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_getspecific() 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.
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.
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 function to
pthread_getspecific() has been designed to favor speed and simplicity
over error reporting.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- 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