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<blockquote cite="mid:20080528062234.GE16797@roseberry" type="cite">
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<pre wrap="">I've got a CGI program that has a problem every once in a while.
The problem code looks like:
open OUT,">>/home/foo/que/$ip" || push @error, "Cant save details";
print OUT "$ip:t:date=",scalar localtime,"\n";
... then it prints to OUT all the rest of ${ENV} and CGI vars.
Sometimes apache will record 2 hits on the page (a double click?)
and most of the time I get two sets of all the data however sometimes
while running perl 5.8.8 I only get the first or second sometimes.
This never happens with perl 5.005_02.
Can anyone explain why perl 5.005 works yet 5.8.8 doesn't?
I was under the impresson that the ">>" means tell the OS to
open in append mode, any data written should go in the file
and not just end up lost.
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<pre wrap=""><!---->
I can't explain why it works on 5.005 and not 5.8.8, but since you have
mentioned it is a very rare occurence, it is possible that it /would/ occur on
5.005 eventually. Maybe the code just runs slower or faster and flukily avoids
a race condition as a result?
Also, it's worth noting that append isn't always safe for use by multiple
processes - it works by seeking to the end of the file before writing, but
according to the man page, this doesn't work reliably on networked file systems
like NFS.
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I suspect this is root of the problem, irrespective of NFS -> the
webserver is using two instances of the script, to execute the request.<br>
<br>
If two processes open the same file for append, they will both
succeed. Both processes will move their file pointer to the "end of
the file" - which both happens to be at the same byte offset. One
starts "print"ing... then the other "print"s -> the second write
will clobber the first write.<br>
<br>
This applies to both mod_perl and CGI environments. If you want
cooperative access to a "shared resource, aka the $ip file, then you
need locking (or something similar).<br>
<br>
<blockquote cite="mid:20080528062234.GE16797@roseberry" type="cite">
<pre wrap="">
Also - I think Apache will send a signal to the CGIs running, to kill them if
the connection dies - is it is simply a case that when someone double-clicked,
one of the cgi instances was killed before it could write to the logfile
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<br>
regards,<br>
Mathew<br>
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