerpt from a debugging session, the `f'

     (`frame') command shows that execution is stopped at line `206';

     yet when we use `until', we get to line `195':

          (gdb) f

          #0  main (argc=4, argv=0xf7fffae8) at m4.c:206

          206                 expand_input();

          (gdb) until

          195             for ( ; argc > 0; NEXTARG) {

     This happened because, for execution efficiency, the compiler had

     generated code for the loop closure test at the end, rather than

     the start, of the loop--even though the test in a C `for'-loop is

     written before the body of the loop.  The `until' command appeared

     to step back to the beginning of the loop when it advanced to this

     expression; however, it has not really gone to an earlier

     statement--not in terms of the actual machine code.

     `until' with no argument works by means of single instruction

     stepping, and hence is slower than `until' with an argument.

`until LOCATION'

`u LOCATION'

     Continue running your program until either the specified location

     is reached, or the current stack frame returns.  LOCATION is any of

     the forms of argument acceptable to `break' (*note Setting

     breakpoints: Set Breaks.).  This form of the command uses

     breakpoints, and hence is quicker than `until' without an argument.

`stepi'

`stepi ARG'

`si'

     Execute one machine instruction, then stop and return to the

     debugger.

     It is often useful to do `display/i $pc' when stepping by machine

     instructions.  This makes GDB automatically display the next

     instruction to be executed, each time your program stops.  *Note

     Automatic display: Auto Display.

     An argument is a repeat count, as in `step'.

`nexti'

`nexti ARG'

`ni'

     Execute one machine instruction, but if it is a function call,

     proceed until the function returns.

     An argument is a repeat count, as in `next'.

File: gdb.info,  Node: Signals,  Next: Thread Stops,  Prev: Continuing and Stepping,  Up: Stopping

Signals

=======

   A signal is an asynchronous event that can happen in a program.  The

operating system defines the possible kinds of signals, and gives each

kind a name and a number.  For example, in Unix `SIGINT' is the signal

a program gets when you type an interrupt character (often `C-c');

`SIGSEGV' is the signal a program gets from referencing a place in

memory far away from all the areas in use; `SIGALRM' occurs when the

alarm clock timer goes off (which happens only if your program has

requested an alarm).

   Some signals, including `SIGALRM', are a normal part of the

functioning of your program.  Others, such as `SIGSEGV', indicate

errors; these signals are "fatal" (they kill your program immediately)

if the program has not specified in advance some other way to handle

the signal.  `SIGINT' does not indicate an error in your program, but

it is normally fatal so it can carry out the purpose of the interrupt:

to kill the program.

   GDB has the ability to detect any occurrence of a signal in your

program.  You can tell GDB in advance what to do for each kind of

signal.

   Normally, GDB is set up to let the non-erroneous signals like

`SIGALRM' be silently passed to your program (so as not to interfere

with their role in the program's functioning) but to stop your program

immediately whenever an error signal happens.  You can change these

settings with the `handle' command.

`info signals'

`info handle'

     Print a table of all the kinds of signals and how GDB has been

     told to handle each one.  You can use this to see the signal

     numbers of all the defined types of signals.

     `info handle' is an alias for `info signals'.

`handle SIGNAL KEYWORDS...'

     Change the way GDB handles signal SIGNAL.  SIGNAL can be the

     number of a signal or its name (with or without the `SIG' at the

     beginning); a list of signal numbers of the form `LOW-HIGH'; or

     the word `all', meaning all the known signals.  The KEYWORDS say

     what change to make.

   The keywords allowed by the `handle' command can be abbreviated.

Their full names are:

`nostop'

     GDB should not stop your program when this signal happens.  It may

     still print a message telling you that the signal has come in.

`stop'

     GDB should stop your program when this signal happens.  This

     implies the `print' keyword as well.

`print'

     GDB should print a message when this signal happens.

`noprint'

     GDB should not mention the occurrence of the signal at all.  This

     implies the `nostop' keyword as well.

