Stopping and Continuing
The principal purposes of using a debugger are so that you can stop your program before it terminates; or so that, if your program runs into trouble, you can investigate and find out why.
Inside GDB, your program may stop for any of several reasons,
such as a signal, a breakpoint, or reaching a new line after a
GDB command such as step
. You may then examine and
change variables, set new breakpoints or remove old ones, and then
continue execution. Usually, the messages shown by GDB provide
ample explanation of the status of your program--but you can also
explicitly request this information at any time.
info program
- Display information about the status of your program: whether it is running or not, what process it is, and why it stopped.
Breakpoints, watchpoints, and catchpoints
A breakpoint makes your program stop whenever a certain point in
the program is reached. For each breakpoint, you can add conditions to
control in finer detail whether your program stops. You can set
breakpoints with the break
command and its variants (see section Setting breakpoints), to specify the place where your program
should stop by line number, function name or exact address in the
program.
On some systems, you can set breakpoints in shared libraries before
the executable is run. There is a minor limitation on HP-UX systems:
you must wait until the executable is run in order to set breakpoints
in shared library routines that are not called directly by the program
(for example, routines that are arguments in a pthread_create
call).
A watchpoint is a special breakpoint that stops your program when the value of an expression changes. You must use a different command to set watchpoints (see section Setting watchpoints), but aside from that, you can manage a watchpoint like any other breakpoint: you enable, disable, and delete both breakpoints and watchpoints using the same commands.
You can arrange to have values from your program displayed automatically whenever GDB stops at a breakpoint. See section Automatic display.
A catchpoint is another special breakpoint that stops your program
when a certain kind of event occurs, such as the throwing of a C++
exception or the loading of a library. As with watchpoints, you use a
different command to set a catchpoint (see section Setting catchpoints), but aside from that, you can manage a catchpoint like any
other breakpoint. (To stop when your program receives a signal, use the
handle
command; see section Signals.)
GDB assigns a number to each breakpoint, watchpoint, or catchpoint when you create it; these numbers are successive integers starting with one. In many of the commands for controlling various features of breakpoints you use the breakpoint number to say which breakpoint you want to change. Each breakpoint may be enabled or disabled; if disabled, it has no effect on your program until you enable it again.
Some GDB commands accept a range of breakpoints on which to operate. A breakpoint range is either a single breakpoint number, like `5', or two such numbers, in increasing order, separated by a hyphen, like `5-7'. When a breakpoint range is given to a command, all breakpoint in that range are operated on.
Setting breakpoints
Breakpoints are set with the break
command (abbreviated
b
). The debugger convenience variable `$bpnum' records the
number of the breakpoint you've set most recently; see section Convenience variables, for a discussion of what you can do with
convenience variables.
You have several ways to say where the breakpoint should go.
break function
- Set a breakpoint at entry to function function. When using source languages that permit overloading of symbols, such as C++, function may refer to more than one possible place to break. See section Breakpoint menus, for a discussion of that situation.
break +offset
break -offset
- Set a breakpoint some number of lines forward or back from the position at which execution stopped in the currently selected stack frame. (See section Stack frames, for a description of stack frames.)
break linenum
- Set a breakpoint at line linenum in the current source file. The current source file is the last file whose source text was printed. The breakpoint will stop your program just before it executes any of the code on that line.
break filename:linenum
- Set a breakpoint at line linenum in source file filename.
break filename:function
- Set a breakpoint at entry to function function found in file filename. Specifying a file name as well as a function name is superfluous except when multiple files contain similarly named functions.
break *address
- Set a breakpoint at address address. You can use this to set breakpoints in parts of your program which do not have debugging information or source files.
break
-
When called without any arguments,
break
sets a breakpoint at the next instruction to be executed in the selected stack frame (see section Examining the Stack). In any selected frame but the innermost, this makes your program stop as soon as control returns to that frame. This is similar to the effect of afinish
command in the frame inside the selected frame--except thatfinish
does not leave an active breakpoint. If you usebreak
without an argument in the innermost frame, GDB stops the next time it reaches the current location; this may be useful inside loops. GDB normally ignores breakpoints when it resumes execution, until at least one instruction has been executed. If it did not do this, you would be unable to proceed past a breakpoint without first disabling the breakpoint. This rule applies whether or not the breakpoint already existed when your program stopped. break ... if cond
- Set a breakpoint with condition cond; evaluate the expression cond each time the breakpoint is reached, and stop only if the value is nonzero--that is, if cond evaluates as true. `...' stands for one of the possible arguments described above (or no argument) specifying where to break. See section Break conditions, for more information on breakpoint conditions.
