5.3 Setting a breakpoint

A breakpoint stops the execution of a program and returns control to the debugger, where its variables and memory can be examined before continuing. Breakpoints can be set for specific functions, lines or memory locations with the break command.

To set a breakpoint on a specific function, use the command break function-name. For example, the following command sets a breakpoint at the start of the main function in the program above:

$ gdb a.out
(gdb) break main
Breakpoint 1 at 0x80483c6: file null.c, line 6.

The debugger will now take control of the program when the function main is called. Since the main function is the first function to be executed in a C program the program will stop immediately when it is run:

(gdb) run
Starting program: a.out 
Breakpoint 1, main () at null.c:6
6         int *p = 0;   /* null pointer */
(gdb)

The display shows the line that will be executed next (the line number is shown on the left). The breakpoint stops the program before the line is executed, so at this stage the pointer p is undefined and has not yet been set to zero.