pthread_spin_init(3)

✨ NAME ✨

pthread_spin_init, pthread_spin_destroy — bestie, these are the ones that initialize or destroy a spin lock 💅

It's a Library Functions Manual moment, and yes we're in section 3 (iykyk).

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📚 LIBRARY 📚

The POSIX threads library, aka libpthread, linked with -lpthread. She's a classic 🎀

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🌸 SYNOPSIS 🌸

#include <pthread.h>

int pthread_spin_init(pthread_spinlock_t *lock, int pshared);
int pthread_spin_destroy(pthread_spinlock_t *lock);

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

pthread_spin_init(), pthread_spin_destroy():
    _POSIX_C_SOURCE >= 200112L

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💖 DESCRIPTION 💖

General note, and this one is important so listen up: most programs should be using mutexes instead of spin locks. Spin locks are mainly useful when you're pairing them up with real-time scheduling policies. More on that down in NOTES 👑

pthread_spin_init() allocates whatever resources the spin lock needs, and it initializes the lock referred to by lock so it's in the unlocked state. The pshared argument has to be one of these two values, no improvising:

• PTHREAD_PROCESS_PRIVATE — the spin lock is only meant to be operated on by threads in the same process as the thread calling pthread_spin_init(). And like… trying to share it between processes is undefined behavior, so don't 🙈
• PTHREAD_PROCESS_SHARED — now the spin lock may be operated on by any thread in any process that has access to the memory holding the lock (basically, the lock can live in a shared memory object that multiple processes are sharing). Iconic for multi-process situations 🫶

Calling pthread_spin_init() on a spin lock that has already been initialized = undefined behavior. One init per lock, bestie, that's the vibe check 💀

pthread_spin_destroy() destroys a previously initialized spin lock and frees up any resources that were allocated for it. But — and stay with me here — destroying a spin lock that hasn't been initialized before is undefined behavior, and destroying one while another thread is still holding the lock is also undefined behavior. Read the room before you destroy 😭

Once a spin lock has been destroyed, doing anything to it other than initializing it once more with pthread_spin_init() is undefined behavior. Periodt.

And the way that… doing operations like pthread_spin_lock(3), pthread_spin_unlock(3), and pthread_spin_destroy() on copies of the object referred to by lock is undefined behavior too. No copy-paste shenanigans 🚫

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🎀 RETURN VALUE 🎀

On success, these functions return zero. On failure, they return an error number. And real talk: if pthread_spin_init() fails, the lock is not initialized. So don't go using it anyway 💅

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⭐ ERRORS ⭐

pthread_spin_init() may fail with these:

• EAGAIN — the system doesn't have enough resources to initialize a new spin lock 🥲
• ENOMEM — not enough memory to initialize the spin lock 😭

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🌈 STANDARDS 🌈

POSIX.1-2008. Slay, standardized ✨

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💜 HISTORY 💜

glibc 2.2. Also POSIX.1-2001.

Support for process-shared spin locks is a POSIX option, and yes, the glibc implementation supports it 🫶

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👑 NOTES 👑

Okay so this is the part where the man page gets a lil stern with you, and honestly? She's right.

Spin locks should be used together with real-time scheduling policies — that's SCHED_FIFO, or possibly SCHED_RR. Using spin locks with nondeterministic scheduling policies like SCHED_OTHER probably indicates a design mistake. And here's why: if a thread running under such a policy gets scheduled off the CPU while it's holding a spin lock, then all the other threads are gonna waste their time spinning on the lock until that lock holder gets rescheduled and releases it. Just spinning. Burning CPU. For nothing. 💀

And if threads create a deadlock situation while using spin locks, those threads will spin forever, consuming CPU time. Forever, bestie. F-o-r-e-v-e-r 😭

User-space spin locks are not a general locking solution, fr fr. They are, by definition, prone to priority inversion and unbounded spin times. Anyone using spin locks has to be exceptionally careful — not just in the code, but also in system configuration, thread placement, and priority assignment. It's giving main character energy with consequences 🌸

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🦋 SEE ALSO 🦋

pthread_mutex_init(3), pthread_mutex_lock(3), pthread_spin_lock(3), pthread_spin_unlock(3), pthreads(7)

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And that's the whole page, bestie 💖 this one came from the Linux man-pages, version 6.20, dated 2026-02-08. Go forth and initialize responsibly, no cap 🎀✨