tbreak args
-
Set a breakpoint enabled only for one stop. args are the
same as for the
break
command, and the breakpoint is set in the same way, but the breakpoint is automatically deleted after the first time your program stops there. See section Disabling breakpoints. hbreak args
-
Set a hardware-assisted breakpoint. args are the same as for the
break
command and the breakpoint is set in the same way, but the breakpoint requires hardware support and some target hardware may not have this support. The main purpose of this is EPROM/ROM code debugging, so you can set a breakpoint at an instruction without changing the instruction. This can be used with the new trap-generation provided by SPARClite DSU and most x86-based targets. These targets will generate traps when a program accesses some data or instruction address that is assigned to the debug registers. However the hardware breakpoint registers can take a limited number of breakpoints. For example, on the DSU, only two data breakpoints can be set at a time, and GDB will reject this command if more than two are used. Delete or disable unused hardware breakpoints before setting new ones (see section Disabling breakpoints). See section Break conditions. For remote targets, you can restrict the number of hardware breakpoints GDB will use, see @xref{set remote hardware-breakpoint-limit}. thbreak args
-
Set a hardware-assisted breakpoint enabled only for one stop. args
are the same as for the
hbreak
command and the breakpoint is set in the same way. However, like thetbreak
command, the breakpoint is automatically deleted after the first time your program stops there. Also, like thehbreak
command, the breakpoint requires hardware support and some target hardware may not have this support. See section Disabling breakpoints. See also section Break conditions. rbreak regex
-
Set breakpoints on all functions matching the regular expression
regex. This command sets an unconditional breakpoint on all
matches, printing a list of all breakpoints it set. Once these
breakpoints are set, they are treated just like the breakpoints set with
the
break
command. You can delete them, disable them, or make them conditional the same way as any other breakpoint. The syntax of the regular expression is the standard one used with tools like `grep'. Note that this is different from the syntax used by shells, so for instancefoo*
matches all functions that include anfo
followed by zero or moreo
s. There is an implicit.*
leading and trailing the regular expression you supply, so to match only functions that begin withfoo
, use^foo
. When debugging C++ programs,rbreak
is useful for setting breakpoints on overloaded functions that are not members of any special classes. Therbreak
command can be used to set breakpoints in all the functions in a program, like this:(gdb) rbreak .
info breakpoints [n]
info break [n]
info watchpoints [n]
-
Print a table of all breakpoints, watchpoints, and catchpoints set and
not deleted, with the following columns for each breakpoint:
- Breakpoint Numbers
- Type
- Breakpoint, watchpoint, or catchpoint.
- Disposition
- Whether the breakpoint is marked to be disabled or deleted when hit.
- Enabled or Disabled
- Enabled breakpoints are marked with `y'. `n' marks breakpoints that are not enabled.
- Address
- Where the breakpoint is in your program, as a memory address. If the breakpoint is pending (see below for details) on a future load of a shared library, the address will be listed as `<PENDING>'.
- What
- Where the breakpoint is in the source for your program, as a file and line number. For a pending breakpoint, the original string passed to the breakpoint command will be listed as it cannot be resolved until the appropriate shared library is loaded in the future.
info break
shows the condition on the line following the affected breakpoint; breakpoint commands, if any, are listed after that. A pending breakpoint is allowed to have a condition specified for it. The condition is not parsed for validity until a shared library is loaded that allows the pending breakpoint to resolve to a valid location.info break
with a breakpoint number n as argument lists only that breakpoint. The convenience variable$_
and the default examining-address for thex
command are set to the address of the last breakpoint listed (see section Examining memory).info break
displays a count of the number of times the breakpoint has been hit. This is especially useful in conjunction with theignore
command. You can ignore a large number of breakpoint hits, look at the breakpoint info to see how many times the breakpoint was hit, and then run again, ignoring one less than that number. This will get you quickly to the last hit of that breakpoint.
