19. Key Manager — RTEMS POSIX API Guide 5.4-rc2 (21st August 2026) documentation

# 19. Key Manager

## 19.1. Introduction

The key manager allows for the creation and deletion of Data keys specific to threads.

The directives provided by the key manager are:

- pthread_key_create - Create Thread Specific Data Key

- pthread_key_delete - Delete Thread Specific Data Key

- pthread_setspecific - Set Thread Specific Key Value

- pthread_getspecific - Get Thread Specific Key Value

## 19.2. Background

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## 19.3. Operations

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## 19.4. Directives

This section details the key manager’s directives. A subsection is dedicated to each of this manager’s directives and describes the calling sequence, related constants, usage, and status codes.

### 19.4.1. pthread_key_create - Create Thread Specific Data Key

CALLING SEQUENCE:

    #include <pthread.h>
    int pthread_key_create(
        pthread_key_t *key,
        void (*destructor)( void )
    );

STATUS CODES:

`EAGAIN` | There were not enough resources available to create another key.
`ENOMEM` | Insufficient memory exists to create the key.

DESCRIPTION

The `pthread_key_create()` function shall create a thread-specific data key visible to all threads in the process. Key values provided by `pthread_key_create()` are opaque objects used to locate thread-specific data. Although the same key value may be used by different threads, the values bound to the key by `pthread_setspecific()` are maintained on a per-thread basis and persist for the life of the calling thread.

Upon key creation, the value `NULL` shall be associated with the new key in all active threads. Upon thread creation, the value `NULL` shall be associated with all defined keys in the new thread.

NOTES

An optional destructor function may be associated with each key value. At thread exit, if a key value has a non-`NULL` destructor pointer, and the thread has a non-`NULL` value associated with that key, the value of the key is set to NULL, and then the function pointed to is called with the previously associated value as its sole argument. The order of destructor calls is unspecified if more than one destructor exists for a thread when it exits.

### 19.4.2. pthread_key_delete - Delete Thread Specific Data Key

CALLING SEQUENCE:

    #include <pthread.h>
    int pthread_key_delete(
        pthread_key_t key
    );

STATUS CODES:

`EINVAL` | The key was invalid

DESCRIPTION:

The `pthread_key_delete()` function shall delete a thread-specific data key previously returned by `pthread_key_create()`. The thread-specific data values associated with key need not be NULL at the time `pthread_key_delete()` is called. It is the responsibility of the application to free any application storage or perform any cleanup actions for data structures related to the deleted key or associated thread-specific data in any threads; this cleanup can be done either before or after `pthread_key_delete()` is called. Any attempt to use key following the call to `pthread_key_delete()` results in undefined behavior.

NOTES:

The `pthread_key_delete()` function shall be callable from within destructor functions. No destructor functions shall be invoked by `pthread_key_delete()`. Any destructor function that may have been associated with key shall no longer be called upon thread exit.

### 19.4.3. pthread_setspecific - Set Thread Specific Key Value

CALLING SEQUENCE:

    #include <pthread.h>
    int pthread_setspecific(
        pthread_key_t  key,
        const void    *value
    );

STATUS CODES:

`EINVAL` | The specified key is invalid.

DESCRIPTION:

The `pthread_setspecific()` function shall associate a thread-specific value with a key obtained via a previous call to `pthread_key_create()`. Different threads may 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.

NOTES:

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.

`pthread_setspecific()` may be called from a thread-specific data destructor function. Calling `pthread_setspecific()` from a thread-specific data destructor routine may result either in lost storage (after at least `PTHREAD_DESTRUCTOR_ITERATIONS` attempts at destruction) or in an infinite loop.

### 19.4.4. pthread_getspecific - Get Thread Specific Key Value

CALLING SEQUENCE:

    #include <pthread.h>
    void *pthread_getspecific(
        pthread_key_t key
    );

STATUS CODES:

`NULL` | There is no thread-specific data associated with the specified key.
`non-NULL` | The data associated with the specified key.

DESCRIPTION:

The `pthread_getspecific()` function shall return the value currently bound to the specified key on behalf of the calling thread.

NOTES:

The effect of calling `pthread_getspecific()` with a key value not obtained from `pthread_key_create()` or after key has been deleted with `pthread_key_delete()` is undefined.

`pthread_getspecific()` may be called from a thread-specific data destructor function. A call to `pthread_getspecific()` for the thread-specific data key being destroyed shall return the value `NULL`, unless the value is changed (after the destructor starts) by a call to `pthread_setspecific()`.