Index: lib/msan/tests/msan_test.cc
===================================================================
--- lib/msan/tests/msan_test.cc
+++ lib/msan/tests/msan_test.cc
@@ -2123,6 +2123,16 @@
   EXPECT_EQ(buff[0], 'a');
 }
 
+TEST(MemorySanitizer, wctomb) {
+  wchar_t x = L'a';
+  char buff[10];
+  wctomb(nullptr, x);
+  int res = wctomb(buff, x);
+  EXPECT_EQ(res, 1);
+  EXPECT_EQ(buff[0], 'a');
+  EXPECT_POISONED(buff[1]);
+}
+
 TEST(MemorySanitizer, wmemset) {
     wchar_t x[25];
     break_optimization(x);
Index: lib/sanitizer_common/sanitizer_common_interceptors.inc
===================================================================
--- lib/sanitizer_common/sanitizer_common_interceptors.inc
+++ lib/sanitizer_common/sanitizer_common_interceptors.inc
@@ -3540,6 +3540,28 @@
 #define INIT_WCRTOMB
 #endif
 
+#if SANITIZER_INTERCEPT_WCTOMB
+INTERCEPTOR(int, wctomb, char *dest, wchar_t src) {
+  void *ctx;
+  COMMON_INTERCEPTOR_ENTER(ctx, wctomb, dest, src);
+  if (!dest)
+    return REAL(wctomb)(dest, src);
+
+  char *local_dest = (char *)WRAP(malloc)(__sanitizer_MB_LEN_MAX);
+  int res = REAL(wctomb)(local_dest, src);
+  if (res != -1) {
+    COMMON_INTERCEPTOR_WRITE_RANGE(ctx, dest, res);
+    REAL(memcpy)(dest, local_dest, res);
+    WRAP(free)(local_dest);
+  }
+  return res;
+}
+
+#define INIT_WCTOMB COMMON_INTERCEPT_FUNCTION(wctomb);
+#else
+#define INIT_WCTOMB
+#endif
+
 #if SANITIZER_INTERCEPT_TCGETATTR
 INTERCEPTOR(int, tcgetattr, int fd, void *termios_p) {
   void *ctx;
@@ -9616,6 +9638,7 @@
   INIT_WCSTOMBS;
   INIT_WCSNRTOMBS;
   INIT_WCRTOMB;
+  INIT_WCTOMB;
   INIT_TCGETATTR;
   INIT_REALPATH;
   INIT_CANONICALIZE_FILE_NAME;
Index: lib/sanitizer_common/sanitizer_platform_interceptors.h
===================================================================
--- lib/sanitizer_common/sanitizer_platform_interceptors.h
+++ lib/sanitizer_common/sanitizer_platform_interceptors.h
@@ -283,6 +283,9 @@
 #define SANITIZER_INTERCEPT_WCRTOMB                                           \
   (SI_FREEBSD || SI_NETBSD || SI_OPENBSD || SI_MAC || SI_LINUX_NOT_ANDROID || \
   SI_SOLARIS)
+#define SANITIZER_INTERCEPT_WCTOMB                                           \
+  (SI_FREEBSD || SI_NETBSD || SI_OPENBSD || SI_MAC || SI_LINUX_NOT_ANDROID || \
+  SI_SOLARIS)
 #define SANITIZER_INTERCEPT_TCGETATTR SI_LINUX_NOT_ANDROID || SI_SOLARIS
 #define SANITIZER_INTERCEPT_REALPATH SI_POSIX
 #define SANITIZER_INTERCEPT_CANONICALIZE_FILE_NAME \
Index: lib/sanitizer_common/sanitizer_platform_limits_posix.h
===================================================================
--- lib/sanitizer_common/sanitizer_platform_limits_posix.h
+++ lib/sanitizer_common/sanitizer_platform_limits_posix.h
@@ -1428,6 +1428,8 @@
 
   extern const int si_SEGV_MAPERR;
   extern const int si_SEGV_ACCERR;
+
+  extern const unsigned __sanitizer_MB_LEN_MAX;
 }  // namespace __sanitizer
 
 #define CHECK_TYPE_SIZE(TYPE) \
Index: lib/sanitizer_common/sanitizer_platform_limits_posix.cc
===================================================================
--- lib/sanitizer_common/sanitizer_platform_limits_posix.cc
+++ lib/sanitizer_common/sanitizer_platform_limits_posix.cc
@@ -913,6 +913,8 @@
 
   const int si_SEGV_MAPERR = SEGV_MAPERR;
   const int si_SEGV_ACCERR = SEGV_ACCERR;
+
+  const unsigned __sanitizer_MB_LEN_MAX = MB_LEN_MAX;
 } // namespace __sanitizer
 
 using namespace __sanitizer;
Index: test/sanitizer_common/TestCases/wctomb.c
===================================================================
--- /dev/null
+++ test/sanitizer_common/TestCases/wctomb.c
@@ -0,0 +1,14 @@
+// RUN: %clang %s -o %t && %run %t 2>&1
+
+#include <assert.h>
+#include <stdlib.h>
+
+int main(int argc, char **argv) {
+  char buff[10];
+  wchar_t x = L'a';
+  wctomb(NULL, x);
+  int res = wctomb(buff, x);
+  assert(res == 1);
+  assert(buff[0] == 'a');
+  return 0;
+}