GDB allows you to set any number of breakpoints at the same place in your program. There is nothing silly or meaningless about this. When the breakpoints are conditional, this is even useful (see section Break conditions).
If a specified breakpoint location cannot be found, it may be due to the fact that the location is in a shared library that is yet to be loaded. In such a case, you may want GDB to create a special breakpoint (known as a pending breakpoint) that attempts to resolve itself in the future when an appropriate shared library gets loaded.
Pending breakpoints are useful to set at the start of your GDB session for locations that you know will be dynamically loaded later by the program being debugged. When shared libraries are loaded, a check is made to see if the load resolves any pending breakpoint locations. If a pending breakpoint location gets resolved, a regular breakpoint is created and the original pending breakpoint is removed.
GDB provides some additional commands for controlling pending breakpoint support:
set breakpoint pending auto
- This is the default behavior. When GDB cannot find the breakpoint location, it queries you whether a pending breakpoint should be created.
set breakpoint pending on
- This indicates that an unrecognized breakpoint location should automatically result in a pending breakpoint being created.
set breakpoint pending off
- This indicates that pending breakpoints are not to be created. Any unrecognized breakpoint location results in an error. This setting does not affect any pending breakpoints previously created.
show breakpoint pending
- Show the current behavior setting for creating pending breakpoints.
Normal breakpoint operations apply to pending breakpoints as well. You may specify a condition for a pending breakpoint and/or commands to run when the breakpoint is reached. You can also enable or disable the pending breakpoint. When you specify a condition for a pending breakpoint, the parsing of the condition will be deferred until the point where the pending breakpoint location is resolved. Disabling a pending breakpoint tells GDB to not attempt to resolve the breakpoint on any subsequent shared library load. When a pending breakpoint is re-enabled, GDB checks to see if the location is already resolved. This is done because any number of shared library loads could have occurred since the time the breakpoint was disabled and one or more of these loads could resolve the location.
GDB itself sometimes sets breakpoints in your program for
special purposes, such as proper handling of longjmp
(in C
programs). These internal breakpoints are assigned negative numbers,
starting with -1
; `info breakpoints' does not display them.
You can see these breakpoints with the GDB maintenance command
`maint info breakpoints' (@xref{maint info breakpoints}).
Setting watchpoints
You can use a watchpoint to stop execution whenever the value of an expression changes, without having to predict a particular place where this may happen.
Depending on your system, watchpoints may be implemented in software or hardware. GDB does software watchpointing by single-stepping your program and testing the variable's value each time, which is hundreds of times slower than normal execution. (But this may still be worth it, to catch errors where you have no clue what part of your program is the culprit.)
On some systems, such as HP-UX, GNU/Linux and most other x86-based targets, GDB includes support for hardware watchpoints, which do not slow down the running of your program.
watch expr
- Set a watchpoint for an expression. GDB will break when expr is written into by the program and its value changes.
rwatch expr
- Set a watchpoint that will break when the value of expr is read by the program.
awatch expr
- Set a watchpoint that will break when expr is either read from or written into by the program.
info watchpoints
-
This command prints a list of watchpoints, breakpoints, and catchpoints;
it is the same as
info break
(see section Setting breakpoints).
GDB sets a hardware watchpoint if possible. Hardware watchpoints execute very quickly, and the debugger reports a change in value at the exact instruction where the change occurs. If GDB cannot set a hardware watchpoint, it sets a software watchpoint, which executes more slowly and reports the change in value at the next statement, not the instruction, after the change occurs.
You can force GDB to use only software watchpoints with the
set can-use-hw-watchpoints 0 command. With this variable set to
zero, GDB will never try to use hardware watchpoints, even if
the underlying system supports them. (Note that hardware-assisted
watchpoints that were set before setting
can-use-hw-watchpoints
to zero will still use the hardware
mechanism of watching expressiion values.)
set can-use-hw-watchpoints
- Set whether or not to use hardware watchpoints.
show can-use-hw-watchpoints
- Show the current mode of using hardware watchpoints.
For remote targets, you can restrict the number of hardware watchpoints GDB will use, see @xref{set remote hardware-breakpoint-limit}.
When you issue the watch
command, GDB reports
Hardware watchpoint num: expr
if it was able to set a hardware watchpoint.
