base/posix/global_descriptors.h

   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
  17 namespace 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.
  36 class 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   static const int kBaseDescriptor = 3;  // 0, 1, 2 are already taken.
  45
  46   // Return the singleton instance of GlobalDescriptors.
  47   static GlobalDescriptors* GetInstance();
  48
  49   // Get a descriptor given a key. It is a fatal error if the key is not known.
  50   int Get(Key key) const;
  51
  52   // Get a descriptor give a key. Returns -1 on error.
  53   int MaybeGet(Key key) const;
  54
  55   // Set the descriptor for the given key.
  56   void Set(Key key, int fd);
  57
  58   void Reset(const Mapping& mapping);
  59
  60  private:
  61   friend struct DefaultSingletonTraits<GlobalDescriptors>;
  62   GlobalDescriptors();
  63   ~GlobalDescriptors();
  64
  65   Mapping descriptors_;
  66 };
  67
  68 }  // namespace base
  69
  70 #endif  // BASE_POSIX_GLOBAL_DESCRIPTORS_H_