When would a blocking call to flock fail?

Question

I was reading about flock. The example has this line:
flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
This call is a blocking call right? So if the lock is already taken the call blocks.
I assume that if the call returns the lock is granted. So when would the die would occur?

Answer 1

flock is a wrapper for the system call of the same name. $! is set by the system. So consult the system's documentation for your answer.

On my system, as per man 2 flock,

• EBADF: fd is not an open file descriptor.

• EINTR: While waiting to acquire a lock, the call was interrupted by delivery of a signal caught by a handler; see signal(7).

• EINVAL: operation is invalid.

• ENOLCK: The kernel ran out of memory for allocating lock records.

• EWOULDBLOCK: The file is locked and the LOCK_NB flag was selected.