Merge pull request #1417 from jens-auer/zero_copy

Zero copy message receive in v2_decoder

src/atomic_counter.hpp

@@ -183,7 +183,7 @@ namespace zmq
 #endif
         }
 
-        inline integer_t get ()
+        inline integer_t get () const
         {
             return value;
         }

src/decoder.hpp

@@ -42,6 +42,45 @@
 
 namespace zmq
 {
+    // Static buffer policy.
+    class c_single_allocator
+    {
+    public:
+        c_single_allocator(size_t bufsize_):
+                buf((unsigned char*) malloc (bufsize) ),
+                bufsize(bufsize_)
+        {
+            alloc_assert (buf);
+        }
+
+        ~c_single_allocator()
+        {
+            free(buf);
+        }
+
+        unsigned char* allocate()
+        {
+            return buf;
+        }
+
+        void deallocate()
+        {
+
+        }
+
+        size_t size() const
+        {
+            return bufsize;
+        }
+
+    private:
+        unsigned char* buf;
+        size_t bufsize;
+
+        c_single_allocator( c_single_allocator const& );
+        c_single_allocator& operator=(c_single_allocator const&);
+    };
+
     //  Helper base class for decoders that know the amount of data to read
     //  in advance at any moment. Knowing the amount in advance is a property
     //  of the protocol used. 0MQ framing protocol is based size-prefixed
@@ -52,31 +91,34 @@ namespace zmq
     //
     //  This class implements the state machine that parses the incoming buffer.
     //  Derived class should implement individual state machine actions.
-
-    template <typename T> class decoder_base_t : public i_decoder
+    //
+    //  Buffer managment is done by an allocator policy.
+    template <typename T, typename A = c_single_allocator>
+    class decoder_base_t : public i_decoder
     {
     public:
 
-        inline decoder_base_t (size_t bufsize_) :
-            next (NULL),
-            read_pos (NULL),
-            to_read (0),
-            bufsize (bufsize_)
+        inline decoder_base_t (A* allocator_) :
+                next (NULL),
+                read_pos (NULL),
+                to_read (0),
+                allocator( allocator_ )
         {
-            buf = (unsigned char*) malloc (bufsize_);
-            alloc_assert (buf);
+            buf = allocator->allocate();
         }
 
         //  The destructor doesn't have to be virtual. It is mad virtual
         //  just to keep ICC and code checking tools from complaining.
         inline virtual ~decoder_base_t ()
         {
-            free (buf);
+            allocator->deallocate();
         }
 
         //  Returns a buffer to be filled with binary data.
         inline void get_buffer (unsigned char **data_, size_t *size_)
         {
+            buf = allocator->allocate();
+
             //  If we are expected to read large message, we'll opt for zero-
             //  copy, i.e. we'll ask caller to fill the data directly to the
             //  message. Note that subsequent read(s) are non-blocking, thus
@@ -85,14 +127,14 @@ namespace zmq
             //  As a consequence, large messages being received won't block
             //  other engines running in the same I/O thread for excessive
             //  amounts of time.
-            if (to_read >= bufsize) {
+            if (to_read >= allocator->size()) {
                 *data_ = read_pos;
                 *size_ = to_read;
                 return;
             }
 
             *data_ = buf;
-            *size_ = bufsize;
+            *size_ = allocator->size();
         }
 
         //  Processes the data in the buffer previously allocated using
@@ -116,7 +158,7 @@ namespace zmq
                 bytes_used_ = size_;
 
                 while (!to_read) {
-                    const int rc = (static_cast <T*> (this)->*next) ();
+                    const int rc = (static_cast <T*> (this)->*next) (data_ + bytes_used_);
                     if (rc != 0)
                         return rc;
                 }
@@ -126,14 +168,20 @@ namespace zmq
             while (bytes_used_ < size_) {
                 //  Copy the data from buffer to the message.
                 const size_t to_copy = std::min (to_read, size_ - bytes_used_);
-                memcpy (read_pos, data_ + bytes_used_, to_copy);
+                // only copy when the destination address is different from the
+                // current address in the buffer
+                if (read_pos != data_ + bytes_used_) {
+                    memcpy(read_pos, data_ + bytes_used_, to_copy);
+                }
+
                 read_pos += to_copy;
                 to_read -= to_copy;
                 bytes_used_ += to_copy;
                 //  Try to get more space in the message to fill in.
                 //  If none is available, return.
                 while (to_read == 0) {
-                    const int rc = (static_cast <T*> (this)->*next) ();
+                    // pass current address in the buffer
+                    const int rc = (static_cast <T*> (this)->*next) (data_ + bytes_used_);
                     if (rc != 0)
                         return rc;
                 }
@@ -146,7 +194,7 @@ namespace zmq
 