Currently, the awatch
and rwatch
commands can only set
hardware watchpoints, because accesses to data that don't change the
value of the watched expression cannot be detected without examining
every instruction as it is being executed, and GDB does not do
that currently. If GDB finds that it is unable to set a
hardware breakpoint with the awatch
or rwatch
command, it
will print a message like this:
Expression cannot be implemented with read/access watchpoint.
Sometimes, GDB cannot set a hardware watchpoint because the data type of the watched expression is wider than what a hardware watchpoint on the target machine can handle. For example, some systems can only watch regions that are up to 4 bytes wide; on such systems you cannot set hardware watchpoints for an expression that yields a double-precision floating-point number (which is typically 8 bytes wide). As a work-around, it might be possible to break the large region into a series of smaller ones and watch them with separate watchpoints.
If you set too many hardware watchpoints, GDB might be unable to insert all of them when you resume the execution of your program. Since the precise number of active watchpoints is unknown until such time as the program is about to be resumed, GDB might not be able to warn you about this when you set the watchpoints, and the warning will be printed only when the program is resumed:
Hardware watchpoint num: Could not insert watchpoint
If this happens, delete or disable some of the watchpoints.
The SPARClite DSU will generate traps when a program accesses some data
or instruction address that is assigned to the debug registers. For the
data addresses, DSU facilitates the watch
command. However the
hardware breakpoint registers can only take two data watchpoints, and
both watchpoints must be the same kind. For example, you can set two
watchpoints with watch
commands, two with rwatch
commands,
or two with awatch
commands, but you cannot set one
watchpoint with one command and the other with a different command.
GDB will reject the command if you try to mix watchpoints.
Delete or disable unused watchpoint commands before setting new ones.
If you call a function interactively using print
or call
,
any watchpoints you have set will be inactive until GDB reaches another
kind of breakpoint or the call completes.
GDB automatically deletes watchpoints that watch local
(automatic) variables, or expressions that involve such variables, when
they go out of scope, that is, when the execution leaves the block in
which these variables were defined. In particular, when the program
being debugged terminates, all local variables go out of scope,
and so only watchpoints that watch global variables remain set. If you
rerun the program, you will need to set all such watchpoints again. One
way of doing that would be to set a code breakpoint at the entry to the
main
function and when it breaks, set all the watchpoints.
Warning: In multi-thread programs, watchpoints have only limited usefulness. With the current watchpoint implementation, GDB can only watch the value of an expression in a single thread. If you are confident that the expression can only change due to the current thread's activity (and if you are also confident that no other thread can become current), then you can use watchpoints as usual. However, GDB may not notice when a non-current thread's activity changes the expression.
HP-UX Warning: In multi-thread programs, software watchpoints have only limited usefulness. If GDB creates a software watchpoint, it can only watch the value of an expression in a single thread. If you are confident that the expression can only change due to the current thread's activity (and if you are also confident that no other thread can become current), then you can use software watchpoints as usual. However, GDB may not notice when a non-current thread's activity changes the expression. (Hardware watchpoints, in contrast, watch an expression in all threads.)
@xref{set remote hardware-watchpoint-limit}.
Setting catchpoints
You can use catchpoints to cause the debugger to stop for certain
kinds of program events, such as C++ exceptions or the loading of a
shared library. Use the catch
command to set a catchpoint.
catch event
-
Stop when event occurs. event can be any of the following:
throw
- The throwing of a C++ exception.
catch
- The catching of a C++ exception.
exec
-
A call to
exec
. This is currently only available for HP-UX. fork
-
A call to
fork
. This is currently only available for HP-UX. vfork
-
A call to
vfork
. This is currently only available for HP-UX. load
load libname
- The dynamic loading of any shared library, or the loading of the library libname. This is currently only available for HP-UX.
unload
unload libname
- The unloading of any dynamically loaded shared library, or the unloading of the library libname. This is currently only available for HP-UX.
tcatch event
- Set a catchpoint that is enabled only for one stop. The catchpoint is automatically deleted after the first time the event is caught.
Use the info break
command to list the current catchpoints.
There are currently some limitations to C++ exception handling
(catch throw
and catch catch
) in GDB:
- If you call a function interactively, GDB normally returns control to you when the function has finished executing. If the call raises an exception, however, the call may bypass the mechanism that returns control to you and cause your program either to abort or to simply continue running until it hits a breakpoint, catches a signal that GDB is listening for, or exits. This is the case even if you set a catchpoint for the exception; catchpoints on exceptions are disabled within interactive calls.
