How does parent process valid the sent signal by child process

Question

Based on this http://man7.org/tlpi/code/online/dist/procexec/fork_sig_sync.c.html

/* fork_sig_sync.c

   Demonstrate how signals can be used to synchronize the actions
   of a parent and child process.
*/
#include <signal.h>
#include "curr_time.h"                  /* Declaration of currTime() */
#include "tlpi_hdr.h"

#define SYNC_SIG SIGUSR1                /* Synchronization signal */

static void             /* Signal handler - does nothing but return */
handler(int sig)
{
}

int
main(int argc, char *argv[])
{
    pid_t childPid;
    sigset_t blockMask, origMask, emptyMask;
    struct sigaction sa;

    setbuf(stdout, NULL);               /* Disable buffering of stdout */

    sigemptyset(&blockMask);
    sigaddset(&blockMask, SYNC_SIG);    /* Block signal */
    if (sigprocmask(SIG_BLOCK, &blockMask, &origMask) == -1)
        errExit("sigprocmask");

    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESTART;
    sa.sa_handler = handler;
    if (sigaction(SYNC_SIG, &sa, NULL) == -1)
        errExit("sigaction");

    switch (childPid = fork()) {
    case -1:
        errExit("fork");

    case 0: /* Child */

        /* Child does some required action here... */

        printf("[%s %ld] Child started - doing some work\n",
                currTime("%T"), (long) getpid());
        sleep(2);               /* Simulate time spent doing some work */

        /* And then signals parent that it's done */

        printf("[%s %ld] Child about to signal parent\n",
                currTime("%T"), (long) getpid());
        if (kill(getppid(), SYNC_SIG) == -1)
            errExit("kill");

        /* Now child can do other things... */

        _exit(EXIT_SUCCESS);

    default: /* Parent */

        /* Parent may do some work here, and then waits for child to
           complete the required action */

        printf("[%s %ld] Parent about to wait for signal\n",
                currTime("%T"), (long) getpid());
        sigemptyset(&emptyMask);
        if (sigsuspend(&emptyMask) == -1 && errno != EINTR)  // <<<<< Question
            errExit("sigsuspend");
        printf("[%s %ld] Parent got signal\n", currTime("%T"), (long) getpid());

        /* If required, return signal mask to its original state */

        if (sigprocmask(SIG_SETMASK, &origMask, NULL) == -1)
            errExit("sigprocmask");

        /* Parent carries on to do other things... */

        exit(EXIT_SUCCESS);
    }
}

Question

When the parent process calls sigsuspend , why it doesn't validate that the sent signal is SYNC_SIG?

http://pubs.opengroup.org/onlinepubs/7908799/xsh/sigsuspend.html

Answer 1

The code assumes no other signal will occur while the program runs, so it doesn't bother to check that it received the right one. Which means that if any other signal occurs the program may fail.

add

There are a number of ways to check that the right signal is sent. You can have the handler set some sig_atomic_t global var and check that its set after sigsuspend returns (looping to repeat the sigsuspend call if it is not). Or you can set the mask passed to sigsuspend to block everything EXCEPT SYNC_SIG . Neither of these will work if some other process sends a spurious SIGUSR1 to your process, but that will only happen if someone is deliberately trying to sabotage your program.