pthread_key_create(3) - Linux manual page (original) (raw)
pthreadkeycreate(3) Library Functions Manual pthreadkeycreate(3)
NAME top
pthread_key_create, pthread_key_delete, pthread_setspecific,
pthread_getspecific - management of thread-specific dataSYNOPSIS top
**#include <pthread.h>**
**int pthread_key_create(pthread_key_t ***_key_**,**
**typeof(void (void *)) ***_destrfunction_**;**
**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 top
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 _destrfunction_ argument, if not **NULL**, specifies a destructor
function associated with the key. When a thread terminates via
**pthread_exit** or by cancelation, _destrfunction_ is called with
arguments the value associated with the key in that thread. The
_destrfunction_ 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 top
**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 top
**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 top
[pthread_create(3)](../man3/pthread%5Fcreate.3.html), [pthread_exit(3)](../man3/pthread%5Fexit.3.html), [pthread_testcancel(3)](../man3/pthread%5Ftestcancel.3.html).EXAMPLE top
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);**
**}**COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2025-01-11 pthreadkeycreate(3)
Pages that refer to this page:pthread_cancel(3), pthreads(7)