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Name

Synopsis

#include <setjmp.h>

int setjmp(jmp_buf env);

int sigsetjmp(sigjmp_buf env, int savemask);

void longjmp(jmp_buf env, int val);

void siglongjmp(sigjmp_buf env, int val);

MT-Level

Unsafe

Description

These functions are useful for dealing with errors and interrupts encountered in a low-level subroutine of a program.

setjmp() saves its stack environment in env for later use by longjmp().

sigsetjmp() saves the calling process’s registers and stack environment (see sigaltstack(2) ) in env for later use by siglongjmp(). If savemask is non-zero, the calling process’s signal mask (see sigprocmask(2) ) and scheduling parameters (see priocntl(2) ) are also saved.

longjmp() restores the environment saved by the last call of setjmp() with the corresponding env argument. After longjmp() is completed, program execution continues as if the corresponding call of setjmp() had just returned the value val. The caller of setjmp() must not have returned in the interim. longjmp() cannot cause setjmp() to return the value 0. If longjmp() is invoked with a second argument of 0, setjmp() will return 1. At the time of the second return from setjmp(), all external and static variables have values as of the time longjmp() is called (see example).

siglongjmp() restores the environment saved by the last call of sigsetjmp() with the corresponding env argument. After siglongjmp() is completed, program execution continues as if the corresponding call of sigsetjmp() had just returned the value val. siglongjmp() cannot cause sigsetjmp() to return the value 0. If siglongjmp() is invoked with a second argument of 0, sigsetjmp() will return 1. At the time of the second return from sigsetjmp(), all external and static variables have values as of the time siglongjmp() is called.

If a signal-catching function interrupts sleep() and calls siglongjmp() to restore an environment saved prior to the sleep() call, the action associated with SIGALRM and time it is scheduled to be generated are unspecified. It is also unspecified whether the SIGALRM signal is blocked, unless the process’s signal mask is restored as part of the environment.

The function siglongjmp() restores the saved signal mask if and only if the env argument was initialized by a call to the sigsetjmp() function with a non-zero savemask argument.

The values of register and automatic variables are undefined. Register or automatic variables whose value must be relied upon must be declared as volatile.

Examples

#include <stdio.h>
#include <setjmp.h>
#include <signal.h>
#include <unistd.h>
jmp_buf env; static void signal_handler();
main()  {
        int returned_from_longjump, processing = 1;
        unsigned int time_interval = 4;
        if ((returned_from_longjump = setjmp(env)) != 0) 
            switch (returned_from_longjump)     {
              case SIGINT: 
                printf("longjumped from interrupt %d\n",SIGINT);   
                break;
              case SIGALRM: 
                printf("longjumped from alarm %d\n",SIGALRM);     
                break;
            }
        (void) signal(SIGINT, signal_handler);
        (void) signal(SIGALRM, signal_handler);
        alarm(time_interval);
        while (processing)        {
          printf(" waiting for you to INTERRUPT (cntrl-C) ...\n");
          sleep(1);
        }         /* end while forever loop */
}
 

static void signal_handler(sig)
int sig; {
        switch (sig)     {
          case SIGINT:    ...       /* process for interrupt */
                          longjmp(env,sig); 
                                        /* break never reached */ 
          case SIGALRM:    ...       /* process for alarm */
                           longjmp(env,sig);
                                        /* break never reached */
          default:         exit(sig);
        }
}

When this example is compiled and executed, and the user sends an interrupt signal, the output will be:

longjumped from interrupt

longjumped from alarm

Return Values

If longjmp() or siglongjmp() are invoked with a second argument of 0, setjmp() and sigsetjmp() , respectively, return 1. Otherwise, setjmp() and sigsetjmp() return 0.

See Also

Warnings

If longjmp() or siglongjmp() are called even though env was never primed by a call to setjmp() or sigsetjmp(), or when the last such call was in a function that has since returned, absolute chaos is guaranteed.

Table of Contents

• Name
• Synopsis
• MT-Level
• Description
• Examples
• Return Values
• See Also
• Warnings