The user is responsible for ensuring that the destination queue pair
has queued a receive request before issuing the send operations. For a list of
supported flags, see ibv_post_send. Unless the send request is using
inline data, the message buffer must have been registered
before being posted, with the mr parameter referencing the registration.
The buffer must remain registered until the send completes.
The user-defined context associated with the send request will be
returned to the user through the work completion wr_id, work request
identifier, field.