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From: "DuckE" <nospam_steved94@home.com>
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Subject: How can I avoid Using a Semaphore? (long)
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Date: Sat, 13 Jan 2001 16:18:16 GMT
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I have an application that receives a streams of data from TCP/IP sockets.
The stream of data is separated into "packets" each beginning with a 32 bit
length value.

For each of these sockets I have a task that receives the length, then gets
a pre-allocated data buffer that is large enough to hold the data, receives
the data into the data buffer, and then puts a reference to the buffer into
a queue for processing by other tasks.

Pseudo code for the main loop for the receive task is something like:
  Loop
    Receive 4 bytes of data from the socket
    Convert the 4 bytes to an integer value
    Get a buffer from available buffers that is large enough to hold
remaining data in message
    Receive remaining part of message into buffer
    Queue the message for processing by other tasks
  End Loop

To avoid memory leaks I am using a controlled type with reference counting
to implement my data buffer.  Since multiple threads may simultaneously have
access to the buffer, I use a protected type for reference counting.

Since I am using a protected type for reference counting, I cannot assign
packets within a protected type (to implement queue's of packets for
instance) since these assignments would be attempting to do a protected
operation within a protected type.

I have worked around this using a semaphore (the semaphore is created using
a protected type) around the assignment of packets:

  lock semaphore
  assign packet
  unlock semaphore

This technique has proven reliable in production code, but I don't find it
to be very "clean".

Suggestions?

Here is a subset of my packet implementation:

package NPUnetworkPacketUtil is

  type aPacketNPU is private;

private
  protected type aRefCounterNPU is
    procedure PlaseHoldNPU;
    procedure ReleaseHoldNPU( lastHoldNPU : out boolean );
  private
    referenceCountNPU : Natural := 0;
  end aRefCounterNPU;

  type aPacketBufNPU( bufSizeNPU : Positive );

  type aPacketBUfPtrNPU is accesss all aPacketBufNPU;

  type aPacketNPU is new Ada.Finalization.Controlled with
    record
      packetBufPtrNPU : aPacketBufPtrNPU;
    end record;
  procedure Initialize( object : in out aPacketNPU );
  procedure Adjust( object : in out aPacketNPU );
  procedure Finalize( object : in out aPacketNPU );

  type aPacketBufNPU( bufSizeNPU : Positive ) is limited
    record
      dataBufferNPU : aliased aDataArrayNPU( 1 .. bufSizeNPU );
      holdLockNPU : aRefCounterNPU;
    end record;

end NPUnetworkPacketUtil ;

package body NPUnetworkPacketUtil  is

  protected body aRefCounterNPU is
    procedure PlaceHoldNPU is
    begin
      referenceCountNPU := referenceCountNPU + 1;
    end PlaceHoldNPU;
    procedure ReleaseHoldNPU( lastHoldNPU : out boolean ) is
    begin
      referenceCountNPU := referenceCountNPU - 1;
      lastHoldNPU := referenceCountNPU = 0;
    end ReleaseHoldNPU;
  end aRefCounterNPU;

  procedure Initialize( object : in out aPacketNPU ) is
  begin
    object.packetBufPtrNPU := null;
  end Initialize;

  procedure Adjust( object : in out aPacketNPU ) is
  begin
    if object.packetBufPtrNPU /= null then
      packetBufPtrNPU.holdLockNPU.PlaceHoldNPU;
    end if;
  end Adjust;

  procedure Finalize( object : in out aPacketNPU ) is
    lastHold : boolean;
  begin
    if object.packetBufPtrNPU /= null then
      packetBufNPU.holdLockNPU.ReleaseHoldNPU( lastHold );
      if lastHold then
        -- Take action to put buffer in the free pool
      end if;
    end if;
  end Finalize;

end NPUnetworkPacketUtil;


Thank's in advance,
SteveD