fcntl.lockf()

fcntl.lockf(fd, operation, length=0, start=0, whence=0)

This is essentially a wrapper around the fcntl() locking calls. fd is the file descriptor of the file to lock or unlock, and operation is one of the following values:

LOCK_UN - unlock LOCK_SH - acquire a shared lock LOCK_EX - acquire an exclusive lock

When operation is LOCK_SH or LOCK_EX, it can also be bitwise ORed with LOCK_NB to avoid blocking on lock acquisition. If LOCK_NB is used and the lock cannot be acquired, an IOError will be raised and the exception will have an errno attribute set to EACCES or EAGAIN (depending on the operating system – for portability, check for either value).

length is the number of bytes to lock, with the default meaning to lock to EOF. start is the byte offset, relative to whence, to that the lock starts. whence is as with fileobj.seek(), specifically:

0 - relative to the start of the file (SEEK_SET) 1 - relative to the current buffer position (SEEK_CUR) 2 - relative to the end of the file (SEEK_END)