'\" et

.TH PTHREAD_ONCE "3P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"

.SH 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.

.SH NAME

pthread_once

\(em dynamic package initialization

.SH SYNOPSIS

.LP

.nf

#include <pthread.h>

.P

int pthread_once(pthread_once_t *\fIonce_control\fP,

    void (*\fIinit_routine\fP)(void));

pthread_once_t \fIonce_control\fP = PTHREAD_ONCE_INIT;

.fi

.SH DESCRIPTION

The first call to

\fIpthread_once\fR()

by any thread in a process, with a given

.IR once_control ,

shall call the

.IR init_routine

with no arguments. Subsequent calls of

\fIpthread_once\fR()

with the same

.IR once_control

shall not call the

.IR init_routine .

On return from

\fIpthread_once\fR(),

.IR init_routine

shall have completed. The

.IR once_control

parameter shall determine whether the associated initialization

routine has been called.

.P

The

\fIpthread_once\fR()

function is not a cancellation point. However, if

.IR init_routine

is a cancellation point and is canceled, the effect on

.IR once_control

shall be as if

\fIpthread_once\fR()

was never called.

.P

The constant PTHREAD_ONCE_INIT is defined in the

.IR <pthread.h> 

header.

.P

The behavior of

\fIpthread_once\fR()

is undefined if

.IR once_control

has automatic storage duration or is not initialized by

PTHREAD_ONCE_INIT.

.SH "RETURN VALUE"

Upon successful completion,

\fIpthread_once\fR()

shall return zero; otherwise, an error number shall be returned to

indicate the error.

.SH ERRORS

The

\fIpthread_once\fR()

function shall not return an error code of

.BR [EINTR] .

.LP

.IR "The following sections are informative."

.SH EXAMPLES

None.

.SH "APPLICATION USAGE"

None.

.SH 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:

.sp

.RS 4

.nf

\fB

static int random_is_initialized = 0;

extern void initialize_random();

.P

int random_function()

{

    if (random_is_initialized == 0) {

        initialize_random();

        random_is_initialized = 1;

    }

    ... /* Operations performed after initialization. */

}

.fi \fR

.P

.RE

.P

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.

.P

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.

.P

The use of

\fIpthread_once\fR()

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:

.sp

.RS 4

.nf

\fB

#include <pthread.h>

static pthread_once_t random_is_initialized = PTHREAD_ONCE_INIT;

extern void initialize_random();

.P

int random_function()

{

    (void) pthread_once(&random_is_initialized, initialize_random);

    ... /* Operations performed after initialization. */

}

.fi \fR

.P

.RE

.P

Note that a

.BR pthread_once_t

cannot be an array because some compilers do not accept the construct

\fB&<array_name>\fP.

.P

If an implementation detects that the value specified by the

.IR once_control

argument to

\fIpthread_once\fR()

does not refer to a

.BR pthread_once_t

object initialized by PTHREAD_ONCE_INIT, it is recommended that the

function should fail and report an

.BR [EINVAL] 

error.

.SH "FUTURE DIRECTIONS"

None.

.SH "SEE ALSO"

The Base Definitions volume of POSIX.1\(hy2008,

.IR "\fB<pthread.h>\fP"

.SH COPYRIGHT

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 .