flock applies or removes an advisory lock on the open file fd. Valid operation types are:
Shared lock. More than one process may hold a shared lock for a given file at a given time.
Exclusive lock. Only one process may hold an exclusive lock for a given file at a given time.
Don't block when locking. May be specified (by oring) along with one of the other operations.
A single file may not simultaneously have both shared and exclusive locks.
On success, 0 is returned. On error, -1 is returned and the global variable errno is set appropriately.