|
HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
|
|
|
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.
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.
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.
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.
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.
None.
The Base Definitions volume of POSIX.1‐2017, pthread.h(0p)
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 man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 PTHREAD_ONCE(3P)
Pages that refer to this page: pthread.h(0p)
|
HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|