#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);

These 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().

The sigsetjmp() function 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.

The longjmp() function restores the environment saved by the last call of setjmp() with the corresponding env argument. After longjmp() completes, program execution continues as if the corresponding call to setjmp() had just returned the value val. The caller of setjmp() must not have returned in the interim. The longjmp() function 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 EXAMPLES).

The siglongjmp() function restores the environment saved by the last call of sigsetjmp() with the corresponding env argument. After siglongjmp() completes, program execution continues as if the corresponding call to sigsetjmp() had just returned the value val. The siglongjmp() function 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() was called.

If a signal-catching function interrupts sleep(3C) 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 siglongjmp() function 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.

If the return is from a direct invocation, setjmp() and sigsetjmp() return 0. If the return is from a call to longjmp(), setjmp() returns a non-zero value. If the return is from a call to siglongjmp(), sigsetjmp() returns a non-zero value.

After longjmp() is completed, program execution continues as if the corresponding invocation of setjmp() had just returned the value specified by val. The longjmp() function cannot cause setjmp() to return 0; if val is 0, setjmp() returns 1.

After siglongjmp() is completed, program execution continues as if the corresponding invocation of sigsetjmp() had just returned the value specified by val. The siglongjmp() function cannot cause sigsetjmp() to return 0; if val is 0, sigsetjmp() returns 1.

Example 1 Example of setjmp() and longjmp() functions.

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\en",SIGINT);
 break;
 case SIGALRM:
 printf("longjumped from alarm %d\en",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) ...\en");
 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

See attributes(5) for descriptions of the following attributes:

getcontext(2), priocntl(2), sigaction(2), sigaltstack(2), sigprocmask(2), signal(3C), attributes(5), standards(5)

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, the results are undefined.