PTHREAD_KEY_CREATE(3)

PTHREAD_KEY_CREATE(3) | Library Functions Manual | PTHREAD_KEY_CREATE(3)

# NAME

`pthread_key_create`, `pthread_key_delete` —
thread-specific data

# LIBRARY

POSIX Threads Library (libpthread, -lpthread)

# SYNOPSIS

`#include <pthread.h>`

int
`pthread_key_create`(pthread_key_t *key, void (*destructor)(void *));

int
`pthread_key_delete`(pthread_key_t key);

# DESCRIPTION

The `pthread_key_create`() function creates a thread-specific data key visible to all threads in the process. Key values are opaque objects used to locate thread-specific data. The same key value may be used by different threads, but 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` is associated with the new key in all active threads. Upon thread creation, the value `NULL` is associated with all defined keys in the new thread.

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 the key, the function pointed to is called with the current 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.

If, after all the destructors have been called for all non-`NULL` values with associated destructors, there are still some non-`NULL` values with associated destructors, then the process is repeated. If, after at least `PTHREAD_DESTRUCTOR_ITERATIONS` iterations of destructor calls for outstanding non-`NULL` values, there are still some non-`NULL` values with associated destructors, the implementation stops calling destructors.

The `pthread_key_delete`() function deletes 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 of the call. 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.

The `pthread_key_delete`() function itself is callable from within destructor functions, but destructor functions are not invoked by the function. Any destructor function that may have been associated with key will no longer be called upon thread exit.

# RETURN VALUES

If successful, the `pthread_key_create`() function will store the newly created key value at the location specified by key and returns zero. Also `pthread_key_delete`() will return zero upon success. Upon failure both functions return an error number to indicate the cause.

# ENVIRONMENT

`PTHREAD_KEYS_MAX`
  Maximum per-process thread-specific data keys. This cannot be set below `_POSIX_THREAD_KEYS_MAX`.

# ERRORS

The `pthread_key_create`() may fail if:

[`EAGAIN`]
  The system lacked the necessary resources to create another thread-specific data key, or the system-imposed limit on the total number of keys per-process `PTHREAD_KEYS_MAX` would be exceeded.
[`ENOMEM`]
  Insufficient memory exists to create the key.

The `pthread_key_delete`() function may fail if:

[`EINVAL`]
  The key value is invalid.

# SEE ALSO

pthread_getspecific(3)

# STANDARDS

These functions conform to IEEE Std 1003.1-2001 (“POSIX.1”).

# BUGS

The current specifications are flawed and do not permit a clean implementation without potential problems. The current implementation in NetBSD addresses these problems by not supporting key reuse.

May 29, 2015 | NetBSD 9.4