longjmp()

Restore the environment saved by setjmp()

Synopsis:

#include <setjmp.h>

void longjmp( jmp_buf env, 
              int return_value );

Arguments:

env: The environment saved by the most recent call to setjmp().
return_value: The value that you want setjmp() to return.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The longjmp() function restores the environment saved in env by the most recent call to the setjmp() function.

Using longjmp() to jump out of a signal handler can cause unpredictable behavior, unless the signal was generated by the raise() function.

After the longjmp() function restores the environment, program execution continues as if the corresponding call to setjmp() had just returned the value specified by return_value. If the value of return_value is 0, the value returned is 1.

Examples:

#include <stdio.h>
#include <stdlib.h>
#include <setjmp.h>

jmp_buf env;

void rtn( void )
{
    printf( "about to longjmp\n" );
    longjmp( env, 14 );
}

int main( void )
{
    int ret_val = 293;

    if( 0 == ( ret_val = setjmp( env ) ) ) {
        printf( "after setjmp %d\n", ret_val );
        rtn();
        printf( "back from rtn %d\n", ret_val );
    } else {
        printf( "back from longjmp %d\n", ret_val );
    }
    
    return EXIT_SUCCESS;
}

produces the following output:

after setjmp 0
about to longjmp
back from longjmp 14

Classification:

ANSI, POSIX 1003.1

Safety:
Cancellation point	No
Interrupt handler	No
Signal handler	Yes
Thread	Yes

Caveats:

A strictly-conforming POSIX application can't assume that the longjmp() function is signal-safe on other platforms.

Don't use longjmp() or siglongjmp() to restore an environment saved by a call to setjmp() or sigsetjmp() in another thread. If you're lucky, your application will crash; if not, it'll look as if it works for a while, until random scribbling on the stack causes it to crash.

longjmp