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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 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. The expression may be a value of a variable, or it could involve values of one or more variables combined by operators, such as ‘a + b’. This is sometimes called data breakpoints. You must use a different command to set watchpoints (see 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 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 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 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 breakpoints in that range are operated on.