Stop multiple simultaneous access to the same file

Question

Can I lock a file to stop it from opening when the GD library accesses it?

What I'm looking for is similar to the isolation level of the "serialzable" database ... I want only one session / user to access the image at a time to stop the dirty reading.

My application allows users to add a larger image.

, eg

• large image is empty
• Raj and Janet upload their images
• The Raj Session opens a large image.
• 1 ms later, Janet opens a large image.
• Raj Session Adds Your Image and Saves a Large Image
• 1 ms after the session, Janet adds his image and saves his version of a large image.
• As a result, the Raj image is not in the final image as the Janet version overwritten it.

I hope this will be clear enough.

Answer 1

Check out flock . You can lock the file you are working with, or create a “lock file” elsewhere in the file system for which all your scripts are checked.