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      1 // Copyright (c) 2009 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_GLOBAL_DESCRIPTORS_POSIX_H_
      6 #define BASE_GLOBAL_DESCRIPTORS_POSIX_H_
      7 
      8 #include "build/build_config.h"
      9 
     10 #include <vector>
     11 #include <utility>
     12 
     13 #include <stdint.h>
     14 
     15 #include "base/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 GlobalDescriptors {
     37  public:
     38   typedef uint32_t Key;
     39   // Often we want a canonical descriptor for a given Key. In this case, we add
     40   // the following constant to the key value:
     41   static const int kBaseDescriptor = 3;  // 0, 1, 2 are already taken.
     42 
     43   // Get a descriptor given a key. It is a fatal error if the key is not known.
     44   int Get(Key key) const;
     45   // Get a descriptor give a key. Returns -1 on error.
     46   int MaybeGet(Key key) const;
     47 
     48   typedef std::vector<std::pair<Key, int> > Mapping;
     49 
     50   // Set the descriptor for the given key.
     51   void Set(Key key, int fd);
     52 
     53   void Reset(const Mapping& mapping) {
     54     descriptors_ = mapping;
     55   }
     56 
     57  private:
     58   GlobalDescriptors() { }
     59   friend struct DefaultSingletonTraits<GlobalDescriptors>;
     60 
     61   Mapping descriptors_;
     62 };
     63 
     64 }  // namespace base
     65 
     66 #endif  // BASE_GLOBAL_DESCRIPTORS_POSIX_H_
     67