Check whether there are outstanding POSIX fcntl or flock locks on files. Written after a stray lock broke my mbox, to see if there were any others hiding around. Does not check for dotlocks (just use ls).

confirm-lock.c

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/file.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

/* Compile with any c99 compiler:
 *
 *   cc -o confirm-lock confirm-lock.c
 *
 * Tested with:
 *
 *   gcc --std=c99 --pedantic -Wall -Wextra -o confirm-lock confirm-lock.c
 *
 */

static int failed_to_lock = 0;
static int other_failure = 0;

static
void
confirm_lock(const char* path)
{
    int fd, err;

    fd = open(path, O_RDWR | O_APPEND);
    if (fd < 0) {
        fprintf(stderr, "open %s: %s\n", path, strerror(errno));
        other_failure = 1;
        return;
    }

    struct flock lock = {
        .l_type = F_WRLCK,
        .l_whence = SEEK_SET, 
        .l_start = 0, /* beginning of file */
        .l_len = 0, /* special meaning: full file */
    };
    err = fcntl(fd, F_SETLK, &lock);
    if (err < 0) {
        fprintf(stderr, "fcntl F_SETLK: %s: %s\n", path, strerror(errno));
        if (errno == EAGAIN || errno == EACCES) {
            failed_to_lock = 1;
        } else {
            other_failure = 1;
        }
    } else {
        lock.l_type = F_UNLCK;
        fcntl(fd, F_SETLK, &lock);
    }

    err = flock(fd, LOCK_EX | LOCK_NB);
    if (err < 0) {
        fprintf(stderr, "flock %s: %s\n", path, strerror(errno));
        if (errno == EWOULDBLOCK) {
            failed_to_lock = 1;
        } else {
            other_failure = 1;
        }
    } else {
        flock(fd, LOCK_UN);
    }

    close(fd);
}

/**
 * Checks whether one or more files have outstanding kernel locks.
 * One example use might be to confirm that nothing has any of
 * a user's mailboxes locked: `cd ~/mail && confirm-lock *`
 *
 * Arguments: one or more paths to existing files
 *
 * Exit codes:
 *   0: no errors and no outstanding locks
 *   1: usage error
 *   2: at least one outstanding lock found (see stderr for details)
 *   3: other error (see stderr for details)
 */
int
main(int argc, char **argv)
{
    int i;

    if (argc < 2) {
        fprintf(stderr, "Usage: %s file ...\n", argv[0]);
        return 1;
    }

    for (i=1; i<argc; i++) {
        confirm_lock(argv[i]);
    }

    if (other_failure) {
        return 3;
    } else if (failed_to_lock) {
        return 2;
    } else {
        return 0;
    }
}