1// Copyright (c) 2012 The Chromium Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5#ifndef BASE_POSIX_GLOBAL_DESCRIPTORS_H_
6#define BASE_POSIX_GLOBAL_DESCRIPTORS_H_
7
8#include "build/build_config.h"
9
10#include <vector>
11#include <utility>
12
13#include <stdint.h>
14
15#include "base/memory/singleton.h"
16
17namespace base {
18
19// It's common practice to install file descriptors into well known slot
20// numbers before execing a child; stdin, stdout and stderr are ubiqutous
21// examples.
22//
23// However, when using a zygote model, this becomes troublesome. Since the
24// descriptors which need to be in these slots generally aren't known, any code
25// could open a resource and take one of the reserved descriptors. Simply
26// overwriting the slot isn't a viable solution.
27//
28// We could try to fill the reserved slots as soon as possible, but this is a
29// fragile solution since global constructors etc are able to open files.
30//
31// Instead, we retreat from the idea of installing descriptors in specific
32// slots and add a layer of indirection in the form of this singleton object.
33// It maps from an abstract key to a descriptor. If independent modules each
34// need to define keys, then values should be chosen randomly so as not to
35// collide.
36class BASE_EXPORT GlobalDescriptors {
37 public:
38  typedef uint32_t Key;
39  typedef std::pair<Key, int> KeyFDPair;
40  typedef std::vector<KeyFDPair> Mapping;
41
42  // Often we want a canonical descriptor for a given Key. In this case, we add
43  // the following constant to the key value:
44#if !defined(OS_ANDROID)
45  static const int kBaseDescriptor = 3;  // 0, 1, 2 are already taken.
46#else
47  static const int kBaseDescriptor = 4;  // 3 used by __android_log_write().
48#endif
49
50  // Return the singleton instance of GlobalDescriptors.
51  static GlobalDescriptors* GetInstance();
52
53  // Get a descriptor given a key. It is a fatal error if the key is not known.
54  int Get(Key key) const;
55
56  // Get a descriptor give a key. Returns -1 on error.
57  int MaybeGet(Key key) const;
58
59  // Set the descriptor for the given key.
60  void Set(Key key, int fd);
61
62  void Reset(const Mapping& mapping);
63
64 private:
65  friend struct DefaultSingletonTraits<GlobalDescriptors>;
66  GlobalDescriptors();
67  ~GlobalDescriptors();
68
69  Mapping descriptors_;
70};
71
72}  // namespace base
73
74#endif  // BASE_POSIX_GLOBAL_DESCRIPTORS_H_
75