---
name: "pthread_once"
section: "3"
platform: "macos"
package: "libpthread"
version: "539.100.4"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/pthread_once"
---

# pthread\_once(3)

dynamic package initialization

## NAME

**pthread\_once** – dynamic package initialization

## SYNOPSIS

**#include \<pthread.h\>**

pthread\_once\_t *once\_control* = PTHREAD\_ONCE\_INIT;

*int*\

**pthread\_once**(*pthread\_once\_t \*once\_control*, *void (\*init\_routine)(void)*);

## DESCRIPTION

The first call to **pthread\_once**() by any thread in a process, with a given *once\_control*, will call the **init\_routine**() with no arguments. Subsequent calls to **pthread\_once**() with the same *once\_control* will not call the **init\_routine**(). On return from **pthread\_once**(), it is guaranteed that **init\_routine**() has completed. The *once\_control* parameter is used to determine whether the associated initialization routine has been called.

The function **pthread\_once**() is not a cancellation point. However, if **init\_routine**() is a cancellation point and is cancelled, the effect on *once\_control* is as if **pthread\_once**() was never called.

The constant *PTHREAD\_ONCE\_INIT* is defined by header ⟨pthread.h⟩.

The behavior of **pthread\_once**() is undefined if *once\_control* has automatic storage duration or is not initialized by *PTHREAD\_ONCE\_INIT*.

## RETURN VALUES

If successful, the **pthread\_once**() function will return zero. Otherwise an error number will be returned to indicate the error.

## ERRORS

None.

## STANDARDS

The **pthread\_once**() function conforms to ISO/IEC 9945-1:1996 (“POSIX.1”).