[libc] Move mbtowc, mbstowcs and inverse functions to stdlib.h (#168455)

These functions should be declared in `stdlib.h`, not `wchar.h`, as
confusing as it is. Move them to the proper header file and matching
directories in src/ and test/ trees.

This was discovered while testing libc++ build against llvm-libc, which
re-declares functions like mbtowc in std-namespace in `<cstdlib>`
header, and then uses those functions in its locale implementation.

1 files changed, 40 insertions, 0 deletions

diff --git a/libc/src/stdlib/mbstowcs.cpp b/libc/src/stdlib/mbstowcs.cpp
new file mode 100644
index 000000000000..6d283ea46e3b
--- /dev/null
+++ b/libc/src/stdlib/mbstowcs.cpp
@@ -0,0 +1,40 @@
+//===-- Implementation of mbstowcs ----------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/stdlib/mbstowcs.h"
+
+#include "hdr/types/size_t.h"
+#include "hdr/types/wchar_t.h"
+#include "src/__support/common.h"
+#include "src/__support/libc_errno.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/macros/null_check.h"
+#include "src/__support/wchar/mbsnrtowcs.h"
+#include "src/__support/wchar/mbstate.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(size_t, mbstowcs,
+                   (wchar_t *__restrict pwcs, const char *__restrict s,
+                    size_t n)) {
+  LIBC_CRASH_ON_NULLPTR(s);
+  // If destination is null, ignore n
+  n = pwcs == nullptr ? SIZE_MAX : n;
+  static internal::mbstate internal_mbstate;
+  const char *temp = s;
+  auto ret = internal::mbsnrtowcs(pwcs, &temp, SIZE_MAX, n, &internal_mbstate);
+
+  if (!ret.has_value()) {
+    // Encoding failure
+    libc_errno = ret.error();
+    return -1;
+  }
+  return ret.value();
+}
+
+} // namespace LIBC_NAMESPACE_DECL