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To: cs551@merlot.usc.edu
Subject: Re: fseek: Illegal seek
Date: Tue, 09 Sep 2008 11:27:28 -0700
From: Bill Cheng <william@bourbon.usc.edu>

Someone wrote:

  > I am a somewhat confused as to why the following command produces
  > an error. I have googled the web for a solution but nothing seems
  > to be coming up that I have not tried.
  > 
  > I am performing a popen to get the MD5 of a given file

You should not do this!  Your server should send the
file over the socket (starting from the given offset)
and your client should calculate MD5 on the fly (without
using a file).

  > so I have written the following
  > 
  > file_size  = fseek(pfp, (long)offset, SEEK_END);
  > 
  > to get the filesize and return this as the data_length.
  > 
  > pfp is a file pointer created from popen. (I checked that pfp is
  > *not* NULL)
  > 
  > the error I get always is (fseek :illegal seek)
  > Any suggestions please?

I don't think you can do a seek on a pipe.  fseek() should be
used on a file stream.

  > By the way I tried using a value of 0L instead of offset but it
  > still same result. Also, I ran "openssl md5 filename" and got the
  > md5  result.

On your server, you can use fopen() to open the file,
use fseek() to skip a given number of bytes.
--
Bill Cheng // bill.cheng@usc.edu <URL:http://merlot.usc.edu/william/usc/>