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diff --git a/src/posix_extras.h b/src/posix_extras.h
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+// Copyright 2016 Benjamin Barenblat
+//
+// 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 POSIX_EXTRAS_H_
+#define POSIX_EXTRAS_H_
+
+#include <stdexcept>
+#include <string>
+
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+namespace scoville {
+
+class IoError : public std::runtime_error {
+ public:
+  explicit IoError(const int number)
+      : std::runtime_error(Message(number)), number_(number) {}
+
+  int number() const noexcept { return number_; }
+
+  // Converts an errno into a human-readable message, a la strerror(3).
+  // Thread-safe.
+  static std::string Message(int) noexcept;
+
+ private:
+  int number_;
+};
+
+// RAII wrapper for Unix file descriptors.
+class File {
+ public:
+  File(const char* path, int flags);
+  virtual ~File() noexcept;
+
+  const std::string& path() const noexcept { return path_; }
+
+  // Calls fstat(2) on the file descriptor.
+  struct stat Stat() const;
+
+  // Calls lstat(2) on the path relative to the file descriptor.  The path must
+  // indeed be relative (i.e., it must not start with '/').
+  struct stat LinkStatAt(const char* path) const;
+
+ private:
+  File(File&) = delete;
+
+  void operator=(File) = delete;
+
+  std::string path_;
+  int fd_;
+};
+
+}  // scoville
+
+#endif  // POSIX_EXTRAS_H_