man pages section 3: Basic Library Functions

_setjmp(3UCB)

Name

setjmp, longjmp, _setjmp, _longjmp– non-local goto

Synopsis

/usr/ucb/cc [ flag ... ] file ...
#include <setjmp.h>

int setjmp(env)
jmp_buf env;

void longjmp(env, val)
jmp_buf env;
int val;

int _setjmp(env)
jmp_buf env;

void _longjmp(env, val)
jmp_buf env;
int val;

Description

The setjmp() and longjmp() functions are useful for dealing with errors and interrupts encountered in a low-level subroutine of a program.

The setjmp() function saves its stack environment in env for later use by longjmp(). A normal call to setjmp() returns zero. setjmp() also saves the register environment. If a longjmp() call will be made, the routine which called setjmp() should not return until after the longjmp() has returned control (see below).

The longjmp() function restores the environment saved by the last call of setjmp(), and then returns in such a way that execution continues as if the call of setjmp() had just returned the value val to the function that invoked setjmp(); however, if val were zero, execution would continue as if the call of setjmp() had returned one. This ensures that a ``return'' from setjmp() caused by a call to longjmp() can be distinguished from a regular return from setjmp(). The calling function must not itself have returned in the interim, otherwise longjmp() will be returning control to a possibly non-existent environment. All memory-bound data have values as of the time longjmp() was called. The CPU and floating-point data registers are restored to the values they had at the time that setjmp() was called. But, because the register storage class is only a hint to the C compiler, variables declared as register variables may not necessarily be assigned to machine registers, so their values are unpredictable after a longjmp(). This is especially a problem for programmers trying to write machine-independent C routines.

The setjmp() and longjmp() functions save and restore the signal mask while _setjmp() and _longjmp() manipulate only the C stack and registers.

None of these functions save or restore any floating-point status or control registers.

Examples

Example 1 Examples of `setjmp()` and `longjmp()`.

The following example uses both setjmp() and longjmp() to return the flow of control to the appropriate instruction block:

#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

Additionally, every 4 seconds the alarm will expire, signalling this process, and the output will be:

longjumped from alarm

Notes

Use of these interfaces should be restricted to only applications written on BSD platforms. Use of these interfaces with any of the system libraries or in multi-thread applications is unsupported.

Bugs

The setjmp() function does not save the current notion of whether the process is executing on the signal stack. The result is that a longjmp() to some place on the signal stack leaves the signal stack state incorrect.

On some systems setjmp() also saves the register environment. Therefore, all data that are bound to registers are restored to the values they had at the time that setjmp() was called. All memory-bound data have values as of the time longjmp() was called. However, because the register storage class is only a hint to the C compiler, variables declared as register variables may not necessarily be assigned to machine registers, so their values are unpredictable after a longjmp(). When using compiler options that specify automatic register allocation (see cc(1B)), the compiler will not attempt to assign variables to registers in routines that call setjmp().

The longjmp() function never causes setjmp() to return 0, so programmers should not depend on longjmp() being able to cause setjmp() to return 0.

SunOS 5.10 Last Revised 7 Apr 1993