'\" et
.TH PTHREAD_ONCE "3P" 2017 "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
If the call to
.IR init_routine
is terminated by a call to
\fIlongjmp\fR(),
\fI_longjmp\fR(),
or
\fIsiglongjmp\fR(),
the behavior is undefined.
.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"
If
.IR init_routine
recursively calls
\fIpthread_once\fR()
with the same
.IR once_control ,
the recursive call will not call the specified
.IR init_routine ,
and thus the specified
.IR init_routine
will not complete, and thus the recursive call to
\fIpthread_once\fR()
will not return. Use of
\fIlongjmp\fR(),
\fI_longjmp\fR(),
or
\fIsiglongjmp\fR()
within an
.IR init_routine
to jump to a point outside of
.IR init_routine
prevents
.IR init_routine
from returning.
.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

static int random_is_initialized = 0;
extern void initialize_random(void);
.P
int random_function()
{
    if (random_is_initialized == 0) {
        initialize_random();
        random_is_initialized = 1;
    }
    ... /* Operations performed after initialization. */
}
.fi
.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, 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
\fIpthread_once\fR()
which is designed for exactly this purpose, as follows:
.sp
.RS 4
.nf

#include <pthread.h>
static pthread_once_t random_is_initialized = PTHREAD_ONCE_INIT;
extern void initialize_random(void);
.P
int random_function()
{
    (void) pthread_once(&random_is_initialized, initialize_random);
    ... /* Operations performed after initialization. */
}
.fi
.P
.RE
.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\(hy2017,
.IR "\fB<pthread.h>\fP"
.\"
.SH 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 .
.PP
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 .