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global_descriptors_posix.h

// Copyright (c) 2009 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_GLOBAL_DESCRIPTORS_POSIX_H_
#define BASE_GLOBAL_DESCRIPTORS_POSIX_H_

#include "build/build_config.h"

#include <vector>
#include <utility>

#include <stdint.h>

#include "base/singleton.h"

namespace base {

// It's common practice to install file descriptors into well known slot
// numbers before execing a child; stdin, stdout and stderr are ubiqutous
// examples.
//
// However, when using a zygote model, this becomes troublesome. Since the
// descriptors which need to be in these slots generally aren't known, any code
// could open a resource and take one of the reserved descriptors. Simply
// overwriting the slot isn't a viable solution.
//
// We could try to fill the reserved slots as soon as possible, but this is a
// fragile solution since global constructors etc are able to open files.
//
// Instead, we retreat from the idea of installing descriptors in specific
// slots and add a layer of indirection in the form of this singleton object.
// It maps from an abstract key to a descriptor. If independent modules each
// need to define keys, then values should be chosen randomly so as not to
// collide.
class GlobalDescriptors {
 public:
  typedef uint32_t Key;
  // Often we want a canonical descriptor for a given Key. In this case, we add
  // the following constant to the key value:
  static const int kBaseDescriptor = 3;  // 0, 1, 2 are already taken.

  // Get a descriptor given a key. It is a fatal error if the key is not known.
  int Get(Key key) const;
  // Get a descriptor give a key. Returns -1 on error.
  int MaybeGet(Key key) const;

  typedef std::vector<std::pair<Key, int> > Mapping;

  // Set the descriptor for the given key.
  void Set(Key key, int fd);

  void Reset(const Mapping& mapping) {
    descriptors_ = mapping;
  }

 private:
  GlobalDescriptors() { }
  friend struct DefaultSingletonTraits<GlobalDescriptors>;

  Mapping descriptors_;
};

}  // namespace base

#endif  // BASE_GLOBAL_DESCRIPTORS_POSIX_H_

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