Add strnlen() wrapper and its test

util/stdlib/strnlen.h

Index: util/stdlib/strnlen.h
diff --git a/util/stdlib/strnlen.h b/util/stdlib/strnlen.h
new file mode 100644
index 0000000000000000000000000000000000000000..f1bbe63829ac30420508dae97c4628a4a72d5bf4
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+++ b/util/stdlib/strnlen.h
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+// Copyright 2014 The Crashpad Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef CRASHPAD_UTIL_STDLIB_STRNLEN_H_
+#define CRASHPAD_UTIL_STDLIB_STRNLEN_H_
+
+#include <string.h>
+
+#include "build/build_config.h"
+
+#if defined(OS_MACOSX)
+#include <AvailabilityMacros.h>
+#endif
+
+namespace crashpad {
+
+//! \brief Returns the length of a string, not to exceed a maximum.
+//!
+//! \param[in] string The string whose length is to be calculated.
+//! \param[in] max_length The maximum length to return.
+//!
+//! \return The length of \a string, determined as the index of the first `NUL`
+//!     byte found, not exceeding \a max_length.
+//!
+//! \note This function is provided because it was introduced in POSIX.1-2008,
+//!     and not all systems’ standard libraries provide an implementation.
+size_t strnlen(const char* string, size_t max_length);
+
+#if !defined(OS_MACOSX) || \
+    MAC_OS_X_VERSION_MIN_REQUIRED >= MAC_OS_X_VERSION_10_7
+inline size_t strnlen(const char* string, size_t max_length) {
+  return ::strnlen(string, max_length);
+}
+#endif
+
+}  // namespace crashpad
+
+#endif  // CRASHPAD_UTIL_STDLIB_STRNLEN_H_