diff --git a/gsl/string_span b/gsl/string_span
index fa3d2ff..415744b 100644
--- a/gsl/string_span
+++ b/gsl/string_span
@@ -56,6 +56,10 @@
 #endif // _MSC_VER <= 1800
 #endif // _MSC_VER
 
+#ifndef __CYGWIN__
+#define GSL_PLATFORM_HAS_STRNLEN
+#endif
+
 // In order to test the library, we need it to throw exceptions that we can catch
 #ifdef GSL_THROW_ON_CONTRACT_VIOLATION
 
@@ -95,6 +99,27 @@ using zstring = basic_zstring<char, Extent>;
 template <std::ptrdiff_t Extent = dynamic_extent>
 using wzstring = basic_zstring<wchar_t, Extent>;
 
+namespace details
+{
+    inline std::size_t string_length(const char *str, std::size_t n)
+    {
+#ifdef GSL_PLATFORM_HAS_STRNLEN
+        return strnlen(str, n);
+#else
+        if (str == nullptr || n == 0)
+            return 0;
+
+        std::size_t len = 0;
+        span<const char> str_span{str, n};
+
+        while (len < n && str_span[len])
+            len++;
+
+        return len;
+#endif
+    }
+}
+
 //
 // ensure_sentinel()
 //
@@ -127,14 +152,14 @@ inline span<T, dynamic_extent> ensure_z(T* const& sz, std::ptrdiff_t max = PTRDI
 // overloads to share an implementation
 inline span<char, dynamic_extent> ensure_z(char* const& sz, std::ptrdiff_t max)
 {
-    auto len = strnlen(sz, narrow_cast<size_t>(max));
+    auto len = details::string_length(sz, narrow_cast<size_t>(max));
     Ensures(sz[len] == 0);
     return {sz, static_cast<std::ptrdiff_t>(len)};
 }
 
 inline span<const char, dynamic_extent> ensure_z(const char* const& sz, std::ptrdiff_t max)
 {
-    auto len = strnlen(sz, narrow_cast<size_t>(max));
+    auto len = details::string_length(sz, narrow_cast<size_t>(max));
     Ensures(sz[len] == 0);
     return {sz, static_cast<std::ptrdiff_t>(len)};
 }
@@ -196,7 +221,7 @@ namespace details
     {
         std::ptrdiff_t operator()(char* const ptr, std::ptrdiff_t length) noexcept
         {
-            return narrow_cast<std::ptrdiff_t>(strnlen(ptr, narrow_cast<size_t>(length)));
+            return narrow_cast<std::ptrdiff_t>(details::string_length(ptr, narrow_cast<size_t>(length)));
         }
     };
 
@@ -214,7 +239,7 @@ namespace details
     {
         std::ptrdiff_t operator()(const char* const ptr, std::ptrdiff_t length) noexcept
         {
-            return narrow_cast<std::ptrdiff_t>(strnlen(ptr, narrow_cast<size_t>(length)));
+            return narrow_cast<std::ptrdiff_t>(details::string_length(ptr, narrow_cast<size_t>(length)));
         }
     };