strnlen is not always available.

runtime/vm/os_macos.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"
 #if defined(TARGET_OS_MACOS)
 
 #include "vm/os.h"
 
 #include <errno.h>  // NOLINT
@@ skipping 261 matching lines @@
     return NULL;
   }
   result[len] = '\0';
   return reinterpret_cast<char*>(memmove(result, s, len));
 #else  // !defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) || ...
   return strndup(s, n);
 #endif  // !defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) || ...
 }
 
 
+intptr_t OS::StrNLen(const char* s, intptr_t n) {
+  // strnlen has only been added to Mac OS X in 10.7. We are supplying
+  // our own copy here if needed.
+#if !defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) || \
+    __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ <= 1060
+  intptr_t len = 0;
+  while ((len <= n) && (*s != '\0')) {
+    s++;
+    len++;
+  }
+  return len;
+#else  // !defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) || ...
+  return strnlen(s, n);
+#endif  // !defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) || ...
+}
+
+
 void OS::Print(const char* format, ...) {
 #if TARGET_OS_IOS
   va_list args;
   va_start(args, format);
   vsyslog(LOG_INFO, format, args);
   va_end(args);
 #else
   va_list args;
   va_start(args, format);
   VFPrint(stdout, format, args);
@@ skipping 118 matching lines @@
 }
 
 
 void OS::Exit(int code) {
   exit(code);
 }
 
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_MACOS)