1 /*
2     Copyright (c) 2007-2016 Contributors as noted in the AUTHORS file
3 
4     This file is part of libzmq, the ZeroMQ core engine in C++.
5 
6     libzmq is free software; you can redistribute it and/or modify it under
7     the terms of the GNU Lesser General Public License (LGPL) as published
8     by the Free Software Foundation; either version 3 of the License, or
9     (at your option) any later version.
10 
11     As a special exception, the Contributors give you permission to link
12     this library with independent modules to produce an executable,
13     regardless of the license terms of these independent modules, and to
14     copy and distribute the resulting executable under terms of your choice,
15     provided that you also meet, for each linked independent module, the
16     terms and conditions of the license of that module. An independent
17     module is a module which is not derived from or based on this library.
18     If you modify this library, you must extend this exception to your
19     version of the library.
20 
21     libzmq is distributed in the hope that it will be useful, but WITHOUT
22     ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
23     FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
24     License for more details.
25 
26     You should have received a copy of the GNU Lesser General Public License
27     along with this program.  If not, see <http://www.gnu.org/licenses/>.
28 */
29 
30 #ifndef __ZMQ_DBUFFER_HPP_INCLUDED__
31 #define __ZMQ_DBUFFER_HPP_INCLUDED__
32 
33 #include <stdlib.h>
34 #include <stddef.h>
35 #include <algorithm>
36 
37 #include "mutex.hpp"
38 #include "msg.hpp"
39 
40 namespace zmq
41 {
42 //  dbuffer is a single-producer single-consumer double-buffer
43 //  implementation.
44 //
45 //  The producer writes to a back buffer and then tries to swap
46 //  pointers between the back and front buffers. If it fails,
47 //  due to the consumer reading from the front buffer, it just
48 //  gives up, which is ok since writes are many and redundant.
49 //
50 //  The reader simply reads from the front buffer.
51 //
52 //  has_msg keeps track of whether there has been a not yet read
53 //  value written, it is used by ypipe_conflate to mimic ypipe
54 //  functionality regarding a reader being asleep
55 
56 template <typename T> class dbuffer_t;
57 
58 template <> class dbuffer_t<msg_t>
59 {
60   public:
dbuffer_t()61     dbuffer_t () : _back (&_storage[0]), _front (&_storage[1]), _has_msg (false)
62     {
63         _back->init ();
64         _front->init ();
65     }
66 
~dbuffer_t()67     ~dbuffer_t ()
68     {
69         _back->close ();
70         _front->close ();
71     }
72 
write(const msg_t & value_)73     void write (const msg_t &value_)
74     {
75         zmq_assert (value_.check ());
76         *_back = value_;
77 
78         zmq_assert (_back->check ());
79 
80         if (_sync.try_lock ()) {
81             _front->move (*_back);
82             _has_msg = true;
83 
84             _sync.unlock ();
85         }
86     }
87 
read(msg_t * value_)88     bool read (msg_t *value_)
89     {
90         if (!value_)
91             return false;
92 
93         {
94             scoped_lock_t lock (_sync);
95             if (!_has_msg)
96                 return false;
97 
98             zmq_assert (_front->check ());
99 
100             *value_ = *_front;
101             _front->init (); // avoid double free
102 
103             _has_msg = false;
104             return true;
105         }
106     }
107 
108 
check_read()109     bool check_read ()
110     {
111         scoped_lock_t lock (_sync);
112 
113         return _has_msg;
114     }
115 
probe(bool (* fn_)(const msg_t &))116     bool probe (bool (*fn_) (const msg_t &))
117     {
118         scoped_lock_t lock (_sync);
119         return (*fn_) (*_front);
120     }
121 
122 
123   private:
124     msg_t _storage[2];
125     msg_t *_back, *_front;
126 
127     mutex_t _sync;
128     bool _has_msg;
129 
130     ZMQ_NON_COPYABLE_NOR_MOVABLE (dbuffer_t)
131 };
132 }
133 
134 #endif
135