Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!uflorida!gatech!udel!burdvax!bbking!bcarb
From: bcarb@KSP.Unisys.COM (Brian Carb)
Newsgroups: comp.windows.x
Subject: Re: unix System V
Summary: Use FIONREAD to Check for Socket Data
Keywords: TCP/IP, socket, non-blocking
Message-ID: <700@bbking.KSP.Unisys.COM>
Date: 8 Jun 89 15:16:44 GMT
References: <8906021126.AA04281@wheat-chex.ai.mit.edu>
Distribution: usa
Organization: Unisys/Knowledge Systems Projects, Frazer, PA
Lines: 33


>>   We are working with Compac 386 using X-window version 11
>>   release III and unix system V of Interactive. When we
>>   use AtAddInput over a pipe there is an error "select failed" with
>>   ...
>You lose. System V doesn't have a general way of implementing a
>select() function that works on all possible kinds of file descriptors
>(there is no "real" non-blocking I/O, i.e., something that would let
>you distinguish EOF from no-data-available, and there is no general
>way of finding out how much data is available on a file descriptor).
>..
>A hack to get around this problem is to do a read() on the file
>descriptor you are interested in and use an alarm() call to time out

We use the following code on a Unisys 6000/50 (Convergent) running
System V to check if data is available on a socket:

 /* Function to check if data exists on a TCP/IP socket */

  #include <sys/types.h>
  #include <sys/socket.h>
  
  int socketlisten(fd)
    int fd;
  {
    int chars_available;
    if(ioctl(fd,FIONREAD,(char *)&chars_available) < 0)
      return(-1);
    else
      return(chars_available);
  }

bcarb@ksp.unisys.com (Brian A. Carb)