'\" 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 .

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