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_ 75
