Implement flow control

This commit introduces the necessary changes necessary
for implementing flow control. None of the socket types
implements the flow control yet. The code will crash when
the flow control is enabled and the thw lwm is reached.

The following commits will add flow-control support for
individual socket types.

1 files changed, 8 insertions, 0 deletions

diff --git a/src/command.hpp b/src/command.hpp
index 150cad1..3d00cd7 100644
--- a/src/command.hpp
+++ b/src/command.hpp
@@ -40,6 +40,7 @@ namespace zmq
             attach,
             bind,
             revive,
+            reader_info,
             pipe_term,
             pipe_term_ack,
             term_req,
@@ -84,6 +85,13 @@ namespace zmq
             struct {
             } revive;
 
+            //  Sent by pipe reader to inform pipe writer
+            //  about how many messages it has read so far.
+            //  Used to implement the flow control.
+            struct {
+                uint64_t msgs_read;
+            } reader_info;
+
             //  Sent by pipe reader to pipe writer to ask it to terminate
             //  its end of the pipe.
             struct {