File System Locking

Locking files in Linux is tricky - not because it is complex, but complicated due to the many variants.

First of all you have different kinds:

mandatory locks are enforced by the Linux kernel and prevent other processes from opening the file while another process has an exclusive lock. This can easily deadlock the system. Linux supports this on file systems explicitly mounted with option mand.
advisory locks require support from all applications: each one has to implement the same mechanism to guarantee proper cooperation.

For advisory locks you have multiple types in Linux:

BSD file lock (<man:flock(2)>)
POSIX record lock (<man:fnctl(2)>; the simplified version <man:lockf(3)> only supporting exclusive locks)
Open file descriptor (OFD) lock (<man:fcntl(2)>)

Basically they are incompatible with each other and your applications should agree to use only one.

Variant	Origin	NFS	range	associated	`fork()`	auto released
FLOCK	BSD	no¹	file	open FD	inherited	last FD closed
POSIX	POSIX	yes	bytes	pid,inode	dropped	any FDs closed
OFDLCK	Linux	yes	bytes	open FD	inherited	last FD closed

BSD file locks

only complete files can be locked, not parts
NFS support may not work, only since Linux 2.6.11 (¹)
mode change is not atomic
inherited on fork()
released when last FD is close()d
Linux type FLOCK in /proc/locks
API: flock(fd, LOCK_{SH,EX,UN}) <man:flock(2)>
for shell scripts there is <man:flock(1)>

POSIX record lock

very strange behavior: lock is associated with (pid,inode) tuple
closing any duplicated file descriptor will release all locks
fork()ed child process does not inherit locks
does not work well with threads
Linux type POSIX in /proc/locks
API: fcntl(fd, F_{GETLK,SETLK,SETLKW}, struct flock *) <man:fcntl(2)>
API: lockf(fd, F_{LOCK,TLOCK,ULOCK,TEST}, len) <man:lockf(3)>

Open File Descriptor locks (file-private POSIX locks)

Linux specific, only since Linux 3.15+
released when last FD is close()d
thread may open files themselves and get their own locks
Linux type OFDLCK in /proc/locks
API: fcntl(fd, F_OFD_{GETLK,SETLK,SETLKW}, struct flock *) <man:fcntl(2)>

Python

In Python fnctl.lockf() is a wrapper using fcntl.ioctl() directly. It is implemented in CPython:fcntlmodule.c. As such despite the name it does not use the implementation from Glibc:lockf.c.

Linux