Detect File Completely Written?

[NRice]

Hi! I've been programming with Java/JSP for a while but I'm new to PHP.

I have a question: how can I detect (in a PHP script) if a file (basically a GIF image) on the server has been fully written (by another program). The other program will write a file on the server and the PHP script will check for its existence. But I want to add an extra check to see if the file has been completely written (so that its valid) before I process the file, in case the program writing procedure and the script reading procedure collide.

I was thinking of maybe checking some sort of a file flag/attribute on the server (linux) or running some shell command from PHP. A file-size check (for zero bytes in case it is being written) doesn't seem to work.

Any clues? Thanks for your help!

[MarkR]

Depends.

If you have control of the program which is doing the writing, consider locking the file with flock() exclusively before writing to it, then unlocking when finished

Have the other program put a nonexclusive lock on the file - it should then be blocked until the file is fully written.

Of course semantics are rather different on Windows I think.

On Unix, unlike DOS, files are NOT shown as 0 bytes while they're open for writing. They are shown at the size they happen to be at approximately that moment.

One option is to wait until the file stops growing - this is primitive. On win32 the other process will probably put an exclusive lock on the file (unless it's told not to).

Mark