Add an implementation of open_memstream() and open_wmemstream(). These

routines provide write-only stdio FILE objects that store their data in a
dynamically allocated buffer.  They are a string builder interface somewhat
akin to a completely dynamic sbuf.

Reviewed by:	bde, jilles (earlier versions)
MFC after:	1 month

5 files changed, 639 insertions, 1 deletions

diff --git a/lib/libc/stdio/Makefile.inc b/lib/libc/stdio/Makefile.inc
index bc3b9e5..0062d3d 100644
--- a/lib/libc/stdio/Makefile.inc
+++ b/lib/libc/stdio/Makefile.inc
@@ -14,6 +14,7 @@ SRCS+=	_flock_stub.c asprintf.c clrerr.c dprintf.c \
 	ftell.c funopen.c fvwrite.c fwalk.c fwide.c fwprintf.c fwscanf.c \
 	fwrite.c getc.c getchar.c getdelim.c getline.c \
 	gets.c getw.c getwc.c getwchar.c makebuf.c mktemp.c \
+	open_memstream.c open_wmemstream.c \
 	perror.c printf.c printf-pos.c putc.c putchar.c \
 	puts.c putw.c putwc.c putwchar.c \
 	refill.c remove.c rewind.c rget.c scanf.c setbuf.c setbuffer.c \
@@ -36,7 +37,7 @@ MAN+=	fclose.3 ferror.3 fflush.3 fgetln.3 fgets.3 fgetwln.3 fgetws.3 \
 	flockfile.3 \
 	fopen.3 fputs.3 \
 	fputws.3 fread.3 fseek.3 funopen.3 fwide.3 getc.3 \
-	getline.3 getwc.3 mktemp.3 \
+	getline.3 getwc.3 mktemp.3 open_memstream.3 \
 	printf.3 printf_l.3 putc.3 putwc.3 remove.3 scanf.3 scanf_l.3 setbuf.3 \
 	stdio.3 tmpnam.3 \
 	ungetc.3 ungetwc.3 wprintf.3 wscanf.3
@@ -60,6 +61,7 @@ MLINKS+=getc.3 fgetc.3 getc.3 getc_unlocked.3 getc.3 getchar.3 \
 MLINKS+=getline.3 getdelim.3
 MLINKS+=getwc.3 fgetwc.3 getwc.3 getwchar.3
 MLINKS+=mktemp.3 mkdtemp.3 mktemp.3 mkstemp.3 mktemp.3 mkstemps.3
+MLINKS+=open_memstream.3 open_wmemstream.3
 MLINKS+=printf.3 asprintf.3 printf.3 dprintf.3 printf.3 fprintf.3 \
 	printf.3 snprintf.3 printf.3 sprintf.3 \
 	printf.3 vasprintf.3 printf.3 vdprintf.3 \
diff --git a/lib/libc/stdio/Symbol.map b/lib/libc/stdio/Symbol.map
index b78de5a..538b29a 100644
--- a/lib/libc/stdio/Symbol.map
+++ b/lib/libc/stdio/Symbol.map
@@ -156,6 +156,8 @@ FBSD_1.3 {
 	putwc_l;
 	putwchar_l;
 	fmemopen;
+	open_memstream;
+	open_wmemstream;
 };
 
 FBSDprivate_1.0 {
diff --git a/lib/libc/stdio/open_memstream.3 b/lib/libc/stdio/open_memstream.3
new file mode 100644
index 0000000..d03ed1f
--- /dev/null
+++ b/lib/libc/stdio/open_memstream.3
@@ -0,0 +1,154 @@
+.\" Copyright (c) 2013 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.
+.\"
+.\" $FreeBSD$
+.\"
+.Dd February 27, 2013
+.Dt OPEN_MEMSTREAM 3
+.Os
+.Sh NAME
+.Nm open_memstream ,
+.Nm open_wmemstream
+.Nd dynamic memory buffer stream open functions
+.Sh LIBRARY
+.Lb libc
+.Sh SYNOPSIS
+.In stdio.h
+.Ft FILE *
+.Fn open_memstream "char **bufp" "size_t **sizep"
+.In wchar.h
+.Ft FILE *
+.Fn open_wmemstream "wchar_t **bufp" "size_t **sizep"
+.Sh DESCRIPTION
+The
+.Fn open_memstream
+and
+.Fn open_wmemstream
+functions create a write-only, seekable stream backed by a dynamically
+allocated memory buffer.