`pass'

`noignore'

     GDB should allow your program to see this signal; your program can

     handle the signal, or else it may terminate if the signal is fatal

     and not handled.  `pass' and `noignore' are synonyms.

`nopass'

`ignore'

     GDB should not allow your program to see this signal.  `nopass'

     and `ignore' are synonyms.

   When a signal stops your program, the signal is not visible to the

program until you continue.  Your program sees the signal then, if

`pass' is in effect for the signal in question _at that time_.  In

other words, after GDB reports a signal, you can use the `handle'

command with `pass' or `nopass' to control whether your program sees

that signal when you continue.

   The default is set to `nostop', `noprint', `pass' for non-erroneous

signals such as `SIGALRM', `SIGWINCH' and `SIGCHLD', and to `stop',

`print', `pass' for the erroneous signals.

   You can also use the `signal' command to prevent your program from

seeing a signal, or cause it to see a signal it normally would not see,

or to give it any signal at any time.  For example, if your program

stopped due to some sort of memory reference error, you might store

correct values into the erroneous variables and continue, hoping to see

more execution; but your program would probably terminate immediately as

a result of the fatal signal once it saw the signal.  To prevent this,

you can continue with `signal 0'.  *Note Giving your program a signal:

Signaling.

File: gdb.info,  Node: Thread Stops,  Prev: Signals,  Up: Stopping

Stopping and starting multi-thread programs

===========================================

   When your program has multiple threads (*note Debugging programs

with multiple threads: Threads.), you can choose whether to set

breakpoints on all threads, or on a particular thread.

`break LINESPEC thread THREADNO'

`break LINESPEC thread THREADNO if ...'

     LINESPEC specifies source lines; there are several ways of writing

     them, but the effect is always to specify some source line.

     Use the qualifier `thread THREADNO' with a breakpoint command to

     specify that you only want GDB to stop the program when a

     particular thread reaches this breakpoint.  THREADNO is one of the

     numeric thread identifiers assigned by GDB, shown in the first

     column of the `info threads' display.

     If you do not specify `thread THREADNO' when you set a breakpoint,

     the breakpoint applies to _all_ threads of your program.

     You can use the `thread' qualifier on conditional breakpoints as

     well; in this case, place `thread THREADNO' before the breakpoint

     condition, like this:

          (gdb) break frik.c:13 thread 28 if bartab > lim

   Whenever your program stops under GDB for any reason, _all_ threads

of execution stop, not just the current thread.  This allows you to

examine the overall state of the program, including switching between

threads, without worrying that things may change underfoot.

   Conversely, whenever you restart the program, _all_ threads start

executing.  _This is true even when single-stepping_ with commands like

`step' or `next'.

   In particular, GDB cannot single-step all threads in lockstep.

Since thread scheduling is up to your debugging target's operating

system (not controlled by GDB), other threads may execute more than one

statement while the current thread completes a single step.  Moreover,

in general other threads stop in the middle of a statement, rather than

at a clean statement boundary, when the program stops.

   You might even find your program stopped in another thread after

continuing or even single-stepping.  This happens whenever some other

thread runs into a breakpoint, a signal, or an exception before the

first thread completes whatever you requested.

   On some OSes, you can lock the OS scheduler and thus allow only a

single thread to run.

`set scheduler-locking MODE'

     Set the scheduler locking mode.  If it is `off', then there is no

     locking and any thread may run at any time.  If `on', then only the

     current thread may run when the inferior is resumed.  The `step'

     mode optimizes for single-stepping.  It stops other threads from

     "seizing the prompt" by preempting the current thread while you are

     stepping.  Other threads will only rarely (or never) get a chance

     to run when you step.  They are more likely to run when you `next'

     over a function call, and they are completely free to run when you

     use commands like `continue', `until', or `finish'.  However,

     unless another thread hits a breakpoint during its timeslice, they

     will never steal the GDB prompt away from the thread that you are

     debugging.

