RE: Tail -f and select - Again

Thanks to all for all of the suggestions.  There seem to be many modules
that address this issue, but I think I will stick to my external tail and
kill strategy.  No external modules to load and I don't need the added
functionality that the modules provide.  I'm sure it isn't the most
efficient, but I think it is the simplest.

That said, I am wondering why nobody has suggested using select and sysread.
I would have thought that this would have been a great way to solve the
problem.  I tried the following code with no success:

#!/usr/local/bin/perl
$datafile="File1";
while () {
   print"Opening $datafile\n";
   open(DATAIN,"<$datafile");
   vec($rin,fileno(DATAIN),1) = 1;
   print "Fileno = ",fileno(DATAIN),"\n";
   $ein=$rin;
   $nfound=select($rout=$rin,undef,$eout=$ein,undef);
   while () {
      printf("nfound=%3x  rout=%3x   eout=%3x\n",$nfound,$rout,$eout);
      sysread DATAIN, $datarec, 10;
      print "rec->$datarec<-\n";
      if (substr($datarec,0,4) eq "SPEC") {
         $datafile=substr($datarec,4,5);
         last;
      }
      $nfound=select($rout=$rin,undef,$eout=$ein,undef);
   }
   close(DATAIN);
}

The output and file data is attached at the bottom.

I don't understand why this wouldn't work.  Shouldn't the select statement
inside the while loop wait only on filehandle 3 for data to be ready.  This
is the behaviour I was looking for.  The fact that nfound = 2 is also
confusing and why didn't the select block at the end of file 3?

I am running the script on an AIX box.  Is there a problem with select on
some platforms or do I just misunderstand how select actually works.

All for now,

Mark Borghardt

Opening File1
Fileno = 3
nfound=  2  rout=  0   eout=  0
rec->Record 01
<-
nfound=  2  rout=  0   eout=  0
rec->Record 02
<-
nfound=  2  rout=  0   eout=  0
rec->SPECFile2
<-
Opening File2
Fileno = 3
nfound=  2  rout=  0   eout=  0
rec->Record 03
<-
nfound=  2  rout=  0   eout=  0
rec->Record 04
<-
nfound=  2  rout=  0   eout=  0
rec->Record 05
<-
nfound=  2  rout=  0   eout=  0
rec->SPECFile3
<-
Opening File3
Fileno = 3
nfound=  2  rout=  0   eout=  0
rec->Record 06
<-
nfound=  2  rout=  0   eout=  0
rec->Record 07
<-
nfound=  2  rout=  0   eout=  0
rec-><-
nfound=  2  rout=  0   eout=  0
rec-><-
nfound=  2  rout=  0   eout=  0
rec-><-
nfound=  2  rout=  0   eout=  0
rec-><-
nfound=  2  rout=  0   eout=  0
rec-><-


Data Files

File1
Record 01
Record 02
SPECFile2

File2
Record 03
Record 04
Record 05
SPECFile3

File3
Record 06
Record 07

-----Original Message-----
From: Uri Guttman [mailto:[email protected]]
Sent: Tuesday, January 22, 2002 11:51 AM
To: [email protected]
Cc: [email protected]; [email protected]
Subject: Re: Tail -f and select - Again


>>>>> "JS" == Jochen Stenzel <[email protected]> writes:

  >> What I want to do is read data from a growing file (tail -f) until
  >> a special record arrives, at which point I would like to close the
  >> file and reopen another one repeating the process.

  JS> there is a module called Event::File which implements tail -f the
  JS> Event way.

and there is a Stem::LogTail module that implements tail the stem
way. :)  the stem log subsystem supports filter rules which can easily
trigger a shutdown or restart of the tailing. no need for a separate
tail process.

uri

-- 
Uri Guttman  ------  [email protected]  --------
http://www.stemsystems.com
-- Stem is an Open Source Network Development Toolkit and Application Suite
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----- Stem and Perl Development, Systems Architecture, Design and Coding
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