filelock/t_flock.c

This is filelock/t_flock.c (Listing 55-1, page 1121), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* 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);
}

 

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