procexec/t_clone.c

+/* t_clone.c
+
+   Demonstrate the use of the Linux-specific clone() system call.
+*/
 #define _GNU_SOURCE
 #include <signal.h>
 #include <sys/wait.h>
 #include <fcntl.h>
 #include <sched.h>
 #include "tlpi_hdr.h"
 
 #ifndef CHILD_SIG
 #define CHILD_SIG SIGUSR1       /* Signal to be generated on termination
                                    of cloned child */
 #endif
 
 static int                      /* Startup function for cloned child */
 childFunc(void *arg)
 {
     if (close(*((int *) arg)) == -1)
         errExit("close");
 
     return 0;                           /* Child terminates now */
 }
 
 int
 main(int argc, char *argv[])
 {
     const int STACK_SIZE = 65536;       /* Stack size for cloned child */
     char *stack;                        /* Start of stack buffer */
     char *stackTop;                     /* End of stack buffer */
     int s, fd, flags;
 
     fd = open("/dev/null", O_RDWR);     /* Child will close this fd */
     if (fd == -1)
         errExit("open");
 
     /* If argc > 1, child shares file descriptor table with parent */
 
     flags = (argc > 1) ? CLONE_FILES : 0;
 
     /* Allocate stack for child */
 
     stack = malloc(STACK_SIZE);
     if (stack == NULL)
         errExit("malloc");
     stackTop = stack + STACK_SIZE;      /* Assume stack grows downward */
 
     /* Ignore CHILD_SIG, in case it is a signal whose default is to
        terminate the process; but don't ignore SIGCHLD (which is ignored
        by default), since that would prevent the creation of a zombie. */
 
     if (CHILD_SIG != 0 && CHILD_SIG != SIGCHLD)
         if (signal(CHILD_SIG, SIG_IGN) == SIG_ERR)          errExit("signal");
 
     /* Create child; child commences execution in childFunc() */
 
     if (clone(childFunc, stackTop, flags | CHILD_SIG, (void *) &fd) == -1)
         errExit("clone");
 
     /* Parent falls through to here. Wait for child; __WCLONE is
        needed for child notifying with signal other than SIGCHLD. */
 
     if (waitpid(-1, NULL, (CHILD_SIG != SIGCHLD) ? __WCLONE : 0) == -1)
         errExit("waitpid");
     printf("child has terminated\n");
 
     /* Did close() of file descriptor in child affect parent? */
 
     s = write(fd, "x", 1);
     if (s == -1 && errno == EBADF)
         printf("file descriptor %d has been closed\n", fd);
     else if (s == -1)
         printf("write() on file descriptor %d failed "
                 "unexpectedly (%s)\n", fd, strerror(errno));
     else
         printf("write() on file descriptor %d succeeded\n", fd);
 
     exit(EXIT_SUCCESS);
 }