[libc] Fixed mbtowc functions (#150118)

mbrtowc was not handling null destination correctly

---------

Co-authored-by: Sriya Pratipati <[email protected]>

Author: sribee8

libc/src/__support/wchar/mbrtowc.cpp

@@ -37,7 +37,8 @@ ErrorOr<size_t> mbrtowc(wchar_t *__restrict pwc, const char *__restrict s,
   }
   auto wc = char_conv.pop_utf32();
   if (wc.has_value()) {
-    *pwc = wc.value();
+    if (pwc != nullptr)
+      *pwc = wc.value();
     // null terminator -> return 0
     if (wc.value() == L'\0')
       return 0;

libc/src/wchar/mbtowc.cpp

@@ -25,10 +25,7 @@ LLVM_LIBC_FUNCTION(int, mbtowc,
   if (s == nullptr)
     return 0;
   internal::mbstate internal_mbstate;
-  // temp ptr to use if pwc is nullptr
-  wchar_t buf[1];
-  auto ret =
-      internal::mbrtowc(pwc == nullptr ? buf : pwc, s, n, &internal_mbstate);
+  auto ret = internal::mbrtowc(pwc, s, n, &internal_mbstate);
   if (!ret.has_value() || static_cast<int>(ret.value()) == -2) {
     // Encoding failure
     libc_errno = EILSEQ;

libc/test/src/wchar/mbrtowc_test.cpp

@@ -190,6 +190,18 @@ TEST_F(LlvmLibcMBRToWCTest, NullString) {
   ASSERT_ERRNO_SUCCESS();
 }
 
+TEST_F(LlvmLibcMBRToWCTest, NullDest) {
+  const char ch[4] = {static_cast<char>(0xF0), static_cast<char>(0x9F),
+                      static_cast<char>(0xA4),
+                      static_cast<char>(0xA1)}; // 🤡 clown emoji
+  mbstate_t *mb;
+  LIBC_NAMESPACE::memset(&mb, 0, sizeof(mbstate_t));
+  // reading nullptr should return correct size
+  size_t n = LIBC_NAMESPACE::mbrtowc(nullptr, ch, 10, mb);
+  ASSERT_EQ(static_cast<int>(n), 4);
+  ASSERT_ERRNO_SUCCESS();
+}
+
 TEST_F(LlvmLibcMBRToWCTest, InvalidMBState) {
   const char ch[4] = {static_cast<char>(0xC2), static_cast<char>(0x8E),
                       static_cast<char>(0xC7), static_cast<char>(0x8C)};