Shrinking file that is opened in read/write

Question

In perl:

I have a file opened in read/write, with an exclusive lock.

open( $f, "+< $filename" ); flock( $f, LOCK_EX );

If I write more data to the file than it previously held, the file will grow.
If I write less data, my new contents are at the beginning, but the old contents are still there at the end of the file.

This isn't surprising, however it's not what I want.

Is there a simple way to shrink the file while it is opened in read/write? Basically I want to tell it to end the file at exactly this byte position.

I know I can open it differently, and I'm considering doing that, but a one line fix would be nice.

I would suggest testing the result of the open, flock, truncate, etc. — hpavc, Nov 23 '10 at 06:56
@hpavc, or use [autodie](http://search.cpan.org/perldoc?autodie). — cjm, Nov 23 '10 at 07:42
I am testing the results of all these commands. Just cut them out for brevity. — Brock, Nov 24 '10 at 21:36

score 1 · Accepted Answer · answered Nov 23 '10 at 00:24

1

truncate

answered Nov 23 '10 at 00:24

Ignacio Vazquez-Abrams

776,304
153
1,341
1,358

You both win a cookie. I can't accept your answers for 9 minutes though (Why is that?). – Brock Nov 23 '10 at 00:28
I even searched for the word truncate, and managed to miss that somehow. Thanks. – Brock Nov 23 '10 at 00:28

score 1 · Answer 2 · answered Nov 23 '10 at 00:25

1

I actually don't know about perl, but since ftruncate(2) would be the C function, maybe this helps?

answered Nov 23 '10 at 00:25

dennycrane

2,301
18
15

Shrinking file that is opened in read/write

2 Answers2