<setjmp.h>: Non-local goto

Functions
int	setjmp (jmp_buf __jmpb)
void	longjmp (jmp_buf __jmpb, int __ret)

Detailed Description

While the C language has the dreaded goto statement, it can only be used to jump to a label in the same (local) function. In order to jump directly to another (non-local) function, the C library provides the setjmp and longjmp functions. setjmp and longjmp are useful for dealing with errors and interrupts encountered in a low-level subroutine of a program.

Note

setjmp and longjmp make programs hard to understand and maintain. If possible, an alternative should be used.

longjmp can destroy changes made to global register variables (see How to permanently bind a variable to a register?).

For a very detailed discussion of setjmp/longjmp, see Chapter 7 of Advanced Programming in the UNIX Environment, by W. Richard Stevens.

Example:

#include <setjmp.h>
 
jmp_buf env;
 
int main (void)
{
    if (setjmp (env))
    {
        // Handle error ...
    }
 
    while (1)
    {
       // Main processing loop which calls foo() somewhere ...
    }
}
 
void foo (void)
{
    // blah, blah, blah ...
 
    if (err)
    {
        longjmp (env, 1);
    }
}

Function Documentation

longjmp()

void longjmp	(	jmp_buf	__jmpb,
		int	__ret
	)

Non-local jump to a saved stack context.

#include <setjmp.h>

longjmp() restores the environment saved by the last call of setjmp() with the corresponding __jmpb argument. After longjmp() is completed, program execution continues as if the corresponding call of setjmp() had just returned the value __ret.

Note

longjmp() cannot cause 0 to be returned. If longjmp() is invoked with a second argument of 0, 1 will be returned instead.

Parameters

__jmpb	Information saved by a previous call to setjmp().
__ret	Value to return to the caller of setjmp().

Returns

This function never returns.

setjmp()

int setjmp

(

jmp_buf

__jmpb

)

Save stack context for non-local goto.

#include <setjmp.h>

setjmp() saves the stack context/environment in __jmpb for later use by longjmp(). The stack context will be invalidated if the function which called setjmp() returns.

Parameters

__jmpb

Variable of type jmp_buf which holds the stack information such that the environment can be restored.

Returns

setjmp() returns 0 if returning directly, and non-zero when returning from longjmp() using the saved context.