PTHREAD_ONCE
Section: POSIX Programmer's Manual (3P)
Updated: 2017
Page Index
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.
NAME
pthread_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. 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.
If the call to
init_routine
is terminated by a call to
longjmp(),
_longjmp(),
or
siglongjmp(),
the behavior is undefined.
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.
RETURN VALUE
Upon successful completion,
pthread_once()
shall return zero; otherwise, an error number shall be returned to
indicate the error.
ERRORS
The
pthread_once()
function shall not return an error code of
[EINTR].
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
If
init_routine
recursively calls
pthread_once()
with the same
once_control,
the recursive call will not call the specified
init_routine,
and thus the specified
init_routine
will not complete, and thus the recursive call to
pthread_once()
will not return. Use of
longjmp(),
_longjmp(),
or
siglongjmp()
within an
init_routine
to jump to a point outside of
init_routine
prevents
init_routine
from returning.
RATIONALE
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 void initialize_random(void);
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, if an
initialization flag is used the flag needs to be protected against
modification by multiple threads simultaneously calling into the
library. This can be done by using a mutex (initialized by assigning
PTHREAD_MUTEX_INITIALIZER). However, the better solution is to use
pthread_once()
which is designed for exactly this purpose, as follows:
-
#include <pthread.h>
static pthread_once_t random_is_initialized = PTHREAD_ONCE_INIT;
extern void initialize_random(void);
int random_function()
{
(void) pthread_once(&random_is_initialized, initialize_random);
... /* Operations performed after initialization. */
}
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.
FUTURE DIRECTIONS
None.
SEE ALSO
The Base Definitions volume of POSIX.1-2017,
<pthread.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 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 .
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
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https://www.kernel.org/doc/man-pages/reporting_bugs.html .