flock allows you to lock a file, so other process that check for locks will wait until the file is unlocked to use it. This way 2 processes will not try to write a file at once, which usually results in one of the writes being lost.

See perldoc -f flock, search for flock on this site and look at KM's RE: File Locking for more information.

1. flock() is a function used to lock files.
2. See the basic info here
3. When to use it. flock() is used whenever two (or more) processes may try to access the same file at the same time. It gives you the abiality to have one process write at a time, and all reads will wait until the wrtie is complete.

1. If there is a process which does not check for a lock, it will not stop it from writting. This means, that for flock() to work correctly, all processes which read/write the data file need to impliment flock().
2. flock() is unimplimented on some OS's.

The name flock is a contraction of "file lock". The syntax for it is available via perldoc -f flock, or here.

Why? Because if two programs, or two instances of the same program (as you may get from a web server) attempt to rewrite the same file at the same time you lose changes.

For example:

• prog 1 reads file
• prog 2 reads file
• prog 1 writes file
• prog 2 writes file, losing prog 1's changes

Worse:

• prog 1 reads file
• prog 1 starts rewriting file, and gets halfway
• prog 2 reads file (but only reads the first half)
• prog 1 finishes writing
• prog 2 rewrites file

By calling flock() before starting to read the file, the problem will be avoided, as prog 2 will simply stall until prog 1 releases the lock.