flock(2)

🦋 flock(2) — apply or remove an advisory lock on an open file 🦋

hi bestie 💖 welcome to the System Calls Manual, the part of the docs where we talk directly to the kernel and the kernel talks back. this one's about flock, and it's genuinely one of the cutest little syscalls you'll ever meet — it's giving file-locking bestie, and yes, you're gonna need it the moment two processes start fighting over the same file like it's the last croissant 🥐

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✨ NAME ✨

flock — apply or remove an advisory lock on an open file 🔒

🎀 LIBRARY 🎀

Standard C library (libc, -lc)

🌸 SYNOPSIS 🌸

#include <sys/file.h>

int flock(int fd, int op);

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💅 DESCRIPTION 💅

so the whole vibe is: you apply or remove an advisory lock on the open file specified by fd. and op? op is literally one of these three icons 👑

• LOCK_SH — Place a shared lock. More than one process may hold a shared lock for a given file at a given time. sharing is caring 🫶
• LOCK_EX — Place an exclusive lock. Only one process may hold an exclusive lock for a given file at a given time. main character energy only, nobody else invited 💅
• LOCK_UN — Remove an existing lock held by this process. bye bestie 👋

now, a call to flock() may block if an incompatible lock is held by another process. want it nonblocking? include LOCK_NB (by ORing it) with any of the above operations. it's that easy, no cap ✨

a single file may not simultaneously have both shared and exclusive locks. pick a lane, babe 🚫

locks created by flock() are associated with an open file description (see open(2)). which means duplicate file descriptors — created by, for example, fork(2) or dup(2) — refer to the same lock, and that lock may be modified or released using any of these file descriptors. furthermore, the lock is released either by an explicit LOCK_UN operation on any of these duplicate file descriptors, or when all such file descriptors have been closed. so it's a whole group chat of file descriptors and they all share one lock 🫂

if a process uses open(2) (or similar) to obtain more than one file descriptor for the same file, these file descriptors are treated independently by flock(). an attempt to lock the file using one of these file descriptors may be denied by a lock that the calling process has already placed via another file descriptor. like, your own self can block your own self. embarrassing but iconic 😭

a process may hold only one type of lock (shared or exclusive) on a file. subsequent flock() calls on an already locked file will convert an existing lock to the new lock mode. glow up, lock edition 💫

locks created by flock() are preserved across an execve(2). they survive the transformation, fr fr 🦋

a shared or exclusive lock can be placed on a file regardless of the mode in which the file was opened. read-only, write-only, whatever — the lock does not care ✨

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🌈 RETURN VALUE 🌈

on success, zero is returned. on error, -1 is returned, and errno is set to indicate the error. simple as 🎀

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💀 ERRORS 💀

• EBADF — fd is not an open file descriptor. bestie that's not a file 😭
• EINTR — While waiting to acquire a lock, the call was interrupted by delivery of a signal caught by a handler; see signal(7). rude interruption honestly 🙈
• EINVAL — op is invalid. that's not one of the three, babes 💅
• ENOLCK — The kernel ran out of memory for allocating lock records. the kernel is full, no room at the inn 💀
• EWOULDBLOCK — The file is locked and the LOCK_NB flag was selected. you asked for nonblocking, you got nonblocking 🚫

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⭐ VERSIONS ⭐

since Linux 2.0, flock() is implemented as a system call in its own right rather than being emulated in the GNU C library as a call to fcntl(2). with this implementation, there is no interaction between the types of lock placed by flock() and fcntl(2), and flock() does not detect deadlock. (note, however, that on some systems, such as the modern BSDs, flock() and fcntl(2) locks do interact with one another.) so it's giving "we don't talk to each other, we're not on speaking terms" 😤

🌐 CIFS details 🌐

up to Linux 5.4, flock() is not propagated over SMB. a file with such locks will not appear locked for remote clients. the remote besties have no idea, they're living in the dark 🫠

since Linux 5.5, flock() locks are emulated with SMB byte-range locks on the entire file. similarly to NFS, this means that fcntl(2) and flock() locks interact with one another. another important side-effect is that the locks are not advisory anymore: any IO on a locked file will always fail with EACCES when done from a separate file descriptor. this difference originates from the design of locks in the SMB protocol, which provides mandatory locking semantics. so yeah, mandatory = the lock actually enforces itself. no ignoring it, periodt 💀

remote and mandatory locking semantics may vary with SMB protocol, mount options and server type. see mount.cifs(8) for additional information. 💜

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🌸 STANDARDS 🌸

BSD. slay, we're keeping the old traditions alive 🎀

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🧚 HISTORY 🧚

4.4BSD (the flock() call first appeared in 4.2BSD). a version of flock(), possibly implemented in terms of fcntl(2), appears on most UNIX systems. she's been around forever and she's still booked and busy 👑

☁️ NFS details ☁️

up to Linux 2.6.11, flock() does not lock files over NFS (i.e., the scope of locks was limited to the local system). instead, one could use fcntl(2) byte-range locking, which does work over NFS, given a sufficiently recent version of Linux and a server which supports locking. workaround era 😭

since Linux 2.6.12, NFS clients support flock() locks by emulating them as fcntl(2) byte-range locks on the entire file. this means that fcntl(2) and flock() locks do interact with one another over NFS. it also means that in order to place an exclusive lock, the file must be opened for writing. noted, bestie 📝

since Linux 2.6.37, the kernel supports a compatibility mode that allows flock() locks (and also fcntl(2) byte region locks) to be treated as local; see the discussion of the local_lock option in nfs(5).

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🙈 NOTES 🙈

flock() places advisory locks only; given suitable permissions on a file, a process is free to ignore the use of flock() and perform I/O on the file. advisory means "please respect this," not "you physically cannot." the lock is a vibe, not a wall 🫶

flock() and fcntl(2) locks have different semantics with respect to forked processes and dup(2). on systems that implement flock() using fcntl(2), the semantics of flock() will be different from those described in this manual page. so read carefully, it depends on the platform 💅

converting a lock (shared to exclusive, or vice versa) is not guaranteed to be atomic: the existing lock is first removed, and then a new lock is established. between these two steps, a pending lock request by another process may be granted, with the result that the conversion either blocks, or fails if LOCK_NB was specified. (this is the original BSD behavior, and occurs on many other implementations.) there's a little gap in the middle and somebody can sneak in 😭

release of a lock when a file descriptor is closed is not sequenced after all observable effects of close(2). for example, if one process writes a file while holding an exclusive lock, then closes that file, and another process blocks placing a shared lock on that file to wait until it is closed, it may observe that a subsequent execve(2) of that file fails with ETXTBSY, and umount(2) of its underlying filesystem fails with EBUSY, as if the file were still open in the first process. spooky ghost file behavior, genuinely 💀

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💖 SEE ALSO 💖

• flock(1)
• close(2)
• dup(2)
• execve(2)
• fcntl(2)
• fork(2)
• open(2)
• lockf(3)
• lslocks(8)
• Documentation/filesystems/locks.rst in the Linux kernel source tree

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and that's the whole page, bestie 🎀 this was Linux man-pages 6.20, dated 2026-02-08 — go lock something responsibly, wrap your file descriptors in a little hug, and remember: advisory means they should respect it, not that they have to 💖✨🦋 periodt.