sem_init(3)

✨ sem_init(3) ✨ — from the Library Functions Manual, bestie 💅

okay so picture this: you've got a semaphore but it's ✨unnamed✨ and it hasn't been born yet. this page is literally the birth announcement. let's get into it 🫶

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🎀 NAME 🎀

sem_init — initialize an unnamed semaphore

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📚 LIBRARY 📚

POSIX threads library — link it with libpthread, -lpthread 🌸

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👑 SYNOPSIS 👑

#include <semaphore.h>

int sem_init(sem_t *sem, int pshared, unsigned int value);

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💖 DESCRIPTION 💖

sem_init() initializes the unnamed semaphore sitting at the address that sem points to. and the value argument? that's the starting value for the semaphore, no cap. 🔥

the pshared argument is the vibe check that tells you whether this semaphore is gonna be shared between the threads of one process, or between processes. main character decisions here ✨

if pshared has the value 0, then the semaphore is shared between the threads of a process, and it should live at some address that's visible to all those threads — think a global variable, or a variable allocated dynamically on the heap. 🧚

if pshared is nonzero, then the semaphore is shared between processes, and it should be located in a region of shared memory — see shm_open(3), mmap(2), and shmget(2). 🦋 (and since a child created by fork(2) inherits its parent's memory mappings, it can also access the semaphore, periodt.) any process that can access the shared memory region can operate on the semaphore using sem_post(3), sem_wait(3), and so on. 💜

⚠️ big scary one: initializing a semaphore that has already been initialized results in undefined behavior. not a vibe. do not do it. 😭

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🌈 RETURN VALUE 🌈

sem_init() returns 0 on success. on error, -1 is returned, and errno is set to indicate the error. iconic behavior honestly 👑

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💀 ERRORS 💀

• EINVAL — value exceeds SEM_VALUE_MAX. she said too much 🍓
• ENOSYS — pshared is nonzero, but the system does not support process-shared semaphores (peep sem_overview(7)). the system simply cannot serve this look 🙈

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⭐ ATTRIBUTES ⭐

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

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

so yeah — sem_init() is MT-Safe, she's thread safe, we love that for her 💅

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🌸 STANDARDS 🌸

POSIX.1-2008. 🎀

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🦋 HISTORY 🦋

POSIX.1-2001.

and here's the tea ☕ — bizarrely, POSIX.1-2001 does not specify the value that should be returned by a successful call to sem_init(). POSIX.1-2008 rectifies this, specifying the zero return on success. glow-up era ✨

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🔥 EXAMPLES 🔥

see shm_open(3) and sem_wait(3).

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💜 SEE ALSO 💜

sem_destroy(3), sem_post(3), sem_wait(3), sem_overview(7)

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okay that's the whole page, you ate that 💅 this was brought to you by Linux man-pages 6.20, dated 2026-02-08, and yes that date is serving future energy. bye bestie, go init responsibly 🫶✨