MFC 199606, 199614:

Add an internal _once() method.  This works identical to pthread_once(3)
with the additional property that it is safe for routines in libc to use
in both single-threaded and multi-threaded processes.  Multi-threaded
processes use the pthread_once() implementation from the threading library
while single-threaded processes use a simplified "stub" version internal
to libc.

lib/libc/gen/Makefile.inc

@@ -5,7 +5,8 @@
 .PATH: ${.CURDIR}/${MACHINE_ARCH}/gen ${.CURDIR}/gen
 
 SRCS+=  __getosreldate.c __xuname.c \
-	_pthread_stubs.c _rand48.c _spinlock_stub.c _thread_init.c \
+	_once_stub.c _pthread_stubs.c _rand48.c _spinlock_stub.c \
+	_thread_init.c \
 	alarm.c arc4random.c assert.c basename.c check_utility_compat.c \
 	clock.c closedir.c confstr.c \
 	crypt.c ctermid.c daemon.c devname.c dirname.c disklabel.c \

lib/libc/gen/_once_stub.c

@@ -0,0 +1,64 @@
+/*-
+ * Copyright (c) 2009 Advanced Computing Technologies LLC
+ * Written by: John H. Baldwin <jhb@FreeBSD.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include "namespace.h"
+#include <pthread.h>
+#include "un-namespace.h"
+#include "libc_private.h"
+
+/* This implements pthread_once() for the single-threaded case. */
+static int
+_libc_once(pthread_once_t *once_control, void (*init_routine)(void))
+{
+
+	if (once_control->state == PTHREAD_DONE_INIT)
+		return (0);
+	init_routine();
+	once_control->state = PTHREAD_DONE_INIT;
+	return (0);
+}
+
+/*
+ * This is the internal interface provided to libc.  It will use
+ * pthread_once() from the threading library in a multi-threaded
+ * process and _libc_once() for a single-threaded library.  Because
+ * _libc_once() uses the same ABI for the values in the pthread_once_t
+ * structure as the threading library, it is safe for a process to
+ * switch from _libc_once() to pthread_once() when threading is
+ * enabled.
+ */
+int
+_once(pthread_once_t *once_control, void (*init_routine)(void))
+{
+
+	if (__isthreaded)
+		return (_pthread_once(once_control, init_routine));
+	return (_libc_once(once_control, init_routine));
+}

lib/libc/include/libc_private.h

@@ -34,6 +34,7 @@
 
 #ifndef _LIBC_PRIVATE_H_
 #define _LIBC_PRIVATE_H_
+#include <sys/_pthreadtypes.h>
 
 /*
  * This global flag is non-zero when a process has created one
@@ -146,6 +147,12 @@ int _yp_check(char **);
  */
 void _init_tls(void);
 
+/*
+ * Provides pthread_once()-like functionality for both single-threaded
+ * and multi-threaded applications.
+ */
+int _once(pthread_once_t *, void (*)(void));
+
 /*
  * Set the TLS thread pointer
  */