         //  Prototype of state machine action. Action should return false if
         //  it is unable to push the data to the system.
-        typedef int (T::*step_t) ();
+        typedef int (T::*step_t) (unsigned char const*);
 
         //  This function should be called from derived class to read data
         //  from the buffer and schedule next state machine action.
@@ -171,8 +219,8 @@ namespace zmq
         size_t to_read;
 
         //  The duffer for data to decode.
-        size_t bufsize;
-        unsigned char *buf;
+        A* allocator;
+        unsigned char* buf;
 
         decoder_base_t (const decoder_base_t&);
         const decoder_base_t &operator = (const decoder_base_t&);

src/msg.cpp

@@ -45,11 +45,30 @@
 typedef char zmq_msg_size_check
     [2 * ((sizeof (zmq::msg_t) == sizeof (zmq_msg_t)) != 0) - 1];
 
+// check whether the size of atomic_counter_t matches the size of the wrapped integer
+// to ensure that the lsmg union is correctly aligned
+typedef char zmq_msg_size_check
+    [2 * ((sizeof (zmq::atomic_counter_t) == sizeof (zmq::atomic_counter_t::integer_t)) != 0) - 1];
+
 bool zmq::msg_t::check ()
 {
      return u.base.type >= type_min && u.base.type <= type_max;
 }
 
+int zmq::msg_t::init (void *data_, size_t size_, msg_free_fn *ffn_, void *hint_)
+{
+    if (size_ <= max_vsm_size)
+    {
+        int rc = init_size(size_);
+        memcpy(data(), data_, size_);
+        return rc;
+    }
+    else
+    {
+        return init_data(data_, size_, ffn_, hint_);
+    }
+}
+
 int zmq::msg_t::init ()
 {
     u.vsm.metadata = NULL;
@@ -76,18 +95,16 @@ int zmq::msg_t::init_size (size_t size_)
         u.lmsg.type = type_lmsg;
         u.lmsg.flags = 0;
         u.lmsg.routing_id = 0;
-        u.lmsg.content =
-            (content_t*) malloc (sizeof (content_t) + size_);
-        if (unlikely (!u.lmsg.content)) {
+        u.lmsg.data = malloc(size_);
+        if (unlikely (!u.lmsg.data)) {
             errno = ENOMEM;
             return -1;
         }
 
-        u.lmsg.content->data = u.lmsg.content + 1;
-        u.lmsg.content->size = size_;
-        u.lmsg.content->ffn = NULL;
-        u.lmsg.content->hint = NULL;
-        new (&u.lmsg.content->refcnt) zmq::atomic_counter_t ();
+        u.lmsg.size = size_;
+        u.lmsg.ffn = NULL;
+        u.lmsg.hint = NULL;
+        new (&u.lmsg.refcnt.counter) zmq::atomic_counter_t ();
     }
     return 0;
 }
@@ -115,17 +132,12 @@ int zmq::msg_t::init_data (void *data_, size_t size_, msg_free_fn *ffn_,
         u.lmsg.type = type_lmsg;
         u.lmsg.flags = 0;
         u.lmsg.routing_id = 0;
-        u.lmsg.content = (content_t*) malloc (sizeof (content_t));
-        if (!u.lmsg.content) {
-            errno = ENOMEM;
-            return -1;
-        }
 
-        u.lmsg.content->data = data_;
-        u.lmsg.content->size = size_;
-        u.lmsg.content->ffn = ffn_;
-        u.lmsg.content->hint = hint_;
-        new (&u.lmsg.content->refcnt) zmq::atomic_counter_t ();
+        u.lmsg.data = data_;
+        u.lmsg.size = size_;
+        u.lmsg.ffn = ffn_;
+        u.lmsg.hint = hint_;
+        new (&u.lmsg.refcnt.counter) zmq::atomic_counter_t ();
     }
     return 0;
 
