[libc-commits] [libc] [libc] Fixed mbtowc functions (PR #150118)

[via libc-commits]

https://github.com/sribee8 created https://github.com/llvm/llvm-project/pull/150118

mbrtowc was not handling null destination correctly

>From 4a5a6a6e57d8656cac666b271edcc810d037876f Mon Sep 17 00:00:00 2001
From: Sriya Pratipati <sriyap at google.com>
Date: Tue, 22 Jul 2025 21:48:06 +0000
Subject: [PATCH] [libc] Fixed mbtowc functions

mbrtowc was not handling null destination correctly
---
 libc/src/__support/wchar/mbrtowc.cpp | 3 ++-
 libc/src/wchar/mbtowc.cpp            | 5 +----
 2 files changed, 3 insertions(+), 5 deletions(-)

diff --git a/libc/src/__support/wchar/mbrtowc.cpp b/libc/src/__support/wchar/mbrtowc.cpp
index 90ba934c42b69..0f730d6e6dbec 100644
--- a/libc/src/__support/wchar/mbrtowc.cpp
+++ b/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;
diff --git a/libc/src/wchar/mbtowc.cpp b/libc/src/wchar/mbtowc.cpp
index eae39ba6081f3..6d099d43da5fa 100644
--- a/libc/src/wchar/mbtowc.cpp
+++ b/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;