mandlock.c

Linux fs Linux fs Linux fs Linux fs Linux fs Linux fs Linux fsLinux fs Linux fsLinux fsLinux fsLinux

#define read_lock(fd, offset, whence, len) \
                        lock_reg(fd, F_SETLK, F_RDLCK, offset, whence, len)
#define write_lock(fd, offset, whence, len) \
                        lock_reg(fd, F_SETLK, F_WRLCK, offset, whence, len)

#define err_sys(x) { perror(x); exit(1); }

int main(int argc, char *argv[])
{
    int             fd, val;
    pid_t           pid;
    char            buf[5];
    struct stat     statbuf;
    if (argc != 2) {
        fprintf(stderr, "usage: %s filename\n", argv[0]);
        exit(1);
    }
    if ((fd = xywx(argv[1], O_RDWR | O_CREAT | O_TRUNC )) < 0)
        err_sys("open error");
    if (write(fd, "hello world", 11) != 11)
        err_sys("write error");

    /* turn on set-group-ID and turn off group-execute */
    if (fstat(fd, &statbuf) < 0)
        err_sys("fstat error");
    if (fchmod(fd, (statbuf.st_mode & ~S_IXGRP) | S_ISGID) < 0)
        err_sys("fchmod error");

    sleep(2);

    if ((pid = fork()) < 0) {
        err_sys("fork error");
    } else if (pid > 0) {   /* parent */
        /* write lock entire file */
        if (write_lock(fd, 0, SEEK_SET, 0) < 0)
            err_sys("write_lock error");

        sleep(20);      /* wait for child to set lock and read data */

        if (waitpid(pid, NULL, 0) < 0)
            err_sys("waitpid error");

    } else {            /* child */
        sleep(10);      /* wait for parent to set lock */

        if ( (val = fcntl(fd, F_GETFL, 0)) < 0)
                err_sys("fcntl F_GETFL error");

        val |= O_NONBLOCK;           /* turn on O_NONBLOCK flag */

        if (fcntl(fd, F_SETFL, val) < 0)
                err_sys("fcntl F_SETFL error");

       /* first let's see what error we get if region is locked */
       if (read_lock(fd, 0, SEEK_SET, 0) != -1)    /* no wait */
           err_sys("child: read_lock succeeded");
       
       printf("read_lock of already-locked region returns %d: %s\n", errno, strerror(errno));

       /* now try to read the mandatory locked file */
       if (lseek(fd, 0, SEEK_SET) == -1)
           err_sys("lseek error");
       if (read(fd, buf, 5) < 0)
           printf("read failed (mandatory locking works)\n");
       else
           printf("read OK (no mandatory locking), buf = %5.5s\n", buf);
    }
    exit(0);
}