excl.c

/*
 * This file is a demonstration of the fact that, in POSIX, exclusive access
 * to a file is dropped once all file descriptors that point to its "open file
 * description" are closed.
 *
 * To compile:
 *     CC excl.c -o excl
 *
 * To run:
 *     ./excl <SERIAL DEVICE> both    # both fds are closed; no error thrown
 *     ./excl <SERIAL DEVICE> one     # only one fd is closed, EBUSY (errno=16) thrown
 *     ./excl <SERIAL DEVICE> neither # neither fd is closed,  EBUSY (errno=16) thrown
 */

#include <stdio.h>
#include <string.h>
#include <termios.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h>

int main(int argc, char **argv) {
    /* ===================== Setup ======================== */
    char *filename;
    int close_original=0;
    int close_dup=0;

    if (argc > 1) {
        filename = argv[1];
    } else {
        filename = "/dev/ttyS0";
    }

    if (argc > 2) {
        if (strcmp(argv[2], "both") == 0) {
            close_original=1;
            close_dup=1;
        } else if (strcmp(argv[2], "one") == 0) {
            close_original=1;
            close_dup=0;
        } else {
            close_original=0;
            close_dup=0;
        }
    }

    /* ======= Open file, create duplicate, get exclusive access =========== */

    int fd = open(filename, 0);
    if (fd < 0) {
        fprintf(stderr, "Failed to create initial file, errno=%d\n", errno);
        exit(1);
    }

    if(ioctl(fd, TIOCEXCL) == -1) {
        fprintf(stderr, "Failed to make file exclusive, errno=%d\n", errno);
        exit(1);
    }

    int dup_fd = dup(fd);
    if (dup_fd < 0) {
        fprintf(stderr, "Failed to dup exclusive file, errno=%d\n", errno);
    }

    /* ============= Optionally close one or both fds (for experiment) ================== */

    if (close_original) {
        if(close(fd)) {
            fprintf(stderr, "Close failed (original), errno=%d\n", errno);
            exit(1);
        }
    }

    if (close_dup) {
        if(close(dup_fd)) {
            fprintf(stderr, "Close failed (dup), errno=%d\n", errno);
            exit(1);
        }
    }

    /* === Check whether opening the same file name fails (i.e. if it is still exclusive) === */

    int fd2 = open(filename, 0);
    if (fd2 < 0) {
        fprintf(stderr, "Failed to open file second time (as expected?), errno=%d\n", errno);
    }

    /* === Finally, verify that once we drop exclusive access that the file can be opened again === */
    if (!close_original) {   
        if(ioctl(fd, TIOCNXCL) == -1) {
            fprintf(stderr, "Failed to make file un-exclusive (original), errno=%d\n", errno);
            exit(1);
        }
    }

    if (!close_dup) {   
        if(ioctl(dup_fd, TIOCNXCL) == -1) {
            fprintf(stderr, "Failed to make file un-exclusive (duplicate), errno=%d\n", errno);
            exit(1);
        }
    }

    int fd3 = open(filename, 0);
    if (fd3 < 0) {
        fprintf(stderr, "Failed to open file third time, errno=%d\n", errno);
    }
}