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PTHREAD_ONCE(3P) POSIX Programmer's Manual PTHREAD_ONCE(3P)
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.
pthread_once — dynamic package initialization
#include <pthread.h>
int pthread_once(pthread_once_t *once_control,
void (*init_routine)(void));
pthread_once_t once_control = PTHREAD_ONCE_INIT;
The first call to pthread_once() by any thread in a process, with a
given once_control, shall call the init_routine with no arguments.
Subsequent calls of pthread_once() with the same once_control shall
not call the init_routine. On return from pthread_once(),
init_routine shall have completed. The once_control parameter shall
determine whether the associated initialization routine has been
called.
The pthread_once() function is not a cancellation point. However, if
init_routine is a cancellation point and is canceled, the effect on
once_control shall be as if pthread_once() was never called.
The constant PTHREAD_ONCE_INIT is defined in the <pthread.h> header.
The behavior of pthread_once() is undefined if once_control has
automatic storage duration or is not initialized by
PTHREAD_ONCE_INIT.
Upon successful completion, pthread_once() shall return zero;
otherwise, an error number shall be returned to indicate the error.
The pthread_once() function shall not return an error code of
[EINTR].
The following sections are informative.
None.
None.
Some C libraries are designed for dynamic initialization. That is,
the global initialization for the library is performed when the first
procedure in the library is called. In a single-threaded program,
this is normally implemented using a static variable whose value is
checked on entry to a routine, as follows:
static int random_is_initialized = 0;
extern int initialize_random();
int random_function()
{
if (random_is_initialized == 0) {
initialize_random();
random_is_initialized = 1;
}
... /* Operations performed after initialization. */
}
To keep the same structure in a multi-threaded program, a new
primitive is needed. Otherwise, library initialization has to be
accomplished by an explicit call to a library-exported initialization
function prior to any use of the library.
For dynamic library initialization in a multi-threaded process, a
simple initialization flag is not sufficient; the flag needs to be
protected against modification by multiple threads simultaneously
calling into the library. Protecting the flag requires the use of a
mutex; however, mutexes have to be initialized before they are used.
Ensuring that the mutex is only initialized once requires a recursive
solution to this problem.
The use of pthread_once() not only supplies an implementation-
guaranteed means of dynamic initialization, it provides an aid to the
reliable construction of multi-threaded and realtime systems. The
preceding example then becomes:
#include <pthread.h>
static pthread_once_t random_is_initialized = PTHREAD_ONCE_INIT;
extern int initialize_random();
int random_function()
{
(void) pthread_once(&random_is_initialized, initialize_random);
... /* Operations performed after initialization. */
}
Note that a pthread_once_t cannot be an array because some compilers
do not accept the construct &<array_name>.
If an implementation detects that the value specified by the
once_control argument to pthread_once() does not refer to a
pthread_once_t object initialized by PTHREAD_ONCE_INIT, it is
recommended that the function should fail and report an [EINVAL]
error.
None.
The Base Definitions volume of POSIX.1‐2008, pthread.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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 .
IEEE/The Open Group 2013 PTHREAD_ONCE(3P)
Pages that refer to this page: pthread.h(0p)
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