This is signals/catch_rtsigs.c (Listing 22-3, page 462), 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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/* catch_rtsigs.c

   Usage: catch_rtsigs [block-time [handler-sleep-time]]
                              default=0   default=1

        block-time specifies an amount of time the program should
                pause after blocking all signals. This allows
                multiple signals to be sent to the process before
                it unblocks all the signals.

        handler-sleep-time specifies the amount of time the signal
                handler should sleep before returning. Using a
                nonzero value here allows us to slow things down
                so that we can see what happens when multiple
                signals are sent.

   After optionally blocking all (possible) signals and sleeping for
   'block-time' seconds, loop continuously using pause() to wait for
   any incoming signals.

   The program can be terminated by typing control-C (which generates
   SIGINT) or sending it SIGTERM.
#define _GNU_SOURCE
#include <string.h>
#include <signal.h>
#include "tlpi_hdr.h"

static volatile int handlerSleepTime;
static volatile int sigCnt = 0;         /* Number of signals received */
static volatile sig_atomic_t allDone = 0;
static void             /* Handler for signals established using SA_SIGINFO */
siginfoHandler(int sig, siginfo_t *si, void *ucontext)
    /* UNSAFE: This handler uses non-async-signal-safe functions
       (printf()); see Section 21.1.2) */

    /* SIGINT or SIGTERM can be used to terminate program */

    if (sig == SIGINT || sig == SIGTERM) {
        allDone = 1;

    printf("caught signal %d\n", sig);

    printf("    si_signo=%d, si_code=%d (%s), ", si->si_signo, si->si_code,
            (si->si_code == SI_USER) ? "SI_USER" :
            (si->si_code == SI_QUEUE) ? "SI_QUEUE" : "other");
    printf("si_value=%d\n", si->si_value.sival_int);
    printf("    si_pid=%ld, si_uid=%ld\n",
            (long) si->si_pid, (long) si->si_uid);

main(int argc, char *argv[])
    struct sigaction sa;
    int sig;
    sigset_t prevMask, blockMask;

    if (argc > 1 && strcmp(argv[1], "--help") == 0)
        usageErr("%s [block-time [handler-sleep-time]]\n", argv[0]);

    printf("%s: PID is %ld\n", argv[0], (long) getpid());

    handlerSleepTime = (argc > 2) ?
                getInt(argv[2], GN_NONNEG, "handler-sleep-time") : 1;

    /* Establish handler for most signals. During execution of the handler,
       mask all other signals to prevent handlers recursively interrupting
       each other (which would make the output hard to read). */

    sa.sa_sigaction = siginfoHandler;
    sa.sa_flags = SA_SIGINFO;

    for (sig = 1; sig < NSIG; sig++)
        if (sig != SIGTSTP && sig != SIGQUIT)
            sigaction(sig, &sa, NULL);

    /* Optionally block signals and sleep, allowing signals to be
       sent to us before they are unblocked and handled */

    if (argc > 1) {
        sigdelset(&blockMask, SIGINT);
        sigdelset(&blockMask, SIGTERM);

        if (sigprocmask(SIG_SETMASK, &blockMask, &prevMask) == -1)

        printf("%s: signals blocked - sleeping %s seconds\n", argv[0], argv[1]);
        sleep(getInt(argv[1], GN_GT_0, "block-time"));
        printf("%s: sleep complete\n", argv[0]);

        if (sigprocmask(SIG_SETMASK, &prevMask, NULL) == -1)

    while (!allDone)                    /* Wait for incoming signals */

    printf("Caught %d signals\n", sigCnt);


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