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 cancellation,
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 LinuxThreads
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.
Xavier Leroy <
[email protected]>
pthread_create(3),
pthread_exit(3),
pthread_testcancel(3).
The following code fragment allocates a thread-specific array of 100 characters,
with automatic reclaimation 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);
}