libc: Implement wcstombs()

libc/include/stdlib.h

@@ -110,6 +110,9 @@ extern "C"
     /* Convert a multibyte character string to a wide character string. */
     size_t mbstowcs(wchar_t* buf, const char* src, size_t max);
 
+    /* Convert a wide character string to a multibyte character string. */
+    size_t wcstombs(char* buf, const wchar_t* src, size_t max);
+
 #ifdef __cplusplus
 }
 #endif

libc/src/stdlib.cpp

@@ -115,6 +115,17 @@ extern "C"
         return decoder.decode(buf, max).value_or((size_t)-1);
     }
 
+    size_t wcstombs(char* buf, const wchar_t* src, size_t max)
+    {
+        if (max == 0) return 0;
+
+        Utf8StringEncoder encoder(src);
+
+        if (!buf) { return encoder.byte_length().value_or((size_t)-1); }
+
+        return encoder.encode(buf, max).value_or((size_t)-1);
+    }
+
     void* malloc(size_t size)
     {
         auto rc = malloc_impl(size);