SLock - A php class to allow safe access to shared resources
by multiple threads of execution
A classic example of where this would be useful would be
a web counter where the current number of hits is stored in
a text file. So long as there is only one thread of
execution, the sequence of events is:
- read counter.txt (value is for example: 42)
- add one to current value (value now: 43)
- write new value to counter.txt
With another thread of execution, the following can happen:
- Thread 1 reads counter.txt (value is for example: 42)
- Thread 2 reads counter.txt (also gets value: 42)
- Thread 1 adds one to current value (value now: 43)
- Thread 2 adds one to current value (value now: 43)
- Thread 1 writes new value to counter.txt
- Thread 2 writes new value to counter.txt
The problem being that counter.txt *should* contain 44, not 43.
By using SLock, these problems can be avoided - before
reading the counter.txt file the script must acquire
an SLock on a specified port and the lock is held until
the file has been written. Any other scripts would be
denied the SLock until the first script finished.
Here is a simple example, run two or more copies
simultaneously to see the effect ...
require('slock.inc');
function basketBallPlayer()
{
print("\nI've got the ball!\n");
for($i = 0; $i < 3; $i ++)
{
print(" I'm dribbling it ...\n");
sleep(1);
}
print("Now I pass it to someone else ...\n");
}
for($i = 0; $i < 5; $i ++)
{
if(!SLock::doCall("basketBallPlayer", 8787, $errMsg))
die($errMsg . "\n");
sleep(2 + (rand() % 3));
}
 Notes
- Since SLock calls: (socket_create, socket_bind, socket_close),
php will need to have socket support. Frequently, this
involves using '--enable-sockets' during the configuration
of php prior to build.
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