sem_getvalue(3)

✨ sem_getvalue(3) ✨

okay bestie, welcome to the sem_getvalue page, straight out of the Unix manual for Library Functions 💖 it lives in the POSIX threads library (libpthread, and you link it with -lpthread) 🧵

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🌸 NAME 🌸

sem_getvalue — get the value of a semaphore 👀 like, you're just peeking at what number the semaphore is holding rn, no cap.

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🦋 LIBRARY 🦋

POSIX threads library (libpthread, -lpthread) — that's the girlies you need to link against, periodt.

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🎀 SYNOPSIS 🎀

#include <semaphore.h>

int sem_getvalue(sem_t *restrict sem, int *restrict sval);

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💅 DESCRIPTION 💅

Okay so here's the tea ☕ sem_getvalue() takes the current value of the semaphore that sem points at, and drops it into the integer that sval points at. That's literally it. It's giving "read the vibe of the semaphore." ✨

Now this next part is important, bestie, don't skip it 🫶 if one or more processes or threads are blocked waiting to lock the semaphore with sem_wait(3), POSIX.1 is like "mmm I'll permit two possibilities for the value returned in sval":

• either 0 is returned 😌
• or a negative number whose absolute value is the count of the number of processes and threads currently blocked in sem_wait(3) 😳

And Linux? Linux adopts the former behavior. The first one. The 0 one. Slay, very decisive of it 👑

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🔥 RETURN VALUE 🔥

• sem_getvalue() returns 0 on success 😇
• on error, -1 is returned and errno is set to indicate the error 😭

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💀 ERRORS 💀

• EINVAL — sem is not a valid semaphore. and okay, dramatic pause, because the glibc implementation currently does not check whether sem is valid 🙈 like, it's not doing a vibe check on your semaphore at all. iconically unbothered.

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👑 ATTRIBUTES 👑

for an explanation of the terms used in this section, go see attributes(7), bestie 📖

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

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🌟 STANDARDS 🌟

POSIX.1-2008 ⭐

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🕰️ HISTORY 🕰️

POSIX.1-2001 💜 (a true elder, still thriving)

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🧚 NOTES 🧚

The value of the semaphore may already have changed by the time sem_getvalue() returns 😭 like, by the time you read the number it's already a different number. it's giving "you took a selfie and the moment passed." no notes, just be aware 🫶

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🌈 SEE ALSO 🌈

• sem_post(3)
• sem_wait(3)
• sem_overview(7)

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and that's the whole page, bestie 🎀 this was Linux man-pages 6.20, dated 2026-02-08, and you just read sem_getvalue(3). go forth and read your semaphores responsibly 💖✨🦋