sem_post(3)

✨ sem_post(3) — bestie's guide to unlocking a semaphore ✨

okay so real quick, this is a page from the Library Functions Manual and it's about sem_post(3), the little function that goes ding 🔔 and lets the next gorl into the club. pull up a chair 💖

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🎀 NAME

sem_post — unlock a semaphore

no fr that's it. that's the whole job. it unlocks the semaphore. icon behavior 💅

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📚 LIBRARY

POSIX threads library — that's libpthread, and you link it with -lpthread. periodt.

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🌸 SYNOPSIS

#include <semaphore.h>

int sem_post(sem_t *sem);

look at her. one include, one call, zero drama 🦋

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💖 DESCRIPTION

so sem_post() increments (aka unlocks) the semaphore that sem points to. that's the vibe.

and here's the juicy part bestie: if the semaphore's value becomes greater than zero after that increment, then some other process or thread that's been sitting there blocked in a sem_wait(3) call gets woken up and proceeds to lock the semaphore. she was waiting in the queue, you opened the door, she walks in. main character handoff 🤝✨

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🫶 RETURN VALUE

• on success → 0. slay.
• on error → the value of the semaphore is left unchanged, -1 is returned, and errno is set to tell you what went wrong.

okay lowkey important: on error the semaphore value stays exactly what it was. no half-updated nonsense. she's respectful like that 😌

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💀 ERRORS

• EINVAL — sem is not a valid semaphore. it's giving "who even are you" 🚫
• EOVERFLOW — the value would exceed SEM_VALUE_MAX. too much. overflowing. the semaphore said "no more, I'm full" 😭

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👑 ATTRIBUTES

for the tea on what all these terms mean, go read attributes(7).

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

translation: she's safe to call from multiple threads. no cap.

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🌈 STANDARDS

POSIX.1-2008. she's certified, babes 💅

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🦋 HISTORY

POSIX.1-2001. she's been here, she's been iconic, she's been doing this 🌟

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🔥 NOTES

sem_post() is async-signal-safe: it may be safely called within a signal handler.

this is a big deal actually!! you can call her inside a signal handler and nothing bad happens. she's that gorl 🎀

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🍓 EXAMPLES

See sem_wait(3) and shm_open(3). they got the receipts, go peek 📖

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💜 SEE ALSO

• sem_getvalue(3)
• sem_wait(3)
• sem_overview(7)
• signal-safety(7)

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okay that's the whole page bestie, you're officially semaphore-literate 🎓✨ this was Linux man-pages 6.20, dated 2026-02-08, and she ate, no crumbs left. go unlock something 💅🫶