PTHREAD_KEY_CREATE(3) Library Functions Manual PTHREAD_KEY_CREATE(3)
pthread_key_create -- thread-specific data
POSIX Threads Library (libpthread, -lpthread)
pthread_key_create(pthread_key_t *key, void (*destructor)(void *));
The pthread_key_create() function creates a thread-specific data key
visible to all threads in the process. Key values are opaque objects
used to locate thread-specific data. The same key value may be used by
different threads, but 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 is associated with the new key in all
active threads. Upon thread creation, the value NULL is 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 the key, the function pointed
to is called with the current 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, the implementation stops calling destructors.
The pthread_key_delete() function deletes a thread-specific data key
previously returned by pthread_key_create(). The thread-specific data
values associated with key need not be NULL at the time of the call. It
is the responsibility of the application to free any application storage
or perform any cleanup actions for data structures related to the deleted
key or associated thread-specific data in any threads; this cleanup can
be done either before or after pthread_key_delete() is called. Any
attempt to use key following the call to pthread_key_delete() results in
The pthread_key_delete() function itself is callable from within
destructor functions, but destructor functions are not invoked by the
function. Any destructor function that may have been associated with key
will no longer be called upon thread exit.
If successful, the pthread_key_create() function will store the newly
created key value at the location specified by key and returns zero.
Also pthread_key_delete() will return zero upon success. Upon failure
both functions return an error number to indicate the cause.
The pthread_key_create() may 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 would be exceeded.
[ENOMEM] Insufficient memory exists to create the key.
The pthread_key_delete() function may fail if:
[EINVAL] The key value is invalid.
These functions conform to IEEE Std 1003.1-2001 (``POSIX.1'').
The current specifications are flawed and do not permit a clean
implementation without potential problems. The current implementation in
NetBSD addresses these problems by not supporting key reuse.
NetBSD 6.1.5 July 9, 2010 NetBSD 6.1.5