SPUG: shell script event loops + wrapping shell utilities

dancerboy dancerboy at strangelight.com
Sun Apr 21 00:44:08 CDT 2002

At 4:20 pm -0700 2002-04-20, Andrew Sweger wrote:
>On Sat, 20 Apr 2002, dancerboy wrote:
>>  And added this to the loop:
>>  >	select( $rin, undef, undef, undef );
>>  But this just blocked forever, even when there was definitely input
>>  on TTY.
>select($rin, undef, undef, 0);
>Having that undef as the timeout means "block until we have something to

Hmmm... was my post not clear?  This is exactly what I *wanted*: 
"block until we have something to read".  The problem is, it seems to 
continue blocking *even when there IS something to read*.  The select 
call never returns, ever, no matter what, and the input just keeps 
piling up in the buffer until I manually kill the process (at which 
point the calling shell gets the input that was intended for the Perl 
script -- which can have "interesting" results, depending on what I 
was typing...)

>  If your application can stand a little idle time in the event loop
>(if it's primary job is waiting for input), it won't hurt to put a timeout
>value greater than zero in. This will reduce the amount of CPU time your
>application will burn too.
>select($rin, undef, undef, 0.01);  # 10ms timeout

As I said, this is exactly what I did:

     select( undef, undef, undef, 0.1);

because that was the *only* way that the select call would *ever* 
return. (And since it was invariably waiting until the timeout, it 
didn't matter what I put as the first argument.)

>(select() is one of those really cool functions that twists your brain
>around the fork tines the first few times, then it's just fun. Do *not*
>mix with buffered IO!)

The concept doesn't seem difficult -- my problem is that as far as I 
can tell, it's not behaving as documented.  And I'm not using 
buffered I/O -- all my input is done with sysread.


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