.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH pthread_key_create 3 2025-05-17 "Linux man-pages (unreleased)"
.
.
.SH NAME
pthread_key_create,
pthread_key_delete,
pthread_setspecific,
pthread_getspecific
\-
management of thread-specific data
.
.
.SH SYNOPSIS
.nf
.B #include <pthread.h>
.P
.BI "int pthread_key_create(pthread_key_t *" key ,
.BI "                       typeof(void (void *)) *" destr_function ;
.BI "int pthread_key_delete(pthread_key_t " key );
.BI "int pthread_setspecific(pthread_key_t " key ", const void *" pointer );
.BI "void * pthread_getspecific(pthread_key_t " key );
.fi
.
.
.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.
.P
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
.I 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.
.P
For concreteness,
the TSD areas can be viewed as arrays of
.I 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.
.P
When a thread is created,
its TSD area initially associates NULL with all keys.
.P
.BR pthread_key_create ()
allocates a new TSD key.
The key is stored in the location pointed to by
.IR key .
There is a limit of
.B 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.
.P
The
.I destr_function
argument,
if not NULL,
specifies a destructor function associated with the key.
When a thread terminates via
.BR pthread_exit ()
or by cancelation,
.I destr_function
is called with arguments
the value associated with the key in that thread.
The
.I destr_function
is not called if that value is NULL.
The order in which destructor functions are called at thread termination time
is unspecified.
.P
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
.B PTHREAD_DESTRUCTOR_ITERATIONS
iterations,
even if some non-NULL values with associated descriptors remain.
Other implementations may loop indefinitely.
.P
.BR 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.
.P
.BR pthread_setspecific ()
changes the value
associated with
.I key
in the calling thread,
storing the given
.I pointer
instead.
.P
.BR pthread_getspecific ()
returns the value
currently associated with
.I key
in the calling thread.
.
.
.SH "RETURN VALUE"
.BR pthread_key_create (),
.BR pthread_key_delete (),
and
.BR pthread_setspecific ()
return 0 on success and a non-zero error code on failure.
If successful,
.BR pthread_key_create ()
stores the newly allocated key
in the location pointed to by its
.I key
argument.
.P
.BR pthread_getspecific ()
returns
the value associated with
.I key
on success,
and NULL on error.
.
.
.SH ERRORS
.BR pthread_key_create ()
returns the following error code on error:
.RS
.TP
.B EAGAIN
.B PTHREAD_KEYS_MAX
keys are already allocated.
.RE
.P
.BR pthread_key_delete ()
and
.BR pthread_setspecific ()
return
the following error code on error:
.RS
.TP
.B EINVAL
.I key
is not a valid, allocated TSD key.
.RE
.P
.BR pthread_getspecific ()
returns NULL if
.I key
is not a valid,
allocated TSD key.
.
.
.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 reclamation at thread exit:
.P
.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
.RE
.fi