Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!think!ames!fxgrp!news
From: grady@fxgrp.fx.com (Steven Grady)
Newsgroups: comp.lang.perl
Subject: do_select() bug
Message-ID: <1989Dec16.044106.7924@fxgrp.fx.com>
Date: 16 Dec 89 04:41:06 GMT
Reply-To: grady@fxgrp.fx.com (Steven Grady)
Organization: FXDevelopment, Mountain View, CA
Lines: 82

do_select(), the function that implements UNIX select(), does not work
properly.  There are two problems.  First, it uses the length of the
vector (represented as a string internally) as the width of the bit
field passed to select().  Unfortunately, it is including the '\0'
terminator of the string as part of the length.  With the width off by
8, the file descriptors examined are 8 higher than they should be.

Second and more important, it does not work on suns.  It does not use
the FD_* macros.  The representation it uses for the file descriptor
bits, namely, a simple string, does not work at all (because the width
on a sun must be a multiple of 32 bits).

Here is a patch to fix do_select().  It is not very pretty, but it gets
the job done.  There is no compile flag to enable it, since it is not a
complete patch anyway.  This version will work on suns, but it won't
work, for instance, on AIX (which uses "sellist" structures).  The
"correct" way to do things would be to avoid explicitly using perl
vecs.  I'd like to see a different interface to select() anyway.
Perhaps something like:

    (@readready, @writeready, @exceptready, $timeleft) = 
	select(@readhandles, @writehandles, @excepthandles, $timeout);

where each of the arrays would be an array of expressions evaluating to
file handles.  The vec interface is pretty bogus (and doesn't work,
under the current implementation).

*** doio.c	Fri Dec 15 20:26:16 1989
--- doio.c.BAK	Fri Dec 15 20:07:44 1989
***************
*** 1510,1537 ****
      else
  	tbuf = Null(struct timeval*);
  
!     {
! 	fd_set fds[4];
! 	register int i, j;
! 	for (j = 1; j <= 3; j++) {
! 	    FD_ZERO(&fds[j]);
! 	    if (st[sp+j]->str_ptr == NULL) continue;
! 	    for (i = 0; i < (maxlen - 1) * 8; i++) {
! 		if (st[sp+j]->str_ptr[i/8] & (1 << (i % 8)))
! 		    FD_SET(i, &fds[j]);
! 	    }
! 	}
! 	nfound = select(
! 	    (maxlen - 1) * 8, &fds[1], &fds[2], &fds[3], tbuf);
! 	for (j = 1; j <= 3; j++) {
! 	    if (st[sp+j]->str_ptr == NULL) continue;
! 	    bzero(st[sp+j]->str_ptr, (maxlen - 1));
! 	    for (i = 0; i < (maxlen - 1) * 8; i++) {
! 		if (FD_ISSET(i, &fds[j]))
! 		    st[sp+j]->str_ptr[i/8] |= (1 << (i % 8));
! 	    }
! 	}
!     }
  
      st[++sp] = str_static(&str_no);
      str_numset(st[sp], (double)nfound);
--- 1510,1521 ----
      else
  	tbuf = Null(struct timeval*);
  
!     nfound = select(
! 	maxlen * 8,
! 	st[sp+1]->str_ptr,
! 	st[sp+2]->str_ptr,
! 	st[sp+3]->str_ptr,
! 	tbuf);
  
      st[++sp] = str_static(&str_no);
      str_numset(st[sp], (double)nfound);
-- 
	Steven
	...!ucbvax!grady
	grady@postgres.berkeley.edu

   Spock was waiting for them when they got to the conference
room.  "Captain, I've run the data we collected through the
computer."
   "Well, Spock, you must be a very proud young man.  So what's
the deal with these council weasels?"