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);
    }
}