filelock/t_flock.c

/* t_flock.c

   Demonstrate the use of flock() to place file locks.
*/
#include <sys/file.h>
#include <fcntl.h>
#include "curr_time.h"                  /* Declaration of currTime() */
#include "tlpi_hdr.h"

int
main(int argc, char *argv[])
{
    int fd, lock;
    const char *lname;

    if (argc < 3 || strcmp(argv[1], "--help") == 0 ||
            strchr("sx", argv[2][0]) == NULL)
        usageErr("%s file lock [sleep-time]\n"
                 "    'lock' is 's' (shared) or 'x' (exclusive)\n"
                 "        optionally followed by 'n' (nonblocking)\n"
                 "    'sleep-time' specifies time to hold lock\n", argv[0]);

    lock = (argv[2][0] == 's') ? LOCK_SH : LOCK_EX;
    if (argv[2][1] == 'n')
        lock |= LOCK_NB;

    fd = open(argv[1], O_RDONLY);               /* Open file to be locked */
    if (fd == -1)
        errExit("open");

    lname = (lock & LOCK_SH) ? "LOCK_SH" : "LOCK_EX";

    printf("PID %ld: requesting %s at %s\n", (long) getpid(), lname,
            currTime("%T"));

    if (flock(fd, lock) == -1) {
        if (errno == EWOULDBLOCK)
            fatal("PID %ld: already locked - bye!", (long) getpid());
        else
            errExit("flock (PID=%ld)", (long) getpid());
    }

    printf("PID %ld: granted    %s at %s\n", (long) getpid(), lname,
            currTime("%T"));

    sleep((argc > 3) ? getInt(argv[3], GN_NONNEG, "sleep-time") : 10);

    printf("PID %ld: releasing  %s at %s\n", (long) getpid(), lname,
            currTime("%T"));
    if (flock(fd, LOCK_UN) == -1)
        errExit("flock");

    exit(EXIT_SUCCESS);
}