@@ -140,6 +152,13 @@ int zmq::msg_t::init_delimiter ()
     return 0;
 }
 
+zmq::atomic_counter_t& zmq::msg_t::msg_counter()
+{
+    zmq_assert( is_lmsg() );
+    void* ptr = static_cast<void*>( &u.lmsg.refcnt.counter );
+    return *static_cast<atomic_counter_t*>( ptr );
+}
+
 int zmq::msg_t::close ()
 {
     //  Check the validity of the message.
@@ -153,16 +172,14 @@ int zmq::msg_t::close ()
         //  If the content is not shared, or if it is shared and the reference
         //  count has dropped to zero, deallocate it.
         if (!(u.lmsg.flags & msg_t::shared) ||
-              !u.lmsg.content->refcnt.sub (1)) {
+              !msg_counter().sub (1)) {
 
-            //  We used "placement new" operator to initialize the reference
-            //  counter so we call the destructor explicitly now.
-            u.lmsg.content->refcnt.~atomic_counter_t ();
-
-            if (u.lmsg.content->ffn)
-                u.lmsg.content->ffn (u.lmsg.content->data,
-                    u.lmsg.content->hint);
-            free (u.lmsg.content);
+            if (u.lmsg.ffn) {
+                u.lmsg.ffn(u.lmsg.data, u.lmsg.hint);
+            }
+            else {
+                free (u.lmsg.data);
+            }
         }
     }
 
@@ -214,10 +231,10 @@ int zmq::msg_t::copy (msg_t &src_)
         //  One reference is added to shared messages. Non-shared messages
         //  are turned into shared messages and reference count is set to 2.
         if (src_.u.lmsg.flags & msg_t::shared)
-            src_.u.lmsg.content->refcnt.add (1);
+            src_.msg_counter().add (1);
         else {
             src_.u.lmsg.flags |= msg_t::shared;
-            src_.u.lmsg.content->refcnt.set (2);
+            src_.msg_counter().set (2);
         }
     }
 
@@ -239,7 +256,7 @@ void *zmq::msg_t::data ()
     case type_vsm:
         return u.vsm.data;
     case type_lmsg:
-        return u.lmsg.content->data;
+        return u.lmsg.data;
     case type_cmsg:
         return u.cmsg.data;
     default:
@@ -257,7 +274,7 @@ size_t zmq::msg_t::size ()
     case type_vsm:
         return u.vsm.size;
     case type_lmsg:
-        return u.lmsg.content->size;
+        return u.lmsg.size;
     case type_cmsg:
         return u.cmsg.size;
     default:
@@ -333,6 +350,11 @@ bool zmq::msg_t::is_vsm ()
     return u.base.type == type_vsm;
 }
 
+bool zmq::msg_t::is_lmsg () const
+{
+    return u.base.type == type_lmsg;
+}
+
 bool zmq::msg_t::is_cmsg ()
 {
     return u.base.type == type_cmsg;
@@ -353,9 +375,9 @@ void zmq::msg_t::add_refs (int refs_)
     //  message type that needs special care are long messages.
     if (u.base.type == type_lmsg) {
         if (u.lmsg.flags & msg_t::shared)
-            u.lmsg.content->refcnt.add (refs_);
+            msg_counter().add (refs_);
         else {
-            u.lmsg.content->refcnt.set (refs_ + 1);
+            msg_counter().set (refs_ + 1);
             u.lmsg.flags |= msg_t::shared;
         }
     }
@@ -379,14 +401,14 @@ bool zmq::msg_t::rm_refs (int refs_)
     }
 
     //  The only message type that needs special care are long messages.
-    if (!u.lmsg.content->refcnt.sub (refs_)) {
+    if (!msg_counter().sub (refs_)) {
         //  We used "placement new" operator to initialize the reference
         //  counter so we call the destructor explicitly now.
-        u.lmsg.content->refcnt.~atomic_counter_t ();
+        msg_counter().~atomic_counter_t ();
 