`show scheduler-locking'

     Display the current scheduler locking mode.

File: gdb.info,  Node: Stack,  Next: Source,  Prev: Stopping,  Up: Top

Examining the Stack

*******************

   When your program has stopped, the first thing you need to know is

where it stopped and how it got there.

   Each time your program performs a function call, information about

the call is generated.  That information includes the location of the

call in your program, the arguments of the call, and the local

variables of the function being called.  The information is saved in a

block of data called a "stack frame".  The stack frames are allocated

in a region of memory called the "call stack".

   When your program stops, the GDB commands for examining the stack

allow you to see all of this information.

   One of the stack frames is "selected" by GDB and many GDB commands

refer implicitly to the selected frame.  In particular, whenever you

ask GDB for the value of a variable in your program, the value is found

in the selected frame.  There are special GDB commands to select

whichever frame you are interested in. *Note Selecting a frame:

Selection.

   When your program stops, GDB automatically selects the currently

executing frame and describes it briefly, similar to the `frame'

command (*note Information about a frame: Frame Info.).

* Menu:

* Frames::                      Stack frames

* Backtrace::                   Backtraces

* Selection::                   Selecting a frame

* Frame Info::                  Information on a frame

File: gdb.info,  Node: Frames,  Next: Backtrace,  Up: Stack

Stack frames

============

   The call stack is divided up into contiguous pieces called "stack

frames", or "frames" for short; each frame is the data associated with

one call to one function.  The frame contains the arguments given to

the function, the function's local variables, and the address at which

the function is executing.

   When your program is started, the stack has only one frame, that of

the function `main'.  This is called the "initial" frame or the

"outermost" frame.  Each time a function is called, a new frame is

made.  Each time a function returns, the frame for that function

invocation is eliminated.  If a function is recursive, there can be

many frames for the same function.  The frame for the function in which

execution is actually occurring is called the "innermost" frame.  This

is the most recently created of all the stack frames that still exist.

   Inside your program, stack frames are identified by their addresses.

A stack frame consists of many bytes, each of which has its own

address; each kind of computer has a convention for choosing one byte

whose address serves as the address of the frame.  Usually this address

is kept in a register called the "frame pointer register" while

execution is going on in that frame.

   GDB assigns numbers to all existing stack frames, starting with zero

for the innermost frame, one for the frame that called it, and so on

upward.  These numbers do not really exist in your program; they are

assigned by GDB to give you a way of designating stack frames in GDB

commands.

   Some compilers provide a way to compile functions so that they

operate without stack frames.  (For example, the gcc option

     `-fomit-frame-pointer'

   generates functions without a frame.)  This is occasionally done

with heavily used library functions to save the frame setup time.  GDB

has limited facilities for dealing with these function invocations.  If

the innermost function invocation has no stack frame, GDB nevertheless

regards it as though it had a separate frame, which is numbered zero as

usual, allowing correct tracing of the function call chain.  However,

GDB has no provision for frameless functions elsewhere in the stack.

`frame ARGS'

     The `frame' command allows you to move from one stack frame to

     another, and to print the stack frame you select.  ARGS may be

     either the address of the frame or the stack frame number.

     Without an argument, `frame' prints the current stack frame.

`select-frame'

     The `select-frame' command allows you to move from one stack frame

     to another without printing the frame.  This is the silent version

     of `frame'.

File: gdb.info,  Node: Backtrace,  Next: Selection,  Prev: Frames,  Up: Stack

Backtraces

==========

   A backtrace is a summary of how your program got where it is.  It

shows one line per frame, for many frames, starting with the currently

executing frame (frame zero), followed by its caller (frame one), and

on up the stack.

`backtrace'

`bt'

     Print a backtrace of the entire stack: one line per frame for all

     frames in the stack.

     You can stop the backtrace at any time by typing the system

     interrupt character, normally `C-c'.

