text: //
  // Copyright 2018 Staysail Systems, Inc. <info@staysail.tech>
  // Copyright 2018 Capitar IT Group BV <info@capitar.com>
  //
  // This software is supplied under the terms of the MIT License, a
  // copy of which should be located in the distribution where this
  // file was obtained (LICENSE.txt).  A copy of the license may also be
  // found online at https://opensource.org/licenses/MIT.
  //
  
  #ifndef CORE_MSGQUEUE_H
  #define CORE_MSGQUEUE_H
  
  #include "nng_impl.h"
  #include "pollable.h"
  
  // Message queues.  Message queues work in some ways like Go channels;
  // they are a thread-safe way to pass messages between subsystems.  They
  // do have additional capabilities though.
  //
  // Message queues can be closed many times safely.
  //
  // Readers & writers in a message queue can be woken either by a timeout
  // or by a specific signal (arranged by the caller).
  typedef struct nni_msgq nni_msgq;
  
  // nni_msgq_init creates a message queue with the given capacity.
  // (If the capacity is zero, then the queue is unbuffered.)
  // It returns NNG_ENOMEM if resources cannot be allocated.
  extern int nni_msgq_init(nni_msgq **, unsigned);
  
  // nni_msgq_fini destroys a message queue.  It will also free any
  // messages that may be in the queue.
  extern void nni_msgq_fini(nni_msgq *);
  
  // nni_msgq_flush discards any messages that are sitting in the queue.
  // It does not wake any writers that might be waiting.
  extern void nni_msgq_flush(nni_msgq *);
  
  extern void nni_msgq_aio_put(nni_msgq *, nni_aio *);
  extern void nni_msgq_aio_get(nni_msgq *, nni_aio *);
  
  // nni_msgq_tryput performs a non-blocking attempt to put a message on
  // the message queue.
  extern int nni_msgq_tryput(nni_msgq *, nni_msg *);
  
  // nni_msgq_set_get_error sets an error condition on the get side of the
  // message queue, and for that side behaves like nni_msgq_set_error.
  // Readers (nni_msgq_put*) are unaffected.
  extern void nni_msgq_set_get_error(nni_msgq *, int);
  
  // nni_msgq_filter is a callback function used to filter messages.
  // The function is called on entry (put) or exit (get).  The void
  // argument is an opaque pointer supplied with the function at registration
  // time.  The primary use for these functions is to support the protocol
  // socket needs.
  typedef nni_msg *(*nni_msgq_filter)(void *, nni_msg *);
  
  // nni_msgq_set_filter sets the filter on the queue.  Messages
  // are filtered through this just before they are returned via the get
  // functions.  If the filter returns NULL, then the message is silently
  // discarded instead, and any get waiters remain waiting.
  extern void nni_msgq_set_filter(nni_msgq *, nni_msgq_filter, void *);
  
  // nni_msgq_close closes the queue.  After this all operates on the
  // message queue will return NNG_ECLOSED.  Messages inside the queue
  // are freed.  Unlike closing a go channel, this operation is idempotent.
  extern void nni_msgq_close(nni_msgq *);
  
  // nni_msgq_resize resizes the message queue; messages already in the queue
  // will be preserved as long as there is room.  Messages that are dropped
  // due to no room are taken from the most recent.  (Oldest messages are
  // preserved.)
  extern int nni_msgq_resize(nni_msgq *, int);
  
  // nni_msgq_cap returns the "capacity" of the message queue.  This does not
  // include the extra room for pushback, nor the extra slot reserved to make
  // zero-length message queues possible.  As a consequence, it is possible
  // for the message queue to contain up to 2 more messages than the capacity.
  extern int nni_msgq_cap(nni_msgq *mq);
  
  // nni_msgq_len returns the number of messages currently in the queue.
  extern int nni_msgq_len(nni_msgq *mq);
  
  extern int nni_msgq_get_recvable(nni_msgq *mq, nni_pollable **);
  extern int nni_msgq_get_sendable(nni_msgq *mq, nni_pollable **);
  
  #endif // CORE_MSQUEUE_H
  