This is signals/sigmask_longjmp.c (Listing 21-2, page 432), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* sigmask_longjmp.c

   Demonstrate the different effects of longjmp() and siglongjmp()
   on the process signal mask.

   By default, this program uses setjmp() + longjmp(). Compile with
   -DUSE_SIGSETJMP to use sigsetjmp() + siglongjmp().
#define _GNU_SOURCE     /* Get strsignal() declaration from <string.h> */
#include <string.h>
#include <setjmp.h>
#include <signal.h>
#include "signal_functions.h"           /* Declaration of printSigMask() */
#include "tlpi_hdr.h"

static volatile sig_atomic_t canJump = 0;
                        /* Set to 1 once "env" buffer has been
                           initialized by [sig]setjmp() */
static sigjmp_buf senv;
static jmp_buf env;
static void
handler(int sig)
    /* UNSAFE: This handler uses non-async-signal-safe functions
       (printf(), strsignal(), printSigMask(); see Section 21.1.2) */

    printf("Received signal %d (%s), signal mask is:\n", sig,
    printSigMask(stdout, NULL);

    if (!canJump) {
        printf("'env' buffer not yet set, doing a simple return\n");

    siglongjmp(senv, 1);
    longjmp(env, 1);
main(int argc, char *argv[])
    struct sigaction sa;

    printSigMask(stdout, "Signal mask at startup:\n");

    sa.sa_flags = 0;
    sa.sa_handler = handler;
    if (sigaction(SIGINT, &sa, NULL) == -1)

    printf("Calling sigsetjmp()\n");
    if (sigsetjmp(senv, 1) == 0)
    printf("Calling setjmp()\n");
    if (setjmp(env) == 0)
        canJump = 1;                    /* Executed after [sig]setjmp() */

    else                                /* Executed after [sig]longjmp() */
        printSigMask(stdout, "After jump from handler, signal mask is:\n" );

    for (;;)                            /* Wait for signals until killed */


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