`backtrace N'

`bt N'

     Similar, but print only the innermost N frames.

`backtrace -N'

`bt -N'

     Similar, but print only the outermost N frames.

   The names `where' and `info stack' (abbreviated `info s') are

additional aliases for `backtrace'.

   Each line in the backtrace shows the frame number and the function

name.  The program counter value is also shown--unless you use `set

print address off'.  The backtrace also shows the source file name and

line number, as well as the arguments to the function.  The program

counter value is omitted if it is at the beginning of the code for that

line number.

   Here is an example of a backtrace.  It was made with the command `bt

3', so it shows the innermost three frames.

     #0  m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)

         at builtin.c:993

     #1  0x6e38 in expand_macro (sym=0x2b600) at macro.c:242

     #2  0x6840 in expand_token (obs=0x0, t=177664, td=0xf7fffb08)

         at macro.c:71

     (More stack frames follow...)

The display for frame zero does not begin with a program counter value,

indicating that your program has stopped at the beginning of the code

for line `993' of `builtin.c'.

File: gdb.info,  Node: Selection,  Next: Frame Info,  Prev: Backtrace,  Up: Stack

Selecting a frame

=================

   Most commands for examining the stack and other data in your program

work on whichever stack frame is selected at the moment.  Here are the

commands for selecting a stack frame; all of them finish by printing a

brief description of the stack frame just selected.

`frame N'

`f N'

     Select frame number N.  Recall that frame zero is the innermost

     (currently executing) frame, frame one is the frame that called the

     innermost one, and so on.  The highest-numbered frame is the one

     for `main'.

`frame ADDR'

`f ADDR'

     Select the frame at address ADDR.  This is useful mainly if the

     chaining of stack frames has been damaged by a bug, making it

     impossible for GDB to assign numbers properly to all frames.  In

     addition, this can be useful when your program has multiple stacks

     and switches between them.

     On the SPARC architecture, `frame' needs two addresses to select

     an arbitrary frame: a frame pointer and a stack pointer.

     On the MIPS and Alpha architecture, it needs two addresses: a stack

     pointer and a program counter.

     On the 29k architecture, it needs three addresses: a register stack

     pointer, a program counter, and a memory stack pointer.

`up N'

     Move N frames up the stack.  For positive numbers N, this advances

     toward the outermost frame, to higher frame numbers, to frames

     that have existed longer.  N defaults to one.

`down N'

     Move N frames down the stack.  For positive numbers N, this

     advances toward the innermost frame, to lower frame numbers, to

     frames that were created more recently.  N defaults to one.  You

     may abbreviate `down' as `do'.

   All of these commands end by printing two lines of output describing

the frame.  The first line shows the frame number, the function name,

the arguments, and the source file and line number of execution in that

frame.  The second line shows the text of that source line.

   For example:

     (gdb) up

     #1  0x22f0 in main (argc=1, argv=0xf7fffbf4, env=0xf7fffbfc)

         at env.c:10

     10              read_input_file (argv[i]);

   After such a printout, the `list' command with no arguments prints

ten lines centered on the point of execution in the frame.  *Note

Printing source lines: List.

`up-silently N'

`down-silently N'

     These two commands are variants of `up' and `down', respectively;

     they differ in that they do their work silently, without causing

     display of the new frame.  They are intended primarily for use in

     GDB command scripts, where the output might be unnecessary and

     distracting.

File: gdb.info,  Node: Frame Info,  Prev: Selection,  Up: Stack

Information about a frame

=========================

   There are several other commands to print information about the

selected stack frame.

`frame'

`f'

     When used without any argument, this command does not change which

     frame is selected, but prints a brief description of the currently

     selected stack frame.  It can be abbreviated `f'.  With an

     argument, this command is used to select a stack frame.  *Note

     Selecting a frame: Selection.

`info frame'

`info f'

     This command prints a verbose description of the selected stack

     frame, including:

        * the address of the frame

        * the address of the next frame down (called by this frame)