- You cannot raise an exception interactively.
- You cannot install an exception handler interactively.
Sometimes catch
is not the best way to debug exception handling:
if you need to know exactly where an exception is raised, it is better to
stop before the exception handler is called, since that way you
can see the stack before any unwinding takes place. If you set a
breakpoint in an exception handler instead, it may not be easy to find
out where the exception was raised.
To stop just before an exception handler is called, you need some
knowledge of the implementation. In the case of GNU C++, exceptions are
raised by calling a library function named __raise_exception
which has the following ANSI C interface:
/* addr is where the exception identifier is stored. id is the exception identifier. */ void __raise_exception (void **addr, void *id);
To make the debugger catch all exceptions before any stack
unwinding takes place, set a breakpoint on __raise_exception
(see section Breakpoints, watchpoints, and catchpoints).
With a conditional breakpoint (see section Break conditions) that depends on the value of id, you can stop your program when a specific exception is raised. You can use multiple conditional breakpoints to stop your program when any of a number of exceptions are raised.
Deleting breakpoints
It is often necessary to eliminate a breakpoint, watchpoint, or catchpoint once it has done its job and you no longer want your program to stop there. This is called deleting the breakpoint. A breakpoint that has been deleted no longer exists; it is forgotten.
With the clear
command you can delete breakpoints according to
where they are in your program. With the delete
command you can
delete individual breakpoints, watchpoints, or catchpoints by specifying
their breakpoint numbers.
It is not necessary to delete a breakpoint to proceed past it. GDB automatically ignores breakpoints on the first instruction to be executed when you continue execution without changing the execution address.
clear
- Delete any breakpoints at the next instruction to be executed in the selected stack frame (see section Selecting a frame). When the innermost frame is selected, this is a good way to delete a breakpoint where your program just stopped.
clear function
clear filename:function
- Delete any breakpoints set at entry to the named function.
clear linenum
clear filename:linenum
- Delete any breakpoints set at or within the code of the specified linenum of the specified filename.
delete [breakpoints] [range...]
-
Delete the breakpoints, watchpoints, or catchpoints of the breakpoint
ranges specified as arguments. If no argument is specified, delete all
breakpoints (GDB asks confirmation, unless you have
set confirm off
). You can abbreviate this command asd
.
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
asdis
. 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.
Breakpoints set by the
tbreak
command start out in this state.
Except for a breakpoint set with tbreak
(see section Setting breakpoints), 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 section Continuing and stepping.)
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 (see section 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 (see section Breakpoint command lists).
Break conditions can be specified when a breakpoint is set, by using
`if' in the arguments to the break
command. See section Setting breakpoints. 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 thecondition
command (or a command that sets a breakpoint with a condition, likebreak if ...
) is given, however. See section 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 tocontinue
, rather than usingignore
. See section 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. See section Convenience variables.
Ignore counts apply to breakpoints, watchpoints, and catchpoints.
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, typecommands
and follow it immediately withend
; 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 RET 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. See section Commands for controlled 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
Breakpoint menus
Some programming languages (notably C++ and Objective-C/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)
"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:
- Remove or disable the breakpoints, then continue.
-
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. - 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.
"Breakpoint address adjusted..."
Some processor architectures place constraints on the addresses at which breakpoints may be placed. For architectures thus constrained, GDB will attempt to adjust the breakpoint's address to comply with the constraints dictated by the architecture.
One example of such an architecture is the Fujitsu FR-V. The FR-V is a VLIW architecture in which a number of RISC-like instructions may be bundled together for parallel execution. The FR-V architecture constrains the location of a breakpoint instruction within such a bundle to the instruction with the lowest address. GDB honors this constraint by adjusting a breakpoint's address to the first in the bundle.
It is not uncommon for optimized code to have bundles which contain instructions from different source statements, thus it may happen that a breakpoint's address will be adjusted from one source statement to another. Since this adjustment may significantly alter GDB's breakpoint related behavior from what the user expects, a warning is printed when the breakpoint is first set and also when the breakpoint is hit.
A warning like the one below is printed when setting a breakpoint that's been subject to address adjustment:
warning: Breakpoint address adjusted from 0x00010414 to 0x00010410.
