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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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PTHREAD_KEY_CREATE(3P) POSIX Programmer's Manual PTHREAD_KEY_CREATE(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
pthread_key_create — thread-specific data key creation
#include <pthread.h>
int pthread_key_create(pthread_key_t *key, void (*destructor)(void*));
The pthread_key_create() function shall create a thread-specific
data key visible to all threads in the process. Key values
provided by pthread_key_create() are opaque objects used to locate
thread-specific data. Although the same key value may be used by
different threads, the values bound to the key by
pthread_setspecific() are maintained on a per-thread basis and
persist for the life of the calling thread.
Upon key creation, the value NULL shall be associated with the new
key in all active threads. Upon thread creation, the value NULL
shall be associated with all defined keys in the new thread.
An optional destructor function may be associated with each key
value. At thread exit, if a key value has a non-NULL destructor
pointer, and the thread has a non-NULL value associated with that
key, the value of the key is set to NULL, and then the function
pointed to is called with the previously associated value as its
sole argument. The order of destructor calls is unspecified if
more than one destructor exists for a thread when it exits.
If, after all the destructors have been called for all non-NULL
values with associated destructors, there are still some non-NULL
values with associated destructors, then the process is repeated.
If, after at least {PTHREAD_DESTRUCTOR_ITERATIONS} iterations of
destructor calls for outstanding non-NULL values, there are still
some non-NULL values with associated destructors, implementations
may stop calling destructors, or they may continue calling
destructors until no non-NULL values with associated destructors
exist, even though this might result in an infinite loop.
If successful, the pthread_key_create() function shall store the
newly created key value at *key and shall return zero. Otherwise,
an error number shall be returned to indicate the error.
The pthread_key_create() function shall fail if:
EAGAIN The system lacked the necessary resources to create another
thread-specific data key, or the system-imposed limit on
the total number of keys per process {PTHREAD_KEYS_MAX} has
been exceeded.
ENOMEM Insufficient memory exists to create the key.
The pthread_key_create() function shall not return an error code
of [EINTR].
The following sections are informative.
The following example demonstrates a function that initializes a
thread-specific data key when it is first called, and associates a
thread-specific object with each calling thread, initializing this
object when necessary.
static pthread_key_t key;
static pthread_once_t key_once = PTHREAD_ONCE_INIT;
static void
make_key()
{
(void) pthread_key_create(&key, NULL);
}
func()
{
void *ptr;
(void) pthread_once(&key_once, make_key);
if ((ptr = pthread_getspecific(key)) == NULL) {
ptr = malloc(OBJECT_SIZE);
...
(void) pthread_setspecific(key, ptr);
}
...
}
Note that the key has to be initialized before
pthread_getspecific() or pthread_setspecific() can be used. The
pthread_key_create() call could either be explicitly made in a
module initialization routine, or it can be done implicitly by the
first call to a module as in this example. Any attempt to use the
key before it is initialized is a programming error, making the
code below incorrect.
static pthread_key_t key;
func()
{
void *ptr;
/* KEY NOT INITIALIZED!!! THIS WILL NOT WORK!!! */
if ((ptr = pthread_getspecific(key)) == NULL &&
pthread_setspecific(key, NULL) != 0) {
pthread_key_create(&key, NULL);
...
}
}
None.
Destructor Functions
Normally, the value bound to a key on behalf of a particular
thread is a pointer to storage allocated dynamically on behalf of
the calling thread. The destructor functions specified with
pthread_key_create() are intended to be used to free this storage
when the thread exits. Thread cancellation cleanup handlers
cannot be used for this purpose because thread-specific data may
persist outside the lexical scope in which the cancellation
cleanup handlers operate.
If the value associated with a key needs to be updated during the
lifetime of the thread, it may be necessary to release the storage
associated with the old value before the new value is bound.
Although the pthread_setspecific() function could do this
automatically, this feature is not needed often enough to justify
the added complexity. Instead, the programmer is responsible for
freeing the stale storage:
pthread_getspecific(key, &old);
new = allocate();
destructor(old);
pthread_setspecific(key, new);
Note: The above example could leak storage if run with
asynchronous cancellation enabled. No such problems occur
in the default cancellation state if no cancellation points
occur between the get and set.
There is no notion of a destructor-safe function. If an
application does not call pthread_exit() from a signal handler, or
if it blocks any signal whose handler may call pthread_exit()
while calling async-unsafe functions, all functions may be safely
called from destructors.
Non-Idempotent Data Key Creation
There were requests to make pthread_key_create() idempotent with
respect to a given key address parameter. This would allow
applications to call pthread_key_create() multiple times for a
given key address and be guaranteed that only one key would be
created. Doing so would require the key value to be previously
initialized (possibly at compile time) to a known null value and
would require that implicit mutual-exclusion be performed based on
the address and contents of the key parameter in order to
guarantee that exactly one key would be created.
Unfortunately, the implicit mutual-exclusion would not be limited
to only pthread_key_create(). On many implementations, implicit
mutual-exclusion would also have to be performed by
pthread_getspecific() and pthread_setspecific() in order to guard
against using incompletely stored or not-yet-visible key values.
This could significantly increase the cost of important
operations, particularly pthread_getspecific().
Thus, this proposal was rejected. The pthread_key_create()
function performs no implicit synchronization. It is the
responsibility of the programmer to ensure that it is called
exactly once per key before use of the key. Several
straightforward mechanisms can already be used to accomplish this,
including calling explicit module initialization functions, using
mutexes, and using pthread_once(). This places no significant
burden on the programmer, introduces no possibly confusing ad hoc
implicit synchronization mechanism, and potentially allows
commonly used thread-specific data operations to be more
efficient.
None.
pthread_getspecific(3p), pthread_key_delete(3p)
The Base Definitions volume of POSIX.1‐2017, pthread.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard,
the original IEEE and The Open Group Standard is the referee
document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 PTHREAD_KEY_CREATE(3P)
Pages that refer to this page: pthread.h(0p), pthread_getspecific(3p), pthread_key_delete(3p)
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HTML rendering created 2025-09-06 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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