PTHREAD_ONCE(3P) POSIX Programmer's Manual PTHREAD_ONCE(3P)PROLOG
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.
NAMEpthread_once - dynamic package initialization
SYNOPSIS
#include <pthread.h>
int pthread_once(pthread_once_t *once_control,
void (*init_routine)(void));
pthread_once_t once_control = PTHREAD_ONCE_INIT;
DESCRIPTION
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. Sub‐
sequent 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 auto‐
matic storage duration or is not initialized by PTHREAD_ONCE_INIT.
RETURN VALUE
Upon successful completion, pthread_once() shall return zero; other‐
wise, an error number shall be returned to indicate the error.
ERRORS
The pthread_once() function may fail if:
EINVAL If either once_control or init_routine is invalid.
The pthread_once() function shall not return an error code of [EINTR].
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
Some C libraries are designed for dynamic initialization. That is, the
global initialization for the library is performed when the first pro‐
cedure 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 sim‐
ple initialization flag is not sufficient; the flag needs to be pro‐
tected against modification by multiple threads simultaneously calling
into the library. Protecting the flag requires the use of a mutex; how‐
ever, 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-guaran‐
teed means of dynamic initialization, it provides an aid to the reli‐
able construction of multi-threaded and realtime systems. The preced‐
ing 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>.
FUTURE DIRECTIONS
None.
SEE ALSO
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 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 .
IEEE/The Open Group 2003 PTHREAD_ONCE(3P)
