procexec/fork_sig_sync.c

+/* 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)
             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);
     }
 }