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Setting breakpoints

Breakpoints are set with the break command (abbreviated b). The debugger convenience variable `$bpnum' records the number of the beakpoint 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 frame.

break linenum

Set a breakpoint at line linenum in the current source file. That file is the last file whose source text was printed. This breakpoint stops 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 a finish command in the frame inside the selected frame--except that finish does not leave an active breakpoint. If you use break 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 disabled after the first time your program stops there. See section Disabling breakpoints.

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.

When debugging C++ programs, rbreak is useful for setting breakpoints on overloaded functions that are not members of any special classes.

info breakpoints [n]

info break [n]

info watchpoints [n]

Print a table of all breakpoints and watchpoints set and not deleted, with the following columns for each breakpoint:

Breakpoint Numbers

Type

Breakpoint or watchpoint.

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

What

Where the breakpoint is in the source for your program, as a file and line number.

If a breakpoint is conditional, info break shows the condition on the line following the affected breakpoint; breakpoint commands, if any, are listed after that.

info break with a breakpoint number n as argument lists only that breakpoint. The convenience variable $_ and the default examining-address for the x command are set to the address of the last breakpoint listed (see section Examining memory).

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).

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'.

maint info breakpoints

Using the same format as `info breakpoints', display both the breakpoints you've set explicitly, and those GDB is using for internal purposes. Internal breakpoints are shown with negative breakpoint numbers. The type column identifies what kind of breakpoint is shown:

breakpoint

Normal, explicitly set breakpoint.

watchpoint

Normal, explicitly set watchpoint.

longjmp

Internal breakpoint, used to handle correctly stepping through longjmp calls.

longjmp resume

Internal breakpoint at the target of a longjmp.

until

Temporary internal breakpoint used by the GDB until command.

finish

Temporary internal breakpoint used by the GDB finish command.

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