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6262 :: Nim zmq binding and poll function

• Clonk (orginal) [2020-04-27T15:16:33+02:00] view original

  Hello,

  I'm trying to use the poll function of the ZMQ (version 4.2) binding. In C (and C++) you just pass an allocated array of zmq_pollitems_t and the length.

  I tried doing the Nim equivalent :

  var p1  = cast[ptr TPollItem](alloc0(2*sizeof(TPollItem)))  # Allocate memory for 2 TPollItem object
  var p2 = cast[ptr TPollItem](cast[int](p1) + cast[int](sizeof(TPollItem))) # Get the start adress of the second object
  p1[] = TPollItem(socket: d1.s, events: ZMQ_POLLIN) # Write first in the memory adress
  p2[] = TPollItem(socket: d2.s, events: ZMQ_POLLIN) # Write second object in the memory adress
  var res : int = poll(p1, 2, 10000)
  echo res
  if res >= 0 :
      var res1 = bitand(p1.revents, ZMQ_POLLIN.cshort)
      var res2 = bitand(p2.revents, ZMQ_POLLIN.cshort)
      echo "res1:", res1, " && res2:", res2
  Run

  But it only works for the first TPollItem (i can swap the socket used in the TPollItem so I know it's not a connexion problem) and I never get a POLLIN flag set in p2.

  Also, on the same subject, the Nim pollitem are defined as follow :

  type
    TPollItem*{.pure, final.} = object
      socket*: PSocket
      fd*: cint
      events*: cshort
      revents*: cshort
      poller*: pointer
  
  proc poll*(items: ptr TPollItem, nitems: cint, timeout: clong): cint{.
    cdecl, importc: "zmq_poll", dynlib: zmqdll.}
  
  Run

  What is the pointer poller ? In C it is just defined as a struct :

  
  typedef struct
  {
      void* //socket//;
      int //fd//;
      short //events//;
      short //revents//;
  } zmq_pollitem_t;
  
  int zmq_poll (zmq_pollitem_t *items, int nitems, long timeout);
  Run

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• Araq (orginal) [2020-04-27T17:18:26+02:00] view original

  Something like this:

  type
    TPollItem* = object
      socket*: PSocket
      fd*: cint
      events*: cshort
      revents*: cshort
      poller*: pointer
  
  proc poll*(items: ptr UncheckedArray[TPollItem], nitems: cint, timeout: clong): cint{.
    cdecl, importc: "zmq_poll", dynlib: zmqdll.}
  
  template asUncheckedArray(a): untyped = cast[ptr UncheckedArray[typeof(a[0])]](addr a[0])
  
  var a: array[2, TPollItem]
  a[0] = TPollItem(socket: d1.s, events: ZMQ_POLLIN)
  a[1] = TPollItem(socket: d2.s, events: ZMQ_POLLIN)
  
  # Poll on 2 element starting at adress p1
  var res : int = poll(asUncheckedArray(a), 2, 10000)
  echo res
  if res >= 0 :
      var res1 = bitand(p1.revents, ZMQ_POLLIN.cshort)
      var res2 = bitand(p2.revents, ZMQ_POLLIN.cshort)
      echo "res1:", res1, " && res2:", res2
  Run

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• Clonk (orginal) [2020-04-27T18:21:01+02:00] view original

  Hi,

  Thanks for the UncheckedArray trick to access underlying C-array.

  asUncheckedArray will return a type :

  ptr UncheckedArray[TPollItem]
  Run

  I tried both :

  var res : int = poll(asUncheckedArray(p)[0].addr, 2, 10000)
  var res : int = poll(cast[ptr TPollItem](asUncheckedArray(p)), 2, 10000)
  Run

  to get a ptr TPollItem back. Both gives me same result as the code I posted.

  I also tried to define a poll function with UncheckedArray ptr parameters :

  proc poll*(items: ptr UncheckedArray[TPollItem], nitems: cint, timeout: clong): cint{.
    cdecl, importc: "zmq_poll", dynlib: zmqdll.}
   
   var res : int = poll(asUncheckedArray(p), 2, -1)
  Run

  In all those case, I only the first TPollItem is receiving data. If I try to receive manually data on the second connexion with no POLLIN flag set (since I know data is present in a trivial) I can receive the data normally.

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• Araq (orginal) [2020-04-27T19:08:42+02:00] view original

  I updated my reply, your definition of TPollItem didn't match its C declaration.

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• Clonk (orginal) [2020-04-27T20:12:27+02:00] view original

  Ah yes that was the cause of the problem.

  I reported the bug here : https://github.com/nim-lang/nim-zmq/issues/14

  I have a fork of nim-zmq where I added some examples, I can fix the problem and submit a Pull Request.

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