pipes/pipe_sync.c

+/* pipe_sync.c
+
+   Show how pipes can be used for synchronizing the actions of a parent and
+   multiple child processes.
+
+   Usage: pipe_sync sleep-time...
+
+   After creating a pipe, the program creates one child for each command-line
+   argument. Each child simulates doing some work by sleeping for the number of
+   seconds specified in the corresponding command-line argument. When it has
+   finished doing its "work", each child closes its file descriptor for the
+   write end of the pipe; the parent can see that all children have finished
+   their work when it sees end-of-file on the read end of the pipe.
+*/
 #include "curr_time.h"                      /* Declaration of currTime() */
 #include "tlpi_hdr.h"
 
 int
 main(int argc, char *argv[])
 {
     int pfd[2];                             /* Process synchronization pipe */
     int j, dummy;
 
     if (argc < 2 || strcmp(argv[1], "--help") == 0)
         usageErr("%s sleep-time...\n", argv[0]);
 
     setbuf(stdout, NULL);                   /* Make stdout unbuffered, since we
                                                terminate child with _exit() */
     printf("%s  Parent started\n", currTime("%T"));
 
     if (pipe(pfd) == -1)
         errExit("pipe");
 
     for (j = 1; j < argc; j++) {
         switch (fork()) {
         case -1:
             errExit("fork %d", j);
 
         case 0: /* Child */
             if (close(pfd[0]) == -1)        /* Read end is unused */
                 errExit("close");
 
             /* Child does some work, and lets parent know it's done */
 
             sleep(getInt(argv[j], GN_NONNEG, "sleep-time"));
                                             /* Simulate processing */
             printf("%s  Child %d (PID=%ld) closing pipe\n",
                     currTime("%T"), j, (long) getpid());
             if (close(pfd[1]) == -1)
                 errExit("close");
 
             /* Child now carries on to do other things... */
 
             _exit(EXIT_SUCCESS);
 
         default: /* Parent loops to create next child */
             break;
         }
     }
 
     /* Parent comes here; close write end of pipe so we can see EOF */
 
     if (close(pfd[1]) == -1)                /* Write end is unused */
         errExit("close");
 
     /* Parent may do other work, then synchronizes with children */
 
     if (read(pfd[0], &dummy, 1) != 0)
         fatal("parent didn't get EOF");
     printf("%s  Parent ready to go\n", currTime("%T"));
 
     /* Parent can now carry on to do other things... */
 
     exit(EXIT_SUCCESS);
 }