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NAME |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |SEE ALSO |EXAMPLE |COLOPHON | |
pthread_key_create(3) Library Functions Manualpthread_key_create(3)pthread_key_create, pthread_key_delete, pthread_setspecific, pthread_getspecific - management of thread-specific data
#include <pthread.h>int pthread_key_create(pthread_key_t *key,typeof(void (void *)) *destr_function;int pthread_key_delete(pthread_key_tkey);int pthread_setspecific(pthread_key_tkey, const void *pointer);void * pthread_getspecific(pthread_key_tkey);
Programs often need global or static variables that have different values in different threads. Since threads share one memory space, this cannot be achieved with regular variables. Thread- specific data is the POSIX threads answer to this need. Each thread possesses a private memory block, the thread-specific data area, or TSD area for short. This area is indexed by TSD keys. The TSD area associates values of typevoid * to TSD keys. TSD keys are common to all threads, but the value associated with a given TSD key can be different in each thread. For concreteness, the TSD areas can be viewed as arrays ofvoid * pointers, TSD keys as integer indices into these arrays, and the value of a TSD key as the value of the corresponding array element in the calling thread. When a thread is created, its TSD area initially associates NULL with all keys.pthread_key_create() allocates a new TSD key. The key is stored in the location pointed to bykey. There is a limit ofPTHREAD_KEYS_MAXon the number of keys allocated at a given time. The value initially associated with the returned key is NULL in all currently executing threads. Thedestr_function argument, if not NULL, specifies a destructor function associated with the key. When a thread terminates viapthread_exit() or by cancelation,destr_function is called with arguments the value associated with the key in that thread. Thedestr_function is not called if that value is NULL. The order in which destructor functions are called at thread termination time is unspecified. Before the destructor function is called, the NULL value is associated with the key in the current thread. A destructor function might, however, re-associate non-NULL values to that key or some other key. To deal with this, if after all the destructors have been called for all non-NULL values, there are still some non-NULL values with associated destructors, then the process is repeated. The glibc implementation stops the process afterPTHREAD_DESTRUCTOR_ITERATIONSiterations, even if some non- NULL values with associated descriptors remain. Other implementations may loop indefinitely.pthread_key_delete() deallocates a TSD key. It does not check whether non-NULL values are associated with that key in the currently executing threads, nor call the destructor function associated with the key.pthread_setspecific() changes the value associated withkey in the calling thread, storing the givenpointer instead.pthread_getspecific() returns the value currently associated withkey in the calling thread.
pthread_key_create(),pthread_key_delete(), andpthread_setspecific() return 0 on success and a non-zero error code on failure. If successful,pthread_key_create() stores the newly allocated key in the location pointed to by itskey argument.pthread_getspecific() returns the value associated withkey on success, and NULL on error.
pthread_key_create() returns the following error code on error:EAGAIN PTHREAD_KEYS_MAXkeys are already allocated.pthread_key_delete() andpthread_setspecific() return the following error code on error:EINVALkey is not a valid, allocated TSD key.pthread_getspecific() returns NULL ifkey is not a valid, allocated TSD key.
pthread_create(3),pthread_exit(3),pthread_testcancel(3).
The following code fragment allocates a thread-specific array of 100 characters, with automatic reclamation at thread exit:/* Key for the thread-specific buffer */static pthread_key_t buffer_key;/* Once-only initialisation of the key */static pthread_once_t buffer_key_once = PTHREAD_ONCE_INIT;/* Allocate the thread-specific buffer */void buffer_alloc(void){pthread_once(&buffer_key_once, buffer_key_alloc);pthread_setspecific(buffer_key, malloc(100));}/* Return the thread-specific buffer */char * get_buffer(void){return (char *) pthread_getspecific(buffer_key);}/* Allocate the key */static void buffer_key_alloc(){pthread_key_create(&buffer_key, buffer_destroy);}/* Free the thread-specific buffer */static void buffer_destroy(void * buf){free(buf);} This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17pthread_key_create(3)Pages that refer to this page:pthread_cancel(3), pthreads(7)
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