+The
+.Fn open_memstream
+function creates a byte-oriented stream,
+while the
+.Fn open_wmemstream
+function creates a wide-oriented stream.
+.Pp
+Each stream maintains a current position and size.
+Initially,
+the position and size are set to zero.
+Each write begins at the current position and advances it the number of
+successfully written bytes for
+.Fn open_memstream
+or wide characters for
+.Fn open_wmemstream .
+If a write moves the current position beyond the length of the buffer,
+the length of the buffer is extended and a null character is appended to the
+buffer.
+.Pp
+A stream's buffer always contains a null character at the end of the buffer
+that is not included in the current length.
+.Pp
+If a stream's current position is moved beyond the current length via a
+seek operation and a write is performed,
+the characters between the current length and the current position are filled
+with null characters before the write is performed.
+.Pp
+After a successful call to
+.Xr fclose 3
+or
+.Xr fflush 3 ,
+the pointer referenced by
+.Fa bufp
+will contain the start of the memory buffer and the variable referenced by
+.Fa sizep
+will contain the smaller of the current position and the current buffer length.
+.Pp
+After a successful call to
+.Xr fflush 3,
+the pointer referenced by
+.Fa bufp
+and the variable referenced by
+.Fa sizep
+are only valid until the next write operation or a call to
+.Xr fclose 3.
+.Pp
+Once a stream is closed,
+the allocated buffer referenced by
+.Fa bufp
+should be released via a call to
+.Xr free 3
+when it is no longer needed.
+.Sh IMPLEMENTATION NOTES
+Internally all I/O streams are effectively byte-oriented,
+so using wide-oriented operations to write to a stream opened via
+.Fn open_wmemstream
+results in wide characters being expanded to a stream of multibyte characters
+in stdio's internal buffers.
+These multibyte characters are then converted back to wide characters when
+written into the stream.
+As a result,
+the wide-oriented streams maintain an internal multibyte character conversion
+state that is cleared on any seek opertion that changes the current position.
+This should have no effect as long as wide-oriented output operations are used
+on a wide-oriented stream.
+.Sh RETURN VALUES
+Upon successful completion,
+.Fn open_memstream
+and
+.Fn open_wmemstream
+return a
+.Tn FILE
+pointer.
+Otherwise,
+.Dv NULL
+is returned and the global variable
+.Va errno
+is set to indicate the error.
+.Sh ERRORS
+.Bl -tag -width Er
+.It Bq Er EINVAL
+The
+.Fa bufp
+or
+.Fa sizep
+argument was
+.Dv NULL .
+.It Bq Er ENOMEM
+Memory for the stream or buffer could not be allocated.
+.Sh SEE ALSO
+.Xr fclose 3 ,
+.Xr fflush 3 ,
+.Xr fopen 3 ,
+.Xr free 3 ,
+.Xr fseek 3 ,
+.Xr sbuf 3 ,
+.Xr stdio 3
+.Sh STANDARDS
+The
+.Fn open_memstream
+and
+.Fn open_wmemstream
+functions conform to
+.St -p1003.1-2008 .
