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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: GNAT implementation of socket sets
Date: Fri, 19 Jan 2018 19:10:36 +0000
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Date: 2018-01-19T19:10:36+00:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> GNAT.Sockets implementation of Socket_Set_Type has no operation to
> walk that set by position. It seemed not a big issue until I noticed
> that the implementation of
>
>    procedure Get (Item : in out Socket_Set_Type; Socket : out Socket_Type);
>    --  Extract a Socket from socket set Item. Socket is set to
>    --  No_Socket when the set is empty.
>
> seems to always return the last element of the set.

Indeed.

Implemented as:

   /* Get last socket and remove it from the socket set SET.  LAST is the
      maximum value of the largest socket.  This hint is used to avoid scanning
      very large socket sets.  On return, LAST is set to the actual largest
      socket in the socket set. */

   void
   __gnat_get_socket_from_set (fd_set *set, int *last, int *socket)
   {
     *socket = *last;
     FD_CLR (*socket, set);
     __gnat_last_socket_in_set (set, last);
   }

where

   void
   __gnat_last_socket_in_set (fd_set *set, int *last)
   {
     int s;
     int l;
     l = -1;

   #ifdef _WIN32
     /* More efficient method for NT. */
     for (s = 0; s < set->fd_count; s++)
       if ((int) set->fd_array[s] > l)
         l = set->fd_array[s];

   #else

     for (s = *last; s != -1; s--)
       if (FD_ISSET (s, set))
         {
           l = s;
           break;
         }
   #endif

     *last = l;
   }