procexec/fork_file_sharing.c

+/* fork_file_sharing.c
+
+   Show that the file descriptors of a forked child refer to the
+   same open file objects as the parent.
+*/
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <sys/wait.h>
 #include "tlpi_hdr.h"
 
 int
 main(int argc, char *argv[])
 {
     int fd, flags;
     char template[] = "/tmp/testXXXXXX";
 
     setbuf(stdout, NULL);                   /* Disable buffering of stdout */
 
+    /* Open a temporary file, set its file offset to some arbitrary value,
+       and change the setting of one of the open file status flags. */
+
     fd = mkstemp(template);
     if (fd == -1)
         errExit("mkstemp");
 
     printf("File offset before fork(): %lld\n",
             (long long) lseek(fd, 0, SEEK_CUR));
 
     flags = fcntl(fd, F_GETFL);
     if (flags == -1)
         errExit("fcntl - F_GETFL");
     printf("O_APPEND flag before fork() is: %s\n",
             (flags & O_APPEND) ? "on" : "off");
 
     switch (fork()) {
     case -1:
         errExit("fork");
 
     case 0:     /* Child: change file offset and status flags */
         if (lseek(fd, 1000, SEEK_SET) == -1)
             errExit("lseek");
 
         flags = fcntl(fd, F_GETFL);         /* Fetch current flags */
         if (flags == -1)
             errExit("fcntl - F_GETFL");
         flags |= O_APPEND;                  /* Turn O_APPEND on */
         if (fcntl(fd, F_SETFL, flags) == -1)
             errExit("fcntl - F_SETFL");
         _exit(EXIT_SUCCESS);
 
     default:    /* Parent: can see file changes made by child */
         if (wait(NULL) == -1)
             errExit("wait");                /* Wait for child exit */
         printf("Child has exited\n");
 
         printf("File offset in parent: %lld\n",
                 (long long) lseek(fd, 0, SEEK_CUR));
 
         flags = fcntl(fd, F_GETFL);
         if (flags == -1)
             errExit("fcntl - F_GETFL");
         printf("O_APPEND flag in parent is: %s\n",
                 (flags & O_APPEND) ? "on" : "off");
         exit(EXIT_SUCCESS);
     }
 }