diff --git a/lib/libc/stdio/open_memstream.c b/lib/libc/stdio/open_memstream.c
new file mode 100644
index 0000000..aa50822
--- /dev/null
+++ b/lib/libc/stdio/open_memstream.c
@@ -0,0 +1,209 @@
+/*-
+ * Copyright (c) 2013 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 <assert.h>
+#include <errno.h>
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <wchar.h>
+#include "un-namespace.h"
+
+/* XXX: There is no FPOS_MAX.  This assumes fpos_t is an off_t. */
+#define	FPOS_MAX	OFF_MAX
+
+struct memstream {
+	char **bufp;
+	size_t *sizep;
+	ssize_t len;
+	fpos_t offset;
+};
+
+static int
+memstream_grow(struct memstream *ms, fpos_t newoff)
+{
+	char *buf;
+	ssize_t newsize;
+
+	if (newoff < 0 || newoff >= SSIZE_MAX)
+		newsize = SSIZE_MAX - 1;
+	else
+		newsize = newoff;
+	if (newsize > ms->len) {
+		buf = realloc(*ms->bufp, newsize + 1);
+		if (buf != NULL) {
+#ifdef DEBUG
+			fprintf(stderr, "MS: %p growing from %zd to %zd\n",
+			    ms, ms->len, newsize);
+#endif
+			memset(buf + ms->len + 1, 0, newsize - ms->len);
+			*ms->bufp = buf;
+			ms->len = newsize;
+			return (1);
+		}
+		return (0);
+	}
+	return (1);
+}
+
+static void
+memstream_update(struct memstream *ms)
+{
+
+	assert(ms->len >= 0 && ms->offset >= 0);
+	*ms->sizep = ms->len < ms->offset ? ms->len : ms->offset;
+}
+
+static int
+memstream_write(void *cookie, const char *buf, int len)
+{
+	struct memstream *ms;
+	ssize_t tocopy;
+
+	ms = cookie;
+	if (!memstream_grow(ms, ms->offset + len))
+		return (-1);
+	tocopy = ms->len - ms->offset;
+	if (len < tocopy)
+		tocopy = len;
+	memcpy(*ms->bufp + ms->offset, buf, tocopy);
+	ms->offset += tocopy;
+	memstream_update(ms);
+#ifdef DEBUG
+	fprintf(stderr, "MS: write(%p, %d) = %zd\n", ms, len, tocopy);
+#endif
+	return (tocopy);
+}
+
+static fpos_t
+memstream_seek(void *cookie, fpos_t pos, int whence)
+{
+	struct memstream *ms;
+#ifdef DEBUG
+	fpos_t old;
+#endif
+
+	ms = cookie;
+#ifdef DEBUG
+	old = ms->offset;
+#endif
+	switch (whence) {
+	case SEEK_SET:
+		/* _fseeko() checks for negative offsets. */
+		assert(pos >= 0);
+		ms->offset = pos;
+		break;
+	case SEEK_CUR:
+		/* This is only called by _ftello(). */
+		assert(pos == 0);
+		break;
+	case SEEK_END:
+		if (pos < 0) {
+			if (pos + ms->len < 0) {
+#ifdef DEBUG
+				fprintf(stderr,
+				    "MS: bad SEEK_END: pos %jd, len %zd\n",
+				    (intmax_t)pos, ms->len);
+#endif
+				errno = EINVAL;
+				return (-1);
+			}
+		} else {
+			if (FPOS_MAX - ms->len < pos) {
+#ifdef DEBUG
+				fprintf(stderr,
+				    "MS: bad SEEK_END: pos %jd, len %zd\n",
+				    (intmax_t)pos, ms->len);
+#endif
+				errno = EOVERFLOW;
+				return (-1);
+			}
+		}
+		ms->offset = ms->len + pos;
+		break;
+	}
+	memstream_update(ms);
+#ifdef DEBUG
+	fprintf(stderr, "MS: seek(%p, %jd, %d) %jd -> %jd\n", ms, (intmax_t)pos,
+	    whence, (intmax_t)old, (intmax_t)ms->offset);
+#endif
+	return (ms->offset);
+}
+
+static int
+memstream_close(void *cookie)
+{
+
+	free(cookie);
+	return (0);
+}
+
+FILE *
+open_memstream(char **bufp, size_t *sizep)
+{
+	struct memstream *ms;
+	int save_errno;
+	FILE *fp;
+
+	if (bufp == NULL || sizep == NULL) {
