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 #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_