Concurrent file access problem !

[gael Jeanniard du Dot]

 Hy all!

I'am working on a webcam application and I enconter a problem. I have a
Camera, a video server and a Web/FTP server (Redhat 7.0, Apache
1.3.12-25, Wu-ftpd 2.6.1-6). My video server send 1 picture/s to the ftp

server and the Web server pushes the pictures to the web client (Ie &
Netscape) by a cgi script. My problem is a concurrent file access
beetween the cgi script, which read the pictures file (1 per second) and

the ftp server which write the picture file. The result of the error is
that the pictures appear truncated to the web client. Furthermore, when
the number of web client increases, the error increases too. I tried to
force simultaneous access to the file by using the perl function sysopen

with the parameter O_NONBLOCK, but my pictures appear mixed. I use a
ramdisk to store the pictures which speeding the application but which
not solve the problem.
Now, I would like to synchronize the cgi file read access in order to
block the script picture reading while the file is being writing. How to

do that ? If you have any other idea, I will be enjoyed ..

Thank you for your future help..

I joined the cgi script :

#!/usr/bin/perl
#
#

use Fcntl;
require 'stat.pl';

#########################################################
# Path to where the image file is stored
$DIR = "/cam/";
#Filename the image is stored as
$fileName = "image.jpg";
#Maximum of images/s sent
$freq = 1;
#Max number of images to send on a connection
$maxImages = 900;
#Max number of seconds until update is considered stopped
#(ie the camera is no longer updating the image)
$maxNoUpdate = 30;
#########################################################

$con_type = "jpeg";

# Unbuffer the output so it streams through faster and better.
 $| = 1;

# No input record separator when reading from file via <>.
undef $/;

# Print HTTP headers...

print "Content-type: multipart/x-mixed-replace;
boundary=--myboundary\n\n";
print "--myboundary\n";

$rounds=0;

while ($rounds < $maxImages)
{
  #$rounds =   $rounds +1;
  $basefile = $DIR . $fileName;

  @fstat = stat($basefile);

  # If the same image time stamp is on the image file for more then
  # X seconds then I presume that the image is no longer updated and
will
  # End the connection

  if ($fstat[$ST_MTIME] ne $oldimagetime)
  {
      $sameCount = 0;
      $oldimagetime = $fstat[$ST_MTIME];
  }

  #We may send the same image multiple times but there is a strict limit

  if ($sameCount > ($maxNoUpdate * $freq))
  {
    die;
  }

  $sameCount = $sameCount +1;

  print "Content-type: image/$con_type\n\n";

  open(PIC,"$basefile");
  print STDOUT <PIC>;
  close(PIC);

  print "\n\n--myboundary\n";

sleep($freq);
}


----- Gaël Jeanniard du Dot -----