+		errno = EINVAL;
+		return (NULL);
+	}
+	*bufp = calloc(1, 1);
+	if (*bufp == NULL)
+		return (NULL);
+	ms = malloc(sizeof(*ms));
+	if (ms == NULL) {
+		save_errno = errno;
+		free(*bufp);
+		*bufp = NULL;
+		errno = save_errno;
+		return (NULL);
+	}
+	ms->bufp = bufp;
+	ms->sizep = sizep;
+	ms->len = 0;
+	ms->offset = 0;
+	memstream_update(ms);
+	fp = funopen(ms, NULL, memstream_write, memstream_seek,
+	    memstream_close);
+	if (fp == NULL) {
+		save_errno = errno;
+		free(ms);
+		free(*bufp);
+		*bufp = NULL;
+		errno = save_errno;
+		return (NULL);
+	}
+	fwide(fp, -1);
+	return (fp);
+}
diff --git a/lib/libc/stdio/open_wmemstream.c b/lib/libc/stdio/open_wmemstream.c
new file mode 100644
index 0000000..cf2968a
--- /dev/null
+++ b/lib/libc/stdio/open_wmemstream.c
@@ -0,0 +1,271 @@
+/*-
+ * Copyright (c) 2013 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 <assert.h>
+#include <errno.h>
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <wchar.h>
+#include "un-namespace.h"
+
+/* XXX: There is no FPOS_MAX.  This assumes fpos_t is an off_t. */
+#define	FPOS_MAX	OFF_MAX
+
+struct wmemstream {
+	wchar_t **bufp;
+	size_t *sizep;
+	ssize_t len;
+	fpos_t offset;
+	mbstate_t mbstate;
+};
+
+static int
+wmemstream_grow(struct wmemstream *ms, fpos_t newoff)
+{
+	wchar_t *buf;
+	ssize_t newsize;
+
+	if (newoff < 0 || newoff >= SSIZE_MAX / sizeof(wchar_t))
+		newsize = SSIZE_MAX / sizeof(wchar_t) - 1;
+	else
+		newsize = newoff;
+	if (newsize > ms->len) {
+		buf = realloc(*ms->bufp, (newsize + 1) * sizeof(wchar_t));
+		if (buf != NULL) {
+#ifdef DEBUG
+			fprintf(stderr, "WMS: %p growing from %zd to %zd\n",
+			    ms, ms->len, newsize);
+#endif
+			wmemset(buf + ms->len + 1, 0, newsize - ms->len);
+			*ms->bufp = buf;
+			ms->len = newsize;
+			return (1);
+		}
+		return (0);
+	}
+	return (1);
+}
+
+static void
+wmemstream_update(struct wmemstream *ms)
+{
+
+	assert(ms->len >= 0 && ms->offset >= 0);
+	*ms->sizep = ms->len < ms->offset ? ms->len : ms->offset;
+}
+
+/*
+ * Based on a starting multibyte state and an input buffer, determine
+ * how many wchar_t's would be output.  This doesn't use mbsnrtowcs()
+ * so that it can handle embedded null characters.
+ */
+static size_t
+wbuflen(const mbstate_t *state, const char *buf, int len)
+{
+	mbstate_t lenstate;
+	size_t charlen, count;
+
+	count = 0;
+	lenstate = *state;
+	while (len > 0) {
+		charlen = mbrlen(buf, len, &lenstate);
+		if (charlen == (size_t)-1)
+			return (-1);
+		if (charlen == (size_t)-2)
+			break;
+		if (charlen == 0)
+			/* XXX: Not sure how else to handle this. */
+			charlen = 1;
+		len -= charlen;
+		buf += charlen;
+		count++;
+	}
+	return (count);
+}
+
+static int
+wmemstream_write(void *cookie, const char *buf, int len)
+{
+	struct wmemstream *ms;
+	ssize_t consumed, wlen;
+	size_t charlen;
+
+	ms = cookie;
+	wlen = wbuflen(&ms->mbstate, buf, len);
+	if (wlen < 0) {
+		errno = EILSEQ;
+		return (-1);
+	}
+	if (!wmemstream_grow(ms, ms->offset + wlen))
+		return (-1);
+
+	/*
+	 * This copies characters one at a time rather than using
+	 * mbsnrtowcs() so it can properly handle embedded null
+	 * characters.
