procexec/fork_whos_on_first.c

+/* fork_whos_on_first.c
+
+   Parent repeatedly creates a child, and then processes both race to be the
+   first to print a message. (Each child terminates after printing its message.)
+   The results of running this program give us an idea of which of the two
+   processes--parent or child--is usually scheduled first after a fork().
+
+   Whether the child or the parent is scheduled first after fork() has
+   changed a number of times across different kernel versions.
+*/
 #include <sys/wait.h>
 #include "tlpi_hdr.h"
 
 int
 main(int argc, char *argv[])
 {
     int numChildren, j;
     pid_t childPid;
 
     if (argc > 1 && strcmp(argv[1], "--help") == 0)
         usageErr("%s [num-children]\n", argv[0]);
 
     numChildren = (argc > 1) ? getInt(argv[1], GN_GT_0, "num-children") : 1;
 
     setbuf(stdout, NULL);               /* Make stdout unbuffered */
 
     for (j = 0; j < numChildren; j++) {
         switch (childPid = fork()) {
         case -1:
             errExit("fork");
 
         case 0:
             printf("%d child\n", j);
             _exit(EXIT_SUCCESS);
 
         default:
             printf("%d parent\n", j);
             wait(NULL);                 /* Wait for child to terminate */
             break;
         }
     }
 
     exit(EXIT_SUCCESS);
 }