diff --git a/src/libimp/codecvt.cpp b/src/libimp/codecvt.cpp
index b10c5b5..d9411be 100644
--- a/src/libimp/codecvt.cpp
+++ b/src/libimp/codecvt.cpp
@@ -3,4 +3,35 @@
 #include "libimp/platform/win/codecvt.h"
 #else
 #include "libimp/platform/posix/codecvt.h"
-#endif
\ No newline at end of file
+#endif
+
+#include <algorithm>
+#include <cstring>
+
+LIBIMP_NAMESPACE_BEG_
+
+namespace {
+
+template <typename CharT>
+std::size_t cpy_cstr(CharT *des, std::size_t dlen, CharT const *src, std::size_t slen) noexcept {
+  if ((des == nullptr) || (dlen == 0)) {
+    return slen;
+  }
+  std::size_t r = (std::min)(slen, dlen);
+  std::memcpy(des, src, r * sizeof(CharT));
+  return r;
+}
+
+} // namespace
+
+template <>
+std::size_t cvt_cstr(char const *src, std::size_t slen, char *des, std::size_t dlen) noexcept {
+  return cpy_cstr(des, dlen, src, slen);
+}
+
+template <>
+std::size_t cvt_cstr(wchar_t const *src, std::size_t slen, wchar_t *des, std::size_t dlen) noexcept {
+  return cpy_cstr(des, dlen, src, slen);
+}
+
+LIBIMP_NAMESPACE_END_
diff --git a/test/test_imp_codecvt.cpp b/test/test_imp_codecvt.cpp
index 722d58c..34d5b12 100644
--- a/test/test_imp_codecvt.cpp
+++ b/test/test_imp_codecvt.cpp
@@ -22,8 +22,22 @@ TEST(codecvt, cvt_cstr) {
   {
     auto cvt_len = imp::cvt_cstr(utf16, std::wcslen(utf16), (char *)nullptr, 0);
     EXPECT_NE(cvt_len, 0);
-    std::string str(cvt_len, L'\0');
+    std::string str(cvt_len, '\0');
     EXPECT_EQ(imp::cvt_cstr(utf16, std::wcslen(utf16), &str[0], str.size()), cvt_len);
     EXPECT_STREQ(str.c_str(), utf8);
   }
+  {
+    auto cvt_len = imp::cvt_cstr(utf8, std::strlen(utf8), (char *)nullptr, 0);
+    EXPECT_EQ(cvt_len, std::strlen(utf8));
+    std::string str(cvt_len, '\0');
+    EXPECT_EQ(imp::cvt_cstr(utf8, cvt_len, &str[0], str.size()), cvt_len);
+    EXPECT_STREQ(str.c_str(), utf8);
+  }
+  {
+    auto cvt_len = imp::cvt_cstr(utf16, std::wcslen(utf16), (wchar_t *)nullptr, 0);
+    EXPECT_EQ(cvt_len, std::wcslen(utf16));
+    std::wstring wstr(cvt_len, L'\0');
+    EXPECT_EQ(imp::cvt_cstr(utf16, cvt_len, &wstr[0], wstr.size()), cvt_len);
+    EXPECT_STREQ(wstr.c_str(), utf16);
+  }
 }
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