---
name: "pthread_setspecific"
section: "3"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/pthread_setspecific"
---

# pthread\_setspecific(3)

set a thread-specific data value

## NAME

**pthread\_setspecific** – set a thread-specific data value

## LIBRARY

POSIX Threads Library (libpthread, -lpthread)

## SYNOPSIS

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

*int*\

**pthread\_setspecific**(*pthread\_key\_t key*, *const void \*value*);

## DESCRIPTION

The **pthread\_setspecific**() function associates a thread-specific value with a *key* obtained via a previous call to **pthread\_key\_create**(). Different threads can bind different values to the same key. These values are typically pointers to blocks of dynamically allocated memory that have been reserved for use by the calling thread.

The effect of calling **pthread\_setspecific**() with a key value not obtained from **pthread\_key\_create**() or after *key* has been deleted with **pthread\_key\_delete**() is undefined.

The **pthread\_setspecific**() function may be called from a thread-specific data destructor function, however this may result in lost storage or infinite loops if doing so causes non-NULL key values to remain after \[PTHREAD\_DESTRUCTOR\_ITERATIONS\] iterations of destructor calls have been made.

## RETURN VALUES

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

## ERRORS

The **pthread\_setspecific**() function will fail if:

- \[ENOMEM\]

  Insufficient memory exists to associate the value with the *key*.

- \[EINVAL\]

  The *key* value is invalid.

## SEE ALSO

[pthread\_getspecific(3)](/pthread_getspecific/3.md), [pthread\_key\_create(3)](/pthread_key_create/3.md), [pthread\_key\_delete(3)](/pthread_key_delete/3.md)

## STANDARDS

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