lockf(3)

💅 lockf(3) — like, the POSIX file-locking girlie

okay bestie, welcome to lockf(3) from the Library Functions Manual ✨ this one lives in the Standard C library (libc, -lc) and she is ALL about putting locks on bits of an open file. apply them, test them, remove them — the whole vibe. 🎀

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✨ SYNOPSIS ✨

#include <unistd.h>

int lockf(int fd, int op, off_t size);

🦋 Feature Test Macro Requirements for glibc (peep feature_test_macros(7))

lockf():
    _XOPEN_SOURCE >= 500
        || /* glibc >= 2.19: */ _DEFAULT_SOURCE
        || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

no cap, you need one of those macros defined or glibc will simply not hand her over 🙈

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🌸 DESCRIPTION 🌸

So here's the tea: lockf() applies, tests, or removes a POSIX lock on a section of an open file. 🫶

• the file is the one behind fd — a file descriptor that's open for writing
• the action is whatever you put in op
• and the section? it's all about byte positions, and it depends on the sign of size:
  • if size is positive, the section is pos..pos+size-1
  • if size is negative, the section is pos-size..pos-1
  • where pos is the current file position
  • and if size is zero, the section stretches from the current file position all the way to infinity, absorbing both the present and future end-of-file positions 🌈

and in every single one of those cases, the section is allowed to extend past the current end-of-file. slay, boundaries are fake.

On Linux, lockf() is literally just an interface sitting on top of fcntl(2) locking. Many other systems do lockf() this exact same way, but — heads up — POSIX.1 leaves the relationship between lockf() and fcntl(2) locks unspecified. So if you want a portable app, you should probably not be mixing calls to these two interfaces. pick a lane, bestie 😭

The valid operations are right here, and there are four of them:

• F_LOCK — set an exclusive lock on the specified section of the file. if (part of) that section is already locked, the call blocks until the previous lock gets released. and if this section overlaps an earlier locked section, the two get merged into one 🫂. also: file locks are released the moment the process holding them closes some file descriptor for the file. and a child process does not inherit these locks. main character energy only, no hand-me-downs 👑
• F_TLOCK — same as F_LOCK, except this one never blocks, and instead returns an error if the file is already locked. non-committal queen 💅
• F_ULOCK — unlock the indicated section of the file. this can cause a locked section to get split into two locked sections, which is honestly kind of iconic 🎀
• F_TEST — test the lock: return 0 if the specified section is unlocked or locked by this process; return -1 and set errno to EAGAIN (EACCES on some other systems) if another process holds a lock. she's just asking questions, not starting drama 🔥

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💖 RETURN VALUE 💖

On success, zero is returned. ✨

On error, -1 is returned, and errno is set to tell you exactly what went wrong. 😭

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💀 ERRORS 💀

• EACCES or EAGAIN — the file is locked and F_TLOCK or F_TEST was specified, or the operation is prohibited because the file has been memory-mapped by another process. ngl, rude.
• EBADF — fd is not an open file descriptor; or op is F_LOCK or F_TLOCK and fd is not a writable file descriptor. you gotta be writable for those two, periodt.
• EDEADLK — op was F_LOCK and this lock operation would cause a deadlock. we do not do deadlocks in this house 🙈
• EINTR — while waiting to acquire a lock, the call was interrupted by delivery of a signal caught by a handler; see signal(7).
• EINVAL — an invalid operation was specified in op.
• ENOLCK — too many segment locks open, lock table is full. the lock table is, like, full full 📚

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⭐ ATTRIBUTES ⭐

For an explanation of the terms used in this section, peep attributes(7). here's the deets:

• Interface: lockf()
• Attribute: Thread safety
• Value: MT-Safe

so yeah, she's MT-Safe 💜

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🌟 STANDARDS 🌟

POSIX.1-2008. certified, official, canon ✨

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🧚 HISTORY 🧚

POSIX.1-2001, SVr4. she's been around, she's seen things 💅

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🔥 SEE ALSO 🔥

• fcntl(2)
• flock(2)

also, for the extra-credit girlies: locks.txt and mandatory-locking.txt live in the Linux kernel source directory Documentation/filesystems — and btw, on older kernels those files sit directly under the Documentation directory, and mandatory-locking.txt is called mandatory.txt there. iykyk 🌸

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and that's the whole page bestie, mwah 💖 this was Linux man-pages 6.20, dated 2026-02-08 — go forth and lock responsibly, no cap 🫶✨