diff --git a/lib/libc/stdio/fflush.c b/lib/libc/stdio/fflush.c
index f7d2fbdc28e5..decc974907f4 100644
--- a/lib/libc/stdio/fflush.c
+++ b/lib/libc/stdio/fflush.c
@@ -1,154 +1,159 @@
 /*-
  * SPDX-License-Identifier: BSD-3-Clause
  *
  * Copyright (c) 1990, 1993
  *	The Regents of the University of California.  All rights reserved.
  *
  * This code is derived from software contributed to Berkeley by
  * Chris Torek.
  *
  * 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.
  * 3. Neither the name of the University nor the names of its contributors
  *    may be used to endorse or promote products derived from this software
  *    without specific prior written permission.
  *
  * THIS SOFTWARE IS PROVIDED BY THE REGENTS 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 REGENTS 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.
  */
 
 #if defined(LIBC_SCCS) && !defined(lint)
 static char sccsid[] = "@(#)fflush.c	8.1 (Berkeley) 6/4/93";
 #endif /* LIBC_SCCS and not lint */
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
 #include "namespace.h"
 #include <errno.h>
 #include <stdio.h>
 #include "un-namespace.h"
 #include "libc_private.h"
 #include "local.h"
 
 static int	sflush_locked(FILE *);
 
 /*
  * Flush a single file, or (if fp is NULL) all files.
  * MT-safe version
  */
 int
 fflush(FILE *fp)
 {
 	int retval;
 
 	if (fp == NULL)
 		return (_fwalk(sflush_locked));
 	FLOCKFILE_CANCELSAFE(fp);
 
 	/*
 	 * There is disagreement about the correct behaviour of fflush()
 	 * when passed a file which is not open for writing.  According to
 	 * the ISO C standard, the behaviour is undefined.
 	 * Under linux, such an fflush returns success and has no effect;
 	 * under Windows, such an fflush is documented as behaving instead
 	 * as fpurge().
 	 * Given that applications may be written with the expectation of
 	 * either of these two behaviours, the only safe (non-astonishing)
 	 * option is to return EBADF and ask that applications be fixed.
 	 * SUSv3 now requires that fflush() returns success on a read-only
 	 * stream.
 	 *
 	 */
 	if ((fp->_flags & (__SWR | __SRW)) == 0)
 		retval = 0;
 	else
 		retval = __sflush(fp);
 	FUNLOCKFILE_CANCELSAFE();
 	return (retval);
 }
 
 /*
  * Flush a single file, or (if fp is NULL) all files.
  * Non-MT-safe version
  */
 int
 __fflush(FILE *fp)
 {
 	int retval;
 
 	if (fp == NULL)
 		return (_fwalk(sflush_locked));
 	if ((fp->_flags & (__SWR | __SRW)) == 0)
 		retval = 0;
 	else
 		retval = __sflush(fp);
 	return (retval);
 }
 
 __weak_reference(__fflush, fflush_unlocked);
 
 int
 __sflush(FILE *fp)
 {
-	unsigned char *p;
-	int n, t;
+	unsigned char *p, *old_p;
+	int n, t, old_w;
 
 	t = fp->_flags;
 	if ((t & __SWR) == 0)
 		return (0);
 
 	if ((p = fp->_bf._base) == NULL)
 		return (0);
 
 	n = fp->_p - p;		/* write this much */
 
 	/*
 	 * Set these immediately to avoid problems with longjmp and to allow
 	 * exchange buffering (via setvbuf) in user write function.
 	 */
+	old_p = fp->_p;
 	fp->_p = p;
+	old_w = fp->_w;
 	fp->_w = t & (__SLBF|__SNBF) ? 0 : fp->_bf._size;
 
 	for (; n > 0; n -= t, p += t) {
 		t = _swrite(fp, (char *)p, n);
 		if (t <= 0) {
 			/* Reset _p and _w. */
 			if (p > fp->_p) {
 				/* Some was written. */
 				memmove(fp->_p, p, n);
 				fp->_p += n;
 				if ((fp->_flags & (__SLBF | __SNBF)) == 0)
 					fp->_w -= n;
+			} else if (p == fp->_p) { /* cond. to handle setvbuf */
+				fp->_p = old_p;
+				fp->_w = old_w;
 			}
 			fp->_flags |= __SERR;
 			return (EOF);
 		}
 	}
 	return (0);
 }
 
 static int
 sflush_locked(FILE *fp)
 {
 	int	ret;
 
 	FLOCKFILE_CANCELSAFE(fp);
 	ret = __sflush(fp);
 	FUNLOCKFILE_CANCELSAFE();
 	return (ret);
 }