fcntl如何知道哪个进程保存锁文件?

作为 man 2 fcntl 页面描述,您可以使用 F_GETLK 获取具有冲突锁的进程ID(如果冲突锁是进程关联锁)。例如,

/* Return 0 if descriptor locked exclusively, positive PID if
   a known process holds a conflicting lock, or -1 if the
   descriptor cannot be locked (and errno has the reason).
*/
static pid_t  lock_exclusively(const int fd)
{
    struct flock  lock;
    int           err = 0;

    if (fd == -1) {
        errno = EINVAL;
        return -1;
    }

    lock.l_type = F_WRLCK;
    lock.l_whence = SEEK_SET;
    lock.l_start = 0;
    lock.l_len = 0;
    if (!fcntl(fd, F_SETLK, &lock))
        return 0;

    /* Remember the cause of the failure */
    err = errno;

    lock.l_type = F_WRLCK;
    lock.l_whence = SEEK_SET;
    lock.l_start = 0;
    lock.l_len = 0;
    lock.l_pid = 0;
    if (fcntl(fd, F_GETLK, &lock) == 0 && lock.l_pid > 0)
        return lock.l_pid;

    errno = err;
    return -1;
}

请注意 fd 必须开放阅读和写作。我建议使用 open(path, O_RDWR | O_NOCTTY) 或 open(path, O_WRONLY | O_NOCTTY) . 关闭 任何 同一文件的文件描述符将释放锁。

有人可能会说,重新设置 lock 在第二个之前 fcntl() 打电话是不必要的,但我宁愿在这里犯错。

至于如何报告,我只需使用

int    fd;
pid_t  p;

fd = open(path, O_RDWR | O_NOCTTY);
if (fd == -1) {
    fprintf(stderr, "%s: Cannot open file: %s.\n",
                    path, strerror(errno));
    exit(EXIT_FAILURE);
}

p = lock_exclusively(fd);
if (p < 0) {
    fprintf(stderr, "%s: Cannot lock file: %s.\n",
                    path, strerror(errno));
    exit(EXIT_FAILURE);
} else
if (p > 0) {
    fprintf(stderr, "%s: File is already locked by process %ld.\n",
                    path, (long)p);
    exit(EXIT_FAILURE);
}

/* fd is now open and exclusive-locked. */

用户始终可以运行,例如 ps -o cmd= -p PID 查看这是什么命令(或者您可以尝试阅读 /proc/PID/cmdline 在Linux中)。

从示例代码中:

printf ("locking\n");
/* Initialize the flock structure. */
memset (&lock, 0, sizeof(lock));
lock.l_type = F_WRLCK;
/* Place a write lock on the file. */
fcntl (fd, F_SETLKW, &lock);

printf ("locked; hit Enter to unlock... ");

你需要改变 fcntl (fd, F_SETLKW, &lock); 到:

if (fcntl (fd, F_SETLK, &lock) == -1) {
  printf ("File is locked by pid %i\n", lock.l_pid);
  return 0;
}

如果无法获取锁,f_setlkw命令将阻止。如果不能获得锁,f_setlk将返回。真的,代码也应该检查 errno == EACCESS 或 errno == EAGAIN 获取-1返回值后。