-        if (u.lmsg.content->ffn)
-            u.lmsg.content->ffn (u.lmsg.content->data, u.lmsg.content->hint);
-        free (u.lmsg.content);
+        if (u.lmsg.ffn)
+            u.lmsg.ffn (u.lmsg.data, u.lmsg.hint);
+        free (u.lmsg.data);
 
         return false;
     }
@@ -404,3 +426,4 @@ int zmq::msg_t::set_routing_id(uint32_t routing_id_)
     u.base.routing_id = routing_id_;
     return 0;
 }
+

src/msg.hpp

@@ -54,7 +54,6 @@ namespace zmq
     class msg_t
     {
     public:
-
         //  Message flags.
         enum
         {
@@ -66,6 +65,8 @@ namespace zmq
         };
 
         bool check ();
+        int init (void *data_, size_t size_, msg_free_fn *ffn_,
+                  void *hint_);
         int init ();
         int init_size (size_t size_);
         int init_data (void *data_, size_t size_, msg_free_fn *ffn_,
@@ -88,6 +89,7 @@ namespace zmq
         bool is_credential () const;
         bool is_delimiter () const;
         bool is_vsm ();
+        bool is_lmsg () const;
         bool is_cmsg ();
         uint32_t get_routing_id();
         int set_routing_id(uint32_t routing_id_);
@@ -107,22 +109,6 @@ namespace zmq
         enum { msg_t_size = 64 };
         enum { max_vsm_size = msg_t_size - (8 + sizeof (metadata_t *) + 3 + sizeof(uint32_t)) };
 
-        //  Shared message buffer. Message data are either allocated in one
-        //  continuous block along with this structure - thus avoiding one
-        //  malloc/free pair or they are stored in used-supplied memory.
-        //  In the latter case, ffn member stores pointer to the function to be
-        //  used to deallocate the data. If the buffer is actually shared (there
-        //  are at least 2 references to it) refcount member contains number of
-        //  references.
-        struct content_t
-        {
-            void *data;
-            size_t size;
-            msg_free_fn *ffn;
-            void *hint;
-            zmq::atomic_counter_t refcnt;
-        };
-
         //  Different message types.
         enum type_t
         {
@@ -138,6 +124,8 @@ namespace zmq
             type_max = 104
         };
 
+        atomic_counter_t& msg_counter();
+
         // the file descriptor where this message originated, needs to be 64bit due to alignment
         int64_t file_desc;
 
@@ -161,10 +149,32 @@ namespace zmq
                 unsigned char flags;
                 uint32_t routing_id;
             } vsm;
-            struct {
+            struct lmsg_t {
                 metadata_t *metadata;
-                content_t *content;
-                unsigned char unused [msg_t_size - (8 + sizeof (metadata_t *) + sizeof (content_t*) + 2 + sizeof(uint32_t))];
+                //  Shared message buffer. Message data are either allocated in one
+                //  continuous block along with this structure - thus avoiding one
+                //  malloc/free pair or they are stored in used-supplied memory.
+                //  In the latter case, ffn member stores pointer to the function to be
+                //  used to deallocate the data. If the buffer is actually shared (there
+                //  are at least 2 references to it) refcount member contains number of
+                //  references.
+                void *data;
+                size_t size;
+                msg_free_fn *ffn;
+                void *hint;
+                // create an aligned block for an atomic_counter_t object
+                union aligned_atomic_counter_storage {
+                    zmq::atomic_counter_t::integer_t maxAlign;
+                    unsigned char counter[ sizeof(zmq::atomic_counter_t) ];
+                } refcnt;
+                unsigned char unused [msg_t_size - (8 + sizeof (metadata_t *)
+                                                    + sizeof(void*)
+                                                    + sizeof(size_t)
+                                                    + sizeof(msg_free_fn*)
+                                                    + sizeof(void*)
+                                                    + sizeof(aligned_atomic_counter_storage)
+                                                    + 2
+                                                    + sizeof(uint32_t))];
                 unsigned char type;
                 unsigned char flags;
                 uint32_t routing_id;

src/v1_decoder.cpp

@@ -43,7 +43,8 @@
 #include "err.hpp"
 
 zmq::v1_decoder_t::v1_decoder_t (size_t bufsize_, int64_t maxmsgsize_) :
-    decoder_base_t <v1_decoder_t> (bufsize_),
+    c_single_allocator(bufsize_),
+    decoder_base_t <v1_decoder_t> (this),
     maxmsgsize (maxmsgsize_)
 {
     int rc = in_progress.init ();
@@ -59,7 +60,7 @@ zmq::v1_decoder_t::~v1_decoder_t ()
     errno_assert (rc == 0);
 }
 
