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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | SEE ALSO | EXAMPLE | COLOPHON |
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pthread_key_create(3) Library Functions Manual pthread_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,
void (*destr_function) (void *));
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);
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.
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.
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.
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 the man-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-6.9.1.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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man-pages@man7.org
Linux man-pages 6.9.1 2024-06-16 pthread_key_create(3)
Pages that refer to this page: pthread_cancel(3), pthreads(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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