+	 */
+	consumed = 0;
+	while (len > 0 && ms->offset < ms->len) {
+		charlen = mbrtowc(*ms->bufp + ms->offset, buf, len,
+		    &ms->mbstate);
+		if (charlen == (size_t)-1) {
+			if (consumed == 0) {
+				errno = EILSEQ;
+				return (-1);
+			}
+			/* Treat it as a successful short write. */
+			break;
+		}
+		if (charlen == 0)
+			/* XXX: Not sure how else to handle this. */
+			charlen = 1;
+		if (charlen == (size_t)-2) {
+			consumed += len;
+			len = 0;
+		} else {
+			consumed += charlen;
+			buf += charlen;
+			len -= charlen;
+			ms->offset++;
+		}
+	}
+	wmemstream_update(ms);
+#ifdef DEBUG
+	fprintf(stderr, "WMS: write(%p, %d) = %zd\n", ms, len, consumed);
+#endif
+	return (consumed);
+}
+
+static fpos_t
+wmemstream_seek(void *cookie, fpos_t pos, int whence)
+{
+	struct wmemstream *ms;
+	fpos_t old;
+
+	ms = cookie;
+	old = ms->offset;
+	switch (whence) {
+	case SEEK_SET:
+		/* _fseeko() checks for negative offsets. */
+		assert(pos >= 0);
+		ms->offset = pos;
+		break;
+	case SEEK_CUR:
+		/* This is only called by _ftello(). */
+		assert(pos == 0);
+		break;
+	case SEEK_END:
+		if (pos < 0) {
+			if (pos + ms->len < 0) {
+#ifdef DEBUG
+				fprintf(stderr,
+				    "WMS: bad SEEK_END: pos %jd, len %zd\n",
+				    (intmax_t)pos, ms->len);
+#endif
+				errno = EINVAL;
+				return (-1);
+			}
+		} else {
+			if (FPOS_MAX - ms->len < pos) {
+#ifdef DEBUG
+				fprintf(stderr,
+				    "WMS: bad SEEK_END: pos %jd, len %zd\n",
+				    (intmax_t)pos, ms->len);
+#endif
+				errno = EOVERFLOW;
+				return (-1);
+			}
+		}
+		ms->offset = ms->len + pos;
+		break;
+	}
+	/* Reset the multibyte state if a seek changes the position. */
+	if (ms->offset != old)
+		memset(&ms->mbstate, 0, sizeof(ms->mbstate));
+	wmemstream_update(ms);
+#ifdef DEBUG
+	fprintf(stderr, "WMS: seek(%p, %jd, %d) %jd -> %jd\n", ms,
+	    (intmax_t)pos, whence, (intmax_t)old, (intmax_t)ms->offset);
+#endif
+	return (ms->offset);
+}
+
+static int
+wmemstream_close(void *cookie)
+{
+
+	free(cookie);
+	return (0);
+}
+
+FILE *
+open_wmemstream(wchar_t **bufp, size_t *sizep)
+{
+	struct wmemstream *ms;
+	int save_errno;
+	FILE *fp;
+
+	if (bufp == NULL || sizep == NULL) {
+		errno = EINVAL;
+		return (NULL);
+	}
+	*bufp = calloc(1, sizeof(wchar_t));
+	if (*bufp == NULL)
+		return (NULL);
+	ms = malloc(sizeof(*ms));
+	if (ms == NULL) {
+		save_errno = errno;
+		free(*bufp);
+		*bufp = NULL;
+		errno = save_errno;
+		return (NULL);
+	}
+	ms->bufp = bufp;
+	ms->sizep = sizep;
+	ms->len = 0;
+	ms->offset = 0;
+	memset(&ms->mbstate, 0, sizeof(mbstate_t));
+	wmemstream_update(ms);
+	fp = funopen(ms, NULL, wmemstream_write, wmemstream_seek,
+	    wmemstream_close);
+	if (fp == NULL) {
+		save_errno = errno;
+		free(ms);
+		free(*bufp);
+		*bufp = NULL;
+		errno = save_errno;
+		return (NULL);
+	}
+	fwide(fp, 1);
+	return (fp);
+}