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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 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 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 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 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 some 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). See section Break conditions.
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 the tbreak
command,
the breakpoint is automatically deleted after the
first time your program stops there. Also, like the hbreak
command, the breakpoint requires hardware support and some target hardware
may not have this support. See section Disabling breakpoints.
See also 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 instance foo*
matches all functions that include
an fo
followed by zero or more o
s. There is an implicit
.*
leading and trailing the regular expression you supply, so to
match only functions that begin with foo
, use ^foo
.
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, watchpoints, and catchpoints set and not deleted, with the following columns for each breakpoint:
Breakpoint, watchpoint, or catchpoint.
Whether the breakpoint is marked to be disabled or deleted when hit.
Enabled breakpoints are marked with `y'. `n' marks breakpoints that are not enabled.
Where the breakpoint is in your program, as a memory address.
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).
info break
displays a count of the number of times the breakpoint
has been hit. This is especially useful in conjunction with the
ignore
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).
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' (see maint info breakpoints).
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