-int zmq::v1_decoder_t::one_byte_size_ready ()
+int zmq::v1_decoder_t::one_byte_size_ready (unsigned char const*)
 {
     //  First byte of size is read. If it is 0xff read 8-byte size.
     //  Otherwise allocate the buffer for message data and read the
@@ -96,7 +97,7 @@ int zmq::v1_decoder_t::one_byte_size_ready ()
     return 0;
 }
 
-int zmq::v1_decoder_t::eight_byte_size_ready ()
+int zmq::v1_decoder_t::eight_byte_size_ready (unsigned char const*)
 {
     //  8-byte payload length is read. Allocate the buffer
     //  for message body and read the message data into it.
@@ -138,7 +139,7 @@ int zmq::v1_decoder_t::eight_byte_size_ready ()
     return 0;
 }
 
-int zmq::v1_decoder_t::flags_ready ()
+int zmq::v1_decoder_t::flags_ready (unsigned char const*)
 {
     //  Store the flags from the wire into the message structure.
     in_progress.set_flags (tmpbuf [0] & msg_t::more);
@@ -149,7 +150,7 @@ int zmq::v1_decoder_t::flags_ready ()
     return 0;
 }
 
-int zmq::v1_decoder_t::message_ready ()
+int zmq::v1_decoder_t::message_ready (unsigned char const*)
 {
     //  Message is completely read. Push it further and start reading
     //  new message. (in_progress is a 0-byte message after this point.)

src/v1_decoder.hpp

@@ -36,7 +36,9 @@ namespace zmq
 {
     //  Decoder for ZMTP/1.0 protocol. Converts data batches into messages.
 
-    class v1_decoder_t : public decoder_base_t <v1_decoder_t>
+    class v1_decoder_t :
+            public zmq::c_single_allocator,
+            public decoder_base_t <v1_decoder_t>
     {
     public:
 
@@ -47,10 +49,10 @@ namespace zmq
 
     private:
 
-        int one_byte_size_ready ();
-        int eight_byte_size_ready ();
-        int flags_ready ();
-        int message_ready ();
+        int one_byte_size_ready (unsigned char const*);
+        int eight_byte_size_ready (unsigned char const*);
+        int flags_ready (unsigned char const*);
+        int message_ready (unsigned char const*);
 
         unsigned char tmpbuf [8];
         msg_t in_progress;

src/v2_decoder.cpp

@@ -41,8 +41,96 @@
 #include "wire.hpp"
 #include "err.hpp"
 
+zmq::shared_message_memory_allocator::shared_message_memory_allocator(size_t bufsize_):
+    buf(NULL),
+    bufsize( bufsize_ )
+{
+
+}
+
+zmq::shared_message_memory_allocator::~shared_message_memory_allocator()
+{
+    deallocate();
+}
+
+unsigned char* zmq::shared_message_memory_allocator::allocate()
+{
+    if (buf)
+    {
+        // release reference count to couple lifetime to messages
+        call_dec_ref(NULL, buf);
+        // release pointer because we are going to create a new buffer
+        release();
+    }
+
+    // @todo aligmnet padding may be needed
+    if (!buf)
+    {
+        buf = (unsigned char *) malloc(bufsize + sizeof(zmq::atomic_counter_t));
+        alloc_assert (buf);
+        new(buf) atomic_counter_t(1);
+    }
+
+    return buf + sizeof( zmq::atomic_counter_t);
+}
+
+void zmq::shared_message_memory_allocator::deallocate()
+{
+    free(buf);
+    buf = NULL;
+}
+
+unsigned char* zmq::shared_message_memory_allocator::release()
+{
+    unsigned char* b = buf;
+    buf = NULL;
+    return b;
+}
+
+void zmq::shared_message_memory_allocator::reset(unsigned char* b)
+{
+    deallocate();
+    buf = b;
+}
+
+void zmq::shared_message_memory_allocator::inc_ref()
+{
+    ((zmq::atomic_counter_t*)buf)->add(1);
+}
+
+void zmq::shared_message_memory_allocator::call_dec_ref(void*, void* hint) {
+    zmq_assert( hint );
+    zmq::atomic_counter_t *c = reinterpret_cast<zmq::atomic_counter_t *>(hint);
+
+    if (!c->sub(1)) {
+        c->~atomic_counter_t();
+        free(hint);
+    }
+}
+
+
+size_t zmq::shared_message_memory_allocator::size() const
+{
+    if (buf)
+    {
+        return bufsize;
+    }
+    else
+    {
+        return  0;
+    }
+}
+
+unsigned char* zmq::shared_message_memory_allocator::data()
+{
+    zmq_assert(buf);
+
+    return buf + sizeof(zmq::atomic_counter_t);
+}
+
 zmq::v2_decoder_t::v2_decoder_t (size_t bufsize_, int64_t maxmsgsize_) :
-    decoder_base_t <v2_decoder_t> (bufsize_),
+    shared_message_memory_allocator( bufsize_),
+    decoder_base_t <v2_decoder_t, shared_message_memory_allocator> (this),
     msg_flags (0),
     maxmsgsize (maxmsgsize_)
 {
@@ -59,7 +147,7 @@ zmq::v2_decoder_t::~v2_decoder_t ()
     errno_assert (rc == 0);
 }
 
