linux man page for pthread_getspecific

PTHREAD_GETSPECIFIC(P)					     POSIX Programmer's Manual					    PTHREAD_GETSPECIFIC(P)

NAME
       pthread_getspecific, pthread_setspecific - thread-specific data management

SYNOPSIS
       #include <pthread.h>

       void *pthread_getspecific(pthread_key_t key);
       int pthread_setspecific(pthread_key_t key, const void *value);

DESCRIPTION
       The pthread_getspecific() function shall return the value currently bound to the specified key on behalf of the calling thread.

       The pthread_setspecific() function shall associate a thread-specific value with a key obtained via a previous call to pthread_key_create().
       Different threads may bind different values to the same key. These values are typically pointers to blocks of dynamically allocated  memory
       that have been reserved for use by the calling thread.

       The  effect  of calling pthread_getspecific() or pthread_setspecific() with a key value not obtained from pthread_key_create() or after key
       has been deleted with pthread_key_delete() is undefined.

       Both pthread_getspecific() and pthread_setspecific() may be called from a thread-specific data destructor function. A call to  pthread_get-
       specific()  for the thread-specific data key being destroyed shall return the value NULL, unless the value is changed (after the destructor
       starts) by a call to pthread_setspecific(). Calling pthread_setspecific() from a thread-specific data destructor routine may result  either
       in lost storage (after at least PTHREAD_DESTRUCTOR_ITERATIONS attempts at destruction) or in an infinite loop.

       Both functions may be implemented as macros.

RETURN VALUE
       The  pthread_getspecific()  function  shall return the thread-specific data value associated with the given key. If no thread-specific data
       value is associated with key, then the value NULL shall be returned.

       If successful, the pthread_setspecific() function shall return zero; otherwise, an error number shall be returned to indicate the error.

ERRORS
       No errors are returned from pthread_getspecific().

       The pthread_setspecific() function shall fail if:

       ENOMEM Insufficient memory exists to associate the value with the key.

       The pthread_setspecific() function may fail if:

       EINVAL The key value is invalid.

       These functions shall not return an error code of [EINTR].

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       None.

RATIONALE
       Performance and ease-of-use of pthread_getspecific() are critical for functions that rely on maintaining  state	in  thread-specific  data.
       Since  no errors are required to be detected by it, and since the only error that could be detected is the use of an invalid key, the func-
       tion to pthread_getspecific() has been designed to favor speed and simplicity over error reporting.

FUTURE DIRECTIONS
       None.

SEE ALSO
       pthread_key_create() , the Base Definitions volume of IEEE Std 1003.1-2001, <pthread.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol-
       ogy -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 .

IEEE
/The Open Group						       2003						    PTHREAD_GETSPECIFIC(P)