summaryrefslogtreecommitdiff
path: root/src/ypipe.hpp
blob: 445b487e37698c11b65208ab0f432e7eaf10f3a3 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
/*
    Copyright (c) 2007-2010 iMatix Corporation

    This file is part of 0MQ.

    0MQ is free software; you can redistribute it and/or modify it under
    the terms of the Lesser GNU General Public License as published by
    the Free Software Foundation; either version 3 of the License, or
    (at your option) any later version.

    0MQ is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    Lesser GNU General Public License for more details.

    You should have received a copy of the Lesser GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#ifndef __ZMQ_YPIPE_HPP_INCLUDED__
#define __ZMQ_YPIPE_HPP_INCLUDED__

#include "atomic_ptr.hpp"
#include "yqueue.hpp"
#include "platform.hpp"

namespace zmq
{

    //  Lock-free queue implementation.
    //  Only a single thread can read from the pipe at any specific moment.
    //  Only a single thread can write to the pipe at any specific moment.
    //  T is the type of the object in the queue.
    //  N is granularity of the pipe, i.e. how many messages are needed to
    //  perform next memory allocation.

    template <typename T, int N> class ypipe_t
    {
    public:

        //  Initialises the pipe.
        inline ypipe_t ()
        {
            //  Insert terminator element into the queue.
            queue.push ();

            //  Let all the pointers to point to the terminator.
            //  (unless pipe is dead, in which case c is set to NULL).
            r = w = &queue.back ();
            c.set (&queue.back ());
        }

        //  Following function (write) deliberately copies uninitialised data
        //  when used with zmq_msg. Initialising the VSM body for
        //  non-VSM messages won't be good for performance.

#ifdef ZMQ_HAVE_OPENVMS
#pragma message save
#pragma message disable(UNINIT)
#endif

        //  Write an item to the pipe.  Don't flush it yet.
        inline void write (const T &value_)
        {
            //  Place the value to the queue, add new terminator element.
            queue.back () = value_;
            queue.push ();
        }

#ifdef ZMQ_HAVE_OPENVMS
#pragma message restore
#endif

        //  Pop an unflushed message from the pipe. Returns true is such
        //  message exists, false otherwise.
        inline bool unwrite (T *value_)
        {
            if (w == &queue.back ())
                return false;
            queue.unpush ();
            *value_ = queue.back ();
            return true;
        }

        //  Flush the messages into the pipe. Returns false if the reader
        //  thread is sleeping. In that case, caller is obliged to wake the
        //  reader up before using the pipe again.
        inline bool flush ()
        {
            //  If there are no un-flushed items, do nothing.
            if (w == &queue.back ())
                return true;

            //  Try to set 'c' to 'back'
            if (c.cas (w, &queue.back ()) != w) {

                //  Compare-and-swap was unseccessful because 'c' is NULL.
                //  This means that the reader is asleep. Therefore we don't
                //  care about thread-safeness and update c in non-atomic
                //  manner. We'll return false to let the caller know
                //  that reader is sleeping.
                c.set (&queue.back ());
                w = &queue.back ();
                return false;
            }

            //  Reader is alive. Nothing special to do now. Just move
            //  the 'first un-flushed item' pointer to the end of the queue.
            w = &queue.back ();
            return true;
        }

        //  Check whether item is available for reading.
        inline bool check_read ()
        {
            //  Was the value prefetched already? If so, return.
            if (&queue.front () != r)
                 return true;

            //  There's no prefetched value, so let us prefetch more values.
            //  Prefetching is to simply retrieve the
            //  pointer from c in atomic fashion. If there are no
            //  items to prefetch, set c to NULL (using compare-and-swap).
            r = c.cas (&queue.front (), NULL);

            //  If there are no elements prefetched, exit.
            //  During pipe's lifetime r should never be NULL, however,
            //  it can happen during pipe shutdown when messages
            //  are being deallocated.
            if (&queue.front () == r || !r)
                return false;

            //  There was at least one value prefetched.
            return true;
        }

        //  Reads an item from the pipe. Returns false if there is no value.
        //  available.
        inline bool read (T *value_)
        {
            //  Try to prefetch a value.
            if (!check_read ())
                return false;

            //  There was at least one value prefetched.
            //  Return it to the caller.
            *value_ = queue.front ();
            queue.pop ();
            return true;
        }

    protected:

        //  Allocation-efficient queue to store pipe items.
        //  Front of the queue points to the first prefetched item, back of
        //  the pipe points to last un-flushed item. Front is used only by
        //  reader thread, while back is used only by writer thread.
        yqueue_t <T, N> queue;

        //  Points to the first un-flushed item. This variable is used
        //  exclusively by writer thread.
        T *w;

        //  Points to the first un-prefetched item. This variable is used
        //  exclusively by reader thread.
        T *r;

        //  The single point of contention between writer and reader thread.
        //  Points past the last flushed item. If it is NULL,
        //  reader is asleep. This pointer should be always accessed using
        //  atomic operations.
        atomic_ptr_t <T> c;

        //  Disable copying of ypipe object.
        ypipe_t (const ypipe_t&);
        void operator = (const ypipe_t&);
    };

}

#endif