pthread_setconcurrency(3)

💖 pthread_setconcurrency(3) — bestie edition 💖

okay so this one lives in the Library Functions Manual, and honestly? it's giving "legacy function that's just vibing here for compatibility." let's get into it 🫶

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✨ NAME ✨

pthread_setconcurrency, pthread_getconcurrency — set/get the concurrency level

📚 LIBRARY 📚

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

🌸 SYNOPSIS 🌸

#include <pthread.h>

int pthread_setconcurrency(int new_level);
int pthread_getconcurrency(void);

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💅 DESCRIPTION 💅

okay so here's the tea. pthread_setconcurrency() is literally just you telling the implementation what concurrency level you'd like, and you pass that wish in new_level.

but like… the implementation takes it as a hint only. that's not me being shady, that's just the facts: POSIX.1 straight up does not specify what level of concurrency you'll actually get back after you call pthread_setconcurrency(). so you ask, and the system does its own thing. manifestation, but make it CPU scheduling ✨

if you pass new_level as 0, you're basically saying "girl, you handle it" — it tells the implementation to manage the concurrency level however it deems appropriate.

pthread_getconcurrency() returns the current value of the concurrency level for this process. simple as that 💁‍♀️

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🎀 RETURN VALUE 🎀

• pthread_setconcurrency() — on success it returns 0; on error it returns a nonzero error number.
• pthread_getconcurrency() — always succeeds. it returns the concurrency level that was set by a previous call to pthread_setconcurrency(), or 0 if pthread_setconcurrency() has never been called before. she has no bad days, no cap 👑

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💀 ERRORS 💀

pthread_setconcurrency() can fail with exactly this:

• EINVAL — new_level is negative. girl. negative concurrency is not a vibe, it's an error 😭

also, POSIX.1 documents an EAGAIN error too — that one means "the value specified by new_level would cause a system resource to be exceeded." (so like, asking for too much. relatable.)

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👑 ATTRIBUTES 👑

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

• pthread_setconcurrency(), pthread_getconcurrency() — Thread safety: MT-Safe 🦋

yes both of them. same attribute. they're matching, it's iconic.

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🌈 STANDARDS 🌈

POSIX.1-2008.

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🕰️ HISTORY 🕰️

• glibc 2.1.
• POSIX.1-2001.

so yeah, these functions have been around. vintage but make it POSIX 💜

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📝 NOTES 📝

okay this is the part where the plot twists, so lean in:

• the default concurrency level is 0.
• concurrency levels are meaningful only for M:N threading implementations. that's the kind where, at any moment, a subset of a process's set of user-level threads may be bound to a smaller number of kernel-scheduling entities. setting the concurrency level lets the application give the system a hint about how many kernel-scheduling entities should be provided for efficient execution of the application. it's a little "hey bestie, maybe this many?" moment 🧚
• but — and this is the big one — both LinuxThreads and NPTL are 1:1 threading implementations, so setting the concurrency level has no meaning. in other words, on Linux these functions merely exist for compatibility with other systems, and they have no effect on the execution of a program.

fr fr: on Linux, calling these does absolutely nothing to how your program runs. it's a decoration. a little ornamental function. aesthetic only 💅

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🔍 SEE ALSO 🔍

• pthread_attr_setscope(3)
• pthreads(7)

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and that's the whole page, babes!! 🌸 this one came to you from Linux man-pages 6.20, dated 2026-02-08 — go forth, set your concurrency to literally anything, and remember: on Linux it changes absolutely nothing. slay anyway 🎀✨💖