Locking files (perl)

[doness]

I am unsure how file locking works, could some kind sole enlighten me: I want to:
open (DAT,"<file"); flock(DAT,LOCK_EX);
#get some data out
$rec=<DAT>;
#then change the data - not shown here, and write it back
#my book says closing a file unlocks it - is this true?
#if it is I cannot close it here but must:-
open (DAT ">file";
print DAT $rec;#write it back
close DAT;
flock (DAT,LOCK_UN);#is this bit required if closing does it anyway?

Help much appreciated

[jeffmott]

my book says closing a file unlocks it - is this true?

Yes.

if it is I cannot close it here but must:-
open (DAT ">file";

Another open will implicitly close the previous, which will also unlock the file.

flock (DAT,LOCK_UN);#is this bit required if closing does it anyway?

No. In fact it is illegal because you place it after the close. It is attempting to perform an operation on a filehandle that is already unlocked and gone.

What you need to do is use seek and truncate so you don't have to close/unlock the filehandle.

use Fcntl qw{:flock :seek};

open FH, '+<file'    or die $!;
flock FH, LOCK_EX    or die $!;
my $data = <FH>;
# ...
seek FH, 0, SEEK_SET or die $!;
truncate FH, 0       or die $!;
print FH $rec        or die $!;
close FH             or die $!;

[doness]

Thanks VERY much for your lucid reply, much appreciated. I will print it all out and study it - if I have any queries I'll be back (like Gen MacArthur!)

I am testing all the programs using Apache on a Windows machine - but I assume I am right in thinking that there is no way the file locking can be tested on this. I assume I shall have to test it when I actually put it on a live ISP.

Thanks

Don - (from the UK)

[jeffmott]

I am testing all the programs using Apache on a Windows machine - but I assume I am right in thinking that there is no way the file locking can be tested on this. I assume I shall have to test it when I actually put it on a live ISP.

Your system should have no problem with perform file locking. You don't even need to have a Web server running. In fact it is a good idea to always check your programs from the command line.

[doness]

Thanks for your reply re testing - I will try again!

Meanwhile,I have had a longer look at the solution you kindly offered. Can you please tell me if my interpretation is correct?

open FH,'+<file'.......opens the file FH for read and write (I didn't know you could do this!)

flock FH,LOCK-EX........locks it

my $data=<FH>;........reads the file data

#....here I amend the data to the new stuff I want to write to the file (ending up as $rec)?

seek FH..............
trucate FH,0.........these 2 together set the file marker back to 0 so that the new data will not be appended?

print FH $rec.............write the new data to disk. NOT appended
close FH.........closes and unlocks

I hope my assumptions are correct!

Can you tell me if ALL of the..."or die $!" instructions are in case the server fails to do the job?

Thanks for everything

Cheers

Don