diff --git a/libc/Makefile b/libc/Makefile
index 92d06d9d..c58ef40f 100644
--- a/libc/Makefile
+++ b/libc/Makefile
@@ -79,6 +79,7 @@ lldiv.o \
 localtime.o \
 localtime_r.o \
 mbrtowc.o \
+mbsrtowcs.o \
 mbtowc.o \
 memccpy.o \
 memchr.o \
diff --git a/libc/include/wchar.h b/libc/include/wchar.h
index bad8a152..7e784baa 100644
--- a/libc/include/wchar.h
+++ b/libc/include/wchar.h
@@ -61,6 +61,7 @@ __BEGIN_DECLS
 
 struct tm;
 
+size_t mbsrtowcs(wchar_t* restrict, const char** restrict, size_t, mbstate_t* restrict);
 size_t wcrtomb(char* restrict, wchar_t, mbstate_t* restrict);
 size_t mbrtowc(wchar_t* restrict, const char* restrict, size_t, mbstate_t* restrict);
 wchar_t* wcscat(wchar_t* restrict, const wchar_t* restrict);
@@ -106,7 +107,6 @@ long double wcstold(const wchar_t* restrict, wchar_t** restrict);
 long long wcstoll(const wchar_t* restrict, wchar_t** restrict, int);
 long wcstol(const wchar_t* restrict, wchar_t** restrict, int);
 size_t mbrlen(const char* restrict, size_t, mbstate_t* restrict);
-size_t mbsrtowcs(wchar_t* restrict, const char** restrict, size_t, mbstate_t* restrict);
 size_t wcsftime(wchar_t* restrict, size_t, const wchar_t* restrict, const struct tm* restrict);
 size_t wcsrtombs(char* restrict, const wchar_t** restrict, size_t, mbstate_t* restrict);
 size_t wcsxfrm(wchar_t* restrict, const wchar_t* restrict, size_t);
diff --git a/libc/mbsrtowcs.cpp b/libc/mbsrtowcs.cpp
new file mode 100644
index 00000000..f056018c
--- /dev/null
+++ b/libc/mbsrtowcs.cpp
@@ -0,0 +1,72 @@
+/*******************************************************************************
+
+    Copyright(C) Jonas 'Sortie' Termansen 2013.
+
+    This file is part of the Sortix C Library.
+
+    The Sortix C Library is free software: you can redistribute it and/or modify
+    it under the terms of the GNU Lesser General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or (at your
+    option) any later version.
+
+    The Sortix C Library is distributed in the hope that it will be useful, but
+    WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+    or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
+    License for more details.
+
+    You should have received a copy of the GNU Lesser General Public License
+    along with the Sortix C Library. If not, see <http://www.gnu.org/licenses/>.
+
+    mbsrtowcs.cpp
+    Convert a multibyte string to a wide-character string.
+
+*******************************************************************************/
+
+#include <assert.h>
+#include <errno.h>
+#include <stdlib.h>
+#include <string.h>
+#include <wchar.h>
+
+extern "C" size_t mbsrtowcs(wchar_t* dst, const char** src_ptr, size_t dst_len,
+                            mbstate_t* ps)
+{
+	assert(src_ptr && *src_ptr);
+	// Avoid changing *src_ptr if dst is NULL.
+	const char* local_src_ptr = *src_ptr;
+	if ( !dst )
+		src_ptr = &local_src_ptr;
+	// For some reason, the standards don't mandate that the secret ps variable
+	// is reset when ps is NULL, unlike mbstowcs that always resets this
+	// variable. We'll avoid resetting the variable here in case any programs
+	// actually take advantage of this fact.
+	static mbstate_t static_ps;
+	if ( !ps )
+		ps = &static_ps;
+	size_t ret = 0;
+	size_t src_len = strlen(*src_ptr);
+	while ( !dst || dst_len )
+	{
+		mbstate_t saved_ps = *ps;
+		size_t consumed = mbrtowc(dst, *src_ptr, src_len, ps);
+		if ( consumed == (size_t) 0 )
+		{
+			*src_ptr = NULL;
+			break;
+		}
+		if ( consumed == (size_t) -1 )
+			return (size_t) -1;
+		if ( consumed == (size_t) -2 )
+		{
+			*ps = saved_ps;
+			break;
+		}
+		*src_ptr += consumed;
+		src_len -= consumed;
+		if ( dst )
+			dst++,
+			dst_len--;
+		ret++;
+	}
+	return ret;
+}