I've just written a couple of simple routines which read an write small text
files to the server:
function readData( $filename, &$data )
{
if (!$fd = @fopen($filename, "r"))
{
return false;
}
flock($fd, LOCK_SH);
$data = fread($fd, filesize($filename));
fclose($fd);
return true;
}
function writeData( $filename, $data )
{
if (!$fd = @fopen($filename, "w"))
{
return false;
}
flock($fd, LOCK_EX);
fwrite($fd, $data);
fclose($fd);
return true;
}
Now, the question is... How much time elapses between the fopen() statement
and the flock() statements?
Looking at the code, it would appear that two separate threads may call
writeData() almost simultaneously giving the following execution:
assume $filename = "sample.txt";
Thread #1 - $fd = fopen($filename, "w"); // succeeds
Thread #2 - $fd = fopen($filename, "w"); // succeeds (I think)
Thread #1 - flock($fd, LOCK_EX); // Does this succeed or fail?
Thread #2 - flock($fd, LOCK_EX); // fails (I think)
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