-int zmq::v2_decoder_t::flags_ready ()
+int zmq::v2_decoder_t::flags_ready (unsigned char const*)
 {
     msg_flags = 0;
     if (tmpbuf [0] & v2_protocol_t::more_flag)
@@ -77,40 +165,20 @@ int zmq::v2_decoder_t::flags_ready ()
     return 0;
 }
 
-int zmq::v2_decoder_t::one_byte_size_ready ()
+int zmq::v2_decoder_t::one_byte_size_ready (unsigned char const* read_from)
 {
-    //  Message size must not exceed the maximum allowed size.
-    if (maxmsgsize >= 0)
-        if (unlikely (tmpbuf [0] > static_cast <uint64_t> (maxmsgsize))) {
-            errno = EMSGSIZE;
-            return -1;
-        }
-
-    //  in_progress is initialised at this point so in theory we should
-    //  close it before calling zmq_msg_init_size, however, it's a 0-byte
-    //  message and thus we can treat it as uninitialised...
-    int rc = in_progress.init_size (tmpbuf [0]);
-    if (unlikely (rc)) {
-        errno_assert (errno == ENOMEM);
-        rc = in_progress.init ();
-        errno_assert (rc == 0);
-        errno = ENOMEM;
-        return -1;
-    }
-
-    in_progress.set_flags (msg_flags);
-    next_step (in_progress.data (), in_progress.size (),
-        &v2_decoder_t::message_ready);
-
-    return 0;
+    return size_ready(tmpbuf[0], read_from);
 }
 
-int zmq::v2_decoder_t::eight_byte_size_ready ()
-{
+int zmq::v2_decoder_t::eight_byte_size_ready (unsigned char const* read_from) {
     //  The payload size is encoded as 64-bit unsigned integer.
     //  The most significant byte comes first.
-    const uint64_t msg_size = get_uint64 (tmpbuf);
+    const uint64_t msg_size = get_uint64(tmpbuf);
 
+    return size_ready(msg_size, read_from);
+}
+
+int zmq::v2_decoder_t::size_ready(uint64_t msg_size, unsigned char const* read_pos) {
     //  Message size must not exceed the maximum allowed size.
     if (maxmsgsize >= 0)
         if (unlikely (msg_size > static_cast <uint64_t> (maxmsgsize))) {
@@ -127,7 +195,31 @@ int zmq::v2_decoder_t::eight_byte_size_ready ()
     //  in_progress is initialised at this point so in theory we should
     //  close it before calling init_size, however, it's a 0-byte
     //  message and thus we can treat it as uninitialised.
-    int rc = in_progress.init_size (static_cast <size_t> (msg_size));
+    int rc = -1;
+
+    // the current message can exceed the current buffer. We have to copy the buffer
+    // data into a new message and complete it in the next receive.
+    if (unlikely ((unsigned char*)read_pos + msg_size > (data() + size())))
+    {
+        // a new message has started, but the size would exceed the pre-allocated arena
+        // this happens everytime when a message does not fit completely into the buffer
+        rc = in_progress.init_size (static_cast <size_t> (msg_size));
+    }
+    else
+    {
+        // construct message using n bytes from the buffer as storage
+        // increase buffer ref count
+        rc = in_progress.init( (unsigned char*)read_pos,
+                               msg_size, shared_message_memory_allocator::call_dec_ref,
+                               buffer() );
+
+        // For small messages, data has been copied and refcount does not have to be increased
+        if (in_progress.is_lmsg())
+        {
+            inc_ref();
+        }
+    }
+
     if (unlikely (rc)) {
         errno_assert (errno == ENOMEM);
         rc = in_progress.init ();
@@ -137,13 +229,19 @@ int zmq::v2_decoder_t::eight_byte_size_ready ()
     }
 
