The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group

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_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.

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 non-NULL value with the key.

The pthread_setspecific() function 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 function to pthread_getspecific() has been designed to favor speed and simplicity over error reporting.

If an implementation detects that the value specified by the key argument to pthread_setspecific() does not refer to a a key value obtained from pthread_key_create() or refers to a key that has been deleted with pthread_key_delete(), it is recommended that the function should fail and report an [EINVAL] error.

FUTURE DIRECTIONS

None.

SEE ALSO

pthread_key_create

XBD <pthread.h>

CHANGE HISTORY

First released in Issue 5. Included for alignment with the POSIX Threads Extension.

Issue 6

The pthread_getspecific() and pthread_setspecific() functions are marked as part of the Threads option.

IEEE PASC Interpretation 1003.1c #3 (Part 6) is applied, updating the DESCRIPTION.

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/96 is applied, updating the ERRORS section so that the [ENOMEM] error case is changed from ``to associate the value with the key'' to ``to associate the non-NULL value with the key''.

Issue 7

Austin Group Interpretation 1003.1-2001 #063 is applied, updating the ERRORS section.

The pthread_getspecific() and pthread_setspecific() functions are moved from the Threads option to the Base.

The [EINVAL] error for a key value not obtained from pthread_key_create() or a key deleted with pthread_key_delete() is removed; this condition results in undefined behavior.

End of informative text.

 

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