Such warnings are printed both for user settable and GDB's internal breakpoints. If you see one of these warnings, you should verify that a breakpoint set at the adjusted address will have the desired affect. If not, the breakpoint in question may be removed and other breakpoints may be set which will have the desired behavior. E.g., it may be sufficient to place the breakpoint at a later instruction. A conditional breakpoint may also be useful in some cases to prevent the breakpoint from triggering too often.
GDB will also issue a warning when stopping at one of these adjusted breakpoints:
warning: Breakpoint 1 address previously adjusted from 0x00010414 to 0x00010410.
When this warning is encountered, it may be too late to take remedial action except in cases where the breakpoint is hit earlier or more frequently than expected.
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. See section 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
(see section Break conditions). The argument ignore-count is meaningful only when your program stopped due to a breakpoint. At other times, the argument tocontinue
is ignored. The synonymsc
andfg
(for foreground, as the debugged program is deemed to be the foreground program) are provided purely for convenience, and have exactly the same behavior ascontinue
.
To resume execution at a different place, you can use return
(see section Returning from a function) to go back to the
calling function; or jump
(see section Continuing at a different address) to go to an arbitrary location in your program.
A typical technique for using stepping is to set a breakpoint (see section Breakpoints, watchpoints, and catchpoints) 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 thestepi
command, described below.step
command only stops at the first instruction of a source line. This prevents the multiple stops that could otherwise occur inswitch
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, thestep
command only enters a function if there is line number information for the function. Otherwise it acts like thenext
command. This avoids problems when usingcc -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 thenext
command. This command is abbreviatedn
. An argument count is a repeat count, as forstep
. Thenext
command only stops at the first instruction of a source line. This prevents multiple stops that could otherwise occur inswitch
statements,for
loops, etc. set step-mode
set step-mode on
-
The
set step-mode on
command causes thestep
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. show step-mode
- Show whether GDB will stop in or step over functions without source line debug information.
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 (see section Returning from a function). 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 whenuntil
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, anext
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, thef
(frame
) command shows that execution is stopped at line206
; yet when we useuntil
, we get to line195
:(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 Cfor
-loop is written before the body of the loop. Theuntil
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 thanuntil
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
(see section Setting breakpoints). This form of the command uses breakpoints, and hence is quicker thanuntil
without an argument. The specified location is actually reached only if it is in the current frame. This implies thatuntil
can be used to skip over recursive function invocations. For instance in the code below, if the current location is line96
, issuinguntil 99
will execute the program up to line99
in the same invocation of factorial, i.e. after the inner invocations have returned.94 int factorial (int value) 95 { 96 if (value > 1) { 97 value *= factorial (value - 1); 98 } 99 return (value); 100 }
advance location
-
Continue running the program up to the given location. An argument is
required, which should be of the same form as arguments for the
break
command. Execution will also stop upon exit from the current stack frame. This command is similar tountil
, butadvance
will not skip over recursive function calls, and the target location doesn't have to be in the same frame as the current one. 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. See section Automatic 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
.
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 forinfo 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
andnoignore
are synonyms. nopass
ignore
-
GDB should not allow your program to see this signal.
nopass
andignore
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'. See section Giving your program a signal.
Stopping and starting multi-thread programs
When your program has multiple threads (see section Debugging programs with multiple 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.
There is an unfortunate side effect. If one thread stops for a breakpoint, or for some other reason, and another thread is blocked in a system call, then the system call may return prematurely. This is a consequence of the interaction between multiple threads and the signals that GDB uses to implement breakpoints and other events that stop execution.
To handle this problem, your program should check the return value of each system call and react appropriately. This is good programming style anyways.
For example, do not write code like this:
sleep (10);
The call to sleep
will return early if a different thread stops
at a breakpoint or for some other reason.
Instead, write this:
int unslept = 10; while (unslept > 0) unslept = sleep (unslept);
A system call is allowed to return early, so the system is still conforming to its specification. But GDB does cause your multi-threaded program to behave differently than it would without GDB.
Also, GDB uses internal breakpoints in the thread library to monitor certain events such as thread creation and thread destruction. When such an event happens, a system call in another thread may return prematurely, even though your program does not appear to stop.
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. Ifon
, then only the current thread may run when the inferior is resumed. Thestep
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.
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