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Showing posts from August, 2008

Difference between for (<STDIN>) and while(<STDIN>) in Perl

Can you tell if there is a difference between "for (<STDIN>) { BLOCK}" and "while(<STDIN>) {BLOCK}" in Perl? They both might look like similar. When I used the first construct and gave a huge file as input, my Perl program crashed. When checked the top output after starting the program, I figured that the RSS size of the program kept growing and eventually crashed. Hmm ... when I switched to the second construct, not only the program ran successfully but also used less memory!

The difference is that for construct reads the entire file before it enters into the loop. But the while construct reads only a part of the file and buffers it. I wrote a program to count the number of lines in an input file in both the flavors. while loop version took only 1MB where as for loop version took 149 MB for an input file of 79 MB. On top of it, the while loop version took on an average 1.36 seconds, and the for loop version took around 2 seconds.

So if you are writing a…

Virtual IP and timeouts

Let us say you are writing a client application that connects to a server and maintains a persistent connection. Whenever there is some user intervention, for e.g. typing some input, the client application sends the user input to the server. And prints back the output produced by the server. (Sounds like telnet?)

One thing your client program doesn't know about is that the server it is connecting to makes use of a virtual IP! Consider the case when your client program initiates a connection and the connection is in ESTABLISHED state. Right in the middle of the connection, the server drops the virtual IP address and for some strange reason it never gets reassigned to another host. In that case, neither the client program nor the server program will know the event of removal of the IP address. The server application would still be listening in the removed IP address and would still be holding the ESTABLISHED connections through removed IP address. Likewise, the client would still be …