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PROLOG
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.
NAME
pthread_key_create — thread-specific data key creation
SYNOPSIS
#include <pthread.h>
int pthread_key_create(pthread_key_t *key, void (*destructor)(void*));
DESCRIPTION
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.
RETURN VALUE
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.
ERRORS
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.
EXAMPLES
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);
...
}
}
APPLICATION USAGE
None.
RATIONALE
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.
FUTURE DIRECTIONS
None.
SEE ALSO
pthread_getspecific(), pthread_key_delete()
The Base Definitions volume of POSIX.1‐2017, <pthread.h>
COPYRIGHT
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 .