NAME

       pthread_key_create,  pthread_key_delete, pthread_setspecific, pthread_getspecific - management of thread-
       specific data

SYNOPSIS

       #include <pthread.h>

       int pthread_key_create(pthread_key_t *key,
                              typeof(void (void *)) *destr_function;
       int pthread_key_delete(pthread_key_t key);
       int pthread_setspecific(pthread_key_t key, const void *pointer);
       void * pthread_getspecific(pthread_key_t key);

DESCRIPTION

       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 type void * 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 of void * 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 by key.
       There is a limit of PTHREAD_KEYS_MAX on the number  of  keys  allocated  at  a  given  time.   The  value
       initially associated with the returned key is NULL in all currently executing threads.

       The  destr_function argument, if not NULL, specifies a destructor function associated with the key.  When
       a thread terminates via pthread_exit() or by cancelation, destr_function is  called  with  arguments  the
       value  associated  with  the key in that thread.  The destr_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 after PTHREAD_DESTRUCTOR_ITERATIONS iterations, 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 with key in the  calling  thread,  storing  the  given
       pointer instead.

       pthread_getspecific() returns the value currently associated with key in the calling thread.

RETURN VALUE

       pthread_key_create(),  pthread_key_delete(), and pthread_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 its key argument.

       pthread_getspecific() returns the value associated with key on success, and NULL on error.

ERRORS

       pthread_key_create() returns the following error code on error:

              EAGAIN PTHREAD_KEYS_MAX keys are already allocated.

       pthread_key_delete() and pthread_setspecific() return the following error code on error:

              EINVAL key is not a valid, allocated TSD key.

       pthread_getspecific() returns NULL if key is not a valid, allocated TSD key.

SEE ALSO

       pthread_create(3), pthread_exit(3), pthread_testcancel(3).

EXAMPLE

       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);
              }

Linux man-pages 6.15                               2025-05-17                              pthread_key_create(3)