Note that to check for a lock, you'll need to try a flock() with LOCK_NB, otherwise, if the file is already locked, the flock() will block untill the lock is released.

But in my case I have some processes running in parallel and accessing the same file. Suppose Process1 is writing to a file, it should lock the file. and when procees2 has to write into the same file, it should check if the file is locked, if so wait till it is released and then write into it. How to implement this?

If you lock a file for writing, it will automatically wait untill any other lock is released. In other words you can do this in both processes and it'll work:

use Fcntl qw(:flock);  # import LOCK_* constants

open F,">>",$somefile or die $!;
flock(F,LOCK_EX);  # lock exclusively - waits for lock

# write stuff here

close(F); # close() will release the lock.
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"What should it profit a man, if he should win a flame war, yet lose his cool?"

http://www.perlmonks.org/?node_id=178572
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If you type code like this; [id://178572] that will expand into a direct link to that node with the title of the node as text, like this: flock() usage.

See also writeup formatting tips.

"What should it profit a man, if he should win a flame war, yet lose his cool?"

http://www.xav.com/perl/lib/Pod/perlthrtut.html
http://www.globalspin.com/thebook/chapter20.html
http://perldoc.perl.org/perlthrtut.html
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#*********************************************************************
+*********
#
# Function      : log_msg
#
# Description   : Log a msg in shared log file ie 1 file for all serve
+r
#                 instances.
#
# Params        : $_[0]     log msg
#
# Returns       : none
#
#*********************************************************************
+*********
sub log_msg
{
    my (
        $log_msg,   # msg to be logged
        $datestamp, # Timestamp in format: Tue Oct 26 12:07:54 2004
        $logfile,   # final logfile name
        $filename,  # filename sans extension
        $filext,    # filename extension
        $filedir,   # current dir
        $error_msg  # error if any
        );

    # Assign input params
    $log_msg = $_[0];

    # Get Timestamp in format: Tue Oct 26 12:07:54 2004
    $datestamp = localtime;

    # Get filename components
    ($filename, $filedir, $filext) = fileparse($0, '\..*');

    # Create filename, open with Exclusive lock
    $logfile = "${cfg::params{'LOG_DIR'}}/${filename}${filext}.log";
    open(CLIENT_LOG, ">>$logfile") or
                $error_msg = "log_msg(): Can't open $logfile: $!";
    if( !$error_msg )
    {
        flock(CLIENT_LOG, LOCK_EX) or
                $error_msg = "log_msg(): Can't lock $logfile: $!";
    }

    # If ok, log msg contents
    if( !$error_msg )
    {
        print CLIENT_LOG "$datestamp : $$ : $log_msg\n";

        # Release lock & close
        flock(CLIENT_LOG, LOCK_UN) or
                $error_msg = "log_msg(): Can't unlock $logfile: $!";
        close(CLIENT_LOG) or
                $error_msg = "log_msg(): Can't close $logfile: $!";
    }

    # Log & email admin if an error occurred
    if( $error_msg )
    {
        print "$error_msg\n";   # desperation / belt & braces

        send_email($error_msg);
    }
}
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