glibc/linuxthreads/man/pthread_key_create.man

.TH PTHREAD_SPECIFIC 3 LinuxThreads

.SH NAME
pthread_key_create, pthread_key_delete, pthread_setspecific, pthread_getspecific \- management of thread-specific data

.SH SYNOPSIS
#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);

.SH 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 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.

.SH "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.

.SH ERRORS
!pthread_key_create! returns the following error code on error:
.RS
.TP
!EAGAIN!
!PTHREAD_KEYS_MAX! keys are already allocated
.RE

!pthread_key_delete! and !pthread_setspecific! return the following
error code on error:
.RS
.TP
!EINVAL!
|key| is not a valid, allocated TSD key
.RE

!pthread_getspecific! returns !NULL! if |key| is not a valid,
allocated TSD key.

.SH AUTHOR
Xavier Leroy <Xavier.Leroy@inria.fr>

.SH "SEE ALSO"
pthread_create(3), pthread_exit(3), pthread_testcancel(3).

.SH EXAMPLE

The following code fragment allocates a thread-specific array of 100
characters, with automatic reclaimation at thread exit:

.RS
.ft 3
.nf
.sp
/* 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);
}
.ft
.LP
.RE
.fi