ypipe.hpp 7.09 KB
Newer Older
Martin Sustrik's avatar
Martin Sustrik committed
1
/*
2
    Copyright (c) 2007-2016 Contributors as noted in the AUTHORS file
Martin Sustrik's avatar
Martin Sustrik committed
3

4
    This file is part of libzmq, the ZeroMQ core engine in C++.
Martin Sustrik's avatar
Martin Sustrik committed
5

6 7 8
    libzmq is free software; you can redistribute it and/or modify it under
    the terms of the GNU Lesser General Public License (LGPL) as published
    by the Free Software Foundation; either version 3 of the License, or
Martin Sustrik's avatar
Martin Sustrik committed
9 10
    (at your option) any later version.

11 12 13 14 15 16 17 18 19 20 21 22 23 24
    As a special exception, the Contributors give you permission to link
    this library with independent modules to produce an executable,
    regardless of the license terms of these independent modules, and to
    copy and distribute the resulting executable under terms of your choice,
    provided that you also meet, for each linked independent module, the
    terms and conditions of the license of that module. An independent
    module is a module which is not derived from or based on this library.
    If you modify this library, you must extend this exception to your
    version of the library.

    libzmq 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 GNU Lesser General Public
    License for more details.
Martin Sustrik's avatar
Martin Sustrik committed
25

26
    You should have received a copy of the GNU Lesser General Public License
Martin Sustrik's avatar
Martin Sustrik committed
27 28 29
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

Martin Sustrik's avatar
Martin Sustrik committed
30 31
#ifndef __ZMQ_YPIPE_HPP_INCLUDED__
#define __ZMQ_YPIPE_HPP_INCLUDED__
Martin Sustrik's avatar
Martin Sustrik committed
32 33 34

#include "atomic_ptr.hpp"
#include "yqueue.hpp"
35
#include "ypipe_base.hpp"
Martin Sustrik's avatar
Martin Sustrik committed
36

Martin Sustrik's avatar
Martin Sustrik committed
37
namespace zmq
Martin Sustrik's avatar
Martin Sustrik committed
38
{
39 40 41 42 43 44 45 46 47 48 49 50
//  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 items are needed to
//  perform next memory allocation.

template <typename T, int N> class ypipe_t : public ypipe_base_t<T>
{
  public:
    //  Initialises the pipe.
    inline ypipe_t ()
Martin Sustrik's avatar
Martin Sustrik committed
51
    {
52 53
        //  Insert terminator element into the queue.
        queue.push ();
Martin Sustrik's avatar
Martin Sustrik committed
54

55 56 57 58 59 60 61 62 63
        //  Let all the pointers to point to the terminator.
        //  (unless pipe is dead, in which case c is set to NULL).
        r = w = f = &queue.back ();
        c.set (&queue.back ());
    }

    //  The destructor doesn't have to be virtual. It is made virtual
    //  just to keep ICC and code checking tools from complaining.
    inline virtual ~ypipe_t () {}
64

65 66 67
        //  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.
Martin Sustrik's avatar
Martin Sustrik committed
68 69 70 71 72 73

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

74 75 76 77 78 79 80 81 82 83 84 85 86 87
    //  Write an item to the pipe.  Don't flush it yet. If incomplete is
    //  set to true the item is assumed to be continued by items
    //  subsequently written to the pipe. Incomplete items are never
    //  flushed down the stream.
    inline void write (const T &value_, bool incomplete_)
    {
        //  Place the value to the queue, add new terminator element.
        queue.back () = value_;
        queue.push ();

        //  Move the "flush up to here" poiter.
        if (!incomplete_)
            f = &queue.back ();
    }
Martin Sustrik's avatar
Martin Sustrik committed
88 89 90 91 92

#ifdef ZMQ_HAVE_OPENVMS
#pragma message restore
#endif

93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110
    //  Pop an incomplete item from the pipe. Returns true if such
    //  item exists, false otherwise.
    inline bool unwrite (T *value_)
    {
        if (f == &queue.back ())
            return false;
        queue.unpush ();
        *value_ = queue.back ();
        return true;
    }

    //  Flush all the completed items 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 == f)
111 112
            return true;

113 114 115 116 117 118 119 120
        //  Try to set 'c' to 'f'.
        if (c.cas (w, f) != 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 (f);
121
            w = f;
122
            return false;
Martin Sustrik's avatar
Martin Sustrik committed
123 124
        }

125 126 127 128 129
        //  Reader is alive. Nothing special to do now. Just move
        //  the 'first un-flushed item' pointer to 'f'.
        w = f;
        return true;
    }
130

131 132 133 134 135
    //  Check whether item is available for reading.
    inline bool check_read ()
    {
        //  Was the value prefetched already? If so, return.
        if (&queue.front () != r && r)
Martin Sustrik's avatar
Martin Sustrik committed
136 137
            return true;

138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208
        //  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 items
        //  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;
    }

    //  Applies the function fn to the first elemenent in the pipe
    //  and returns the value returned by the fn.
    //  The pipe mustn't be empty or the function crashes.
    inline bool probe (bool (*fn) (const T &))
    {
        bool rc = check_read ();
        zmq_assert (rc);

        return (*fn) (queue.front ());
    }

  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;

    //  Points to the first item to be flushed in the future.
    T *f;

    //  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 &);
    const ypipe_t &operator= (const ypipe_t &);
};
Martin Sustrik's avatar
Martin Sustrik committed
209 210 211
}

#endif