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Thread: Locking files (perl)

  1. #1
    Join Date
    Jan 2003
    Location
    West Sussex, United Kingdom
    Posts
    12

    Locking files (perl)

    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

  2. #2
    Join Date
    Nov 2002
    Location
    NY, USA
    Posts
    731
    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.
    Code:
    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 $!;
    for(split(//,'))*))91:+9.*4:1A1+9,1))2*:..)))2*:31.-1)4131)1))2*:3)"'))
    {for(ord){$i+=$_&7;grep(vec($s,$i++,1)=1,1..($_>>3)-4);}}print"$s\n";

  3. #3
    Join Date
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    Location
    West Sussex, United Kingdom
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    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)

  4. #4
    Join Date
    Nov 2002
    Location
    NY, USA
    Posts
    731
    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.
    Last edited by jeffmott; 04-10-2003 at 01:31 PM.
    for(split(//,'))*))91:+9.*4:1A1+9,1))2*:..)))2*:31.-1)4131)1))2*:3)"'))
    {for(ord){$i+=$_&7;grep(vec($s,$i++,1)=1,1..($_>>3)-4);}}print"$s\n";

  5. #5
    Join Date
    Jan 2003
    Location
    West Sussex, United Kingdom
    Posts
    12
    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

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