C Pocket Reference

Non-Local Jumps

Local jumps, or jumps within a function, are performed by the goto statement. The macro setjmp(), on the other hand, marks a location in the program (by storing the pertinent process information) so that execution can be resumed at that point at any time by a call to the function longjmp(). The longjmp() function and the setjmp() macro are declared in the header file setjmp.h .

int setjmp( jmp_buf env );

Saves the current calling environment (CPU registers and stack) in the buffer env, which has the type jmp_buf.

void longjmp( jmp_buf env, int retval );

Restores the saved environment, so that program execution continues at the point where setjmp() was called.

The program can use the return value of setjmp() to determine whether setjmp() itself was just called, or whether a jump to this point by means of longjmp() has just occurred. setjmp() itself returns the value 0, but after a call to longjmp() the apparent return value of setjmp() is the value of the argument retval. If retval is equal to 0, the apparent return value is 1.