     in_progress.set_flags (msg_flags);
+    // this sets read_pos to
+    // the message data address if the data needs to be copied
+    // for small message / messages exceeding the current buffer
+    // or
+    // to the current start address in the buffer because the message
+    // was constructed to use n bytes from the address passed as argument
     next_step (in_progress.data (), in_progress.size (),
         &v2_decoder_t::message_ready);
 
     return 0;
 }
 
-int zmq::v2_decoder_t::message_ready ()
+int zmq::v2_decoder_t::message_ready (unsigned char const*)
 {
     //  Message is completely read. Signal this to the caller
     //  and prepare to decode next message.

src/v2_decoder.hpp

@@ -34,11 +34,66 @@
 
 namespace zmq
 {
-    //  Decoder for ZMTP/2.x framing protocol. Converts data stream into messages.
-    class v2_decoder_t : public decoder_base_t <v2_decoder_t>
+    // This allocater allocates a reference counted buffer which is used by v2_decoder_t
+    // to use zero-copy msg::init_data to create messages with memory from this buffer as
+    // data storage.
+    //
+    // The buffer is allocated with a reference count of 1 to make sure that is is alive while
+    // decoding messages. Otherwise, it is possible that e.g. the first message increases the count
+    // from zero to one, gets passed to the user application, processed in the user thread and deleted
+    // which would then deallocate the buffer. The drawback is that the buffer may be allocated longer
+    // than necessary because it is only deleted when allocate is called the next time.
+    class shared_message_memory_allocator
     {
     public:
+        shared_message_memory_allocator(size_t bufsize_);
 
+        ~shared_message_memory_allocator();
+
+        // Allocate a new buffer
+        //
+        // This releases the current buffer to be bound to the lifetime of the messages
+        // created on this bufer.
+        unsigned char* allocate();
+
+        // force deallocation of buffer.
+        void deallocate();
+
+        // Give up ownership of the buffer. The buffer's lifetime is now coupled to
+        // the messages constructed on top of it.
+        unsigned char* release();
+
+        void reset(unsigned char* b);
+
+        void inc_ref();
+
+        static void call_dec_ref(void*, void* buffer);
+
+        size_t size() const;
+
+        // Return pointer to the first message data byte.
+        unsigned char* data();
+
+        // Return pointer to the first byte of the buffer.
+        unsigned char* buffer()
+        {
+            return buf;
+        }
+
+    private:
+        unsigned char* buf;
+        size_t bufsize;
+    };
+
+    //  Decoder for ZMTP/2.x framing protocol. Converts data stream into messages.
+    //  The class has to inherit from shared_message_memory_allocator because
+    //  the base class calls allocate in its constructor.
+    class v2_decoder_t :
+            // inherit first from allocator to ensure that it is constructed before decoder_base_t
+            public shared_message_memory_allocator,
+            public decoder_base_t <v2_decoder_t, shared_message_memory_allocator>
+    {
+    public:
         v2_decoder_t (size_t bufsize_, int64_t maxmsgsize_);
         virtual ~v2_decoder_t ();
 
@@ -47,10 +102,12 @@ namespace zmq
 
     private:
 
-        int flags_ready ();
-        int one_byte_size_ready ();
-        int eight_byte_size_ready ();
-        int message_ready ();
+        int flags_ready (unsigned char const*);
+        int one_byte_size_ready (unsigned char const*);
+        int eight_byte_size_ready (unsigned char const*);
+        int message_ready (unsigned char const*);
+
+        int size_ready(uint64_t size_, unsigned char const*);
 
         unsigned char tmpbuf [8];
         unsigned char msg_flags;