5.1.2 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. (This is sometimes called a data breakpoint.) The expression may be as simple as the value of a single variable, or as complex as many variables combined by operators. Examples include:
- A reference to the value of a single variable.
- An address cast to an appropriate data type. For example,
*(int *)0x12345678 will watch a 4-byte region at the specified
address (assuming an
intoccupies 4 bytes).
- An arbitrarily complex expression, such as a*b + c/d. The expression can use any operators valid in the program's native language (see Languages).
You can set a watchpoint on an expression even if the expression can
not be evaluated yet. For instance, you can set a watchpoint on
*global_ptr before global_ptr is initialized.
gdb will stop when your program sets global_ptr and
the expression produces a valid value. If the expression becomes
valid in some other way than changing a variable (e.g. if the memory
pointed to by *global_ptr becomes readable as the result of a
malloc call), gdb may not stop until the next time
the expression changes.
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.)
- Set a watchpoint for an expression. gdb will break when the
expression expr is written into by the program and its value
changes. The simplest (and the most popular) use of this command is
to watch the value of a single variable:
(gdb) watch foo
If the command includes a [
threadthreadnum] clause, gdb breaks only when the thread identified by threadnum changes the value of expr. If any other threads change the value of expr, gdb will not break. Note that watchpoints restricted to a single thread in this way only work with Hardware Watchpoints.
- Set a watchpoint that will break when the value of expr is read by the program.
- Set a watchpoint that will break when expr is either read from or written into by the program.
- This command prints a list of watchpoints, using the same format as
info break(see Set Breaks).
If you watch for a change in a numerically entered address you need to dereference it, as the address itself is just a constant number which will never change. gdb refuses to create a watchpoint that watches a never-changing value:
(gdb) watch 0x600850 Cannot watch constant value 0x600850. (gdb) watch *(int *) 0x600850 Watchpoint 1: *(int *) 6293584
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 expression values.)
- Set whether or not to use hardware watchpoints.
- Show the current mode of using hardware watchpoints.
For remote targets, you can restrict the number of hardware watchpoints gdb will use, see 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.
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
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.
Watching complex expressions that reference many variables can also exhaust the resources available for hardware-assisted watchpoints. That's because gdb needs to watch every variable in the expression with separately allocated resources.
If you call a function interactively using
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-threaded 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.)