some easy question.

Posted on 2000-03-29
Last Modified: 2010-03-05
1.what's mean about this section?

if ($lock){                <---?
flock(LIST, $lock);        <---?


print qq~
Question by:li_fang
LVL 19

Expert Comment

by:Kim Ryan
ID: 2668232
If $lock is true ( > 0), get a lock on the file handle LIST. Lock modes are:

$LOCK_SH = 1;
$LOCK_EX = 2;
$LOCK_NB = 4;
$LOCK_UN = 8;

qq means  print the followong literla string, don't understand the ~

LVL 10

Expert Comment

ID: 2668346
# some crude ways to lock a section of critical code
my $FILEH;

sub lockSection()
      open(FILEH,$com."common/") or die("cannt open the lock file : ".$!);
      flock(FILEH,2) or die("cannt lock the file : ".$!);

sub unlockSection()
      flock(FILEH,8) or die("cannt unlock file : ".$!);
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Expert Comment

ID: 2668350
sorry i meant to edit that code before submitting..
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LVL 16

Accepted Solution

maneshr earned 20 total points
ID: 2669235

ABOUT flock(LIST, $lock);

Operating systems vary greatly in the type and reliability of locking techniques available. Perl tries hard to give you something
that works, even if your operating system uses its own underlying technique.

The flock function takes two arguments: a filehandle and a number representing what to do with the lock on that filehandle or the Lock mode.

The different Lock modes are:

$LOCK_SH  = 1      #  Shared lock (for reading)
$LOCK_EX  = 2      #  Exclusive lock (for writing)
$LOCK_NB  = 4      #  Non-blocking request (don't stall)
$LOCK_UN  = 8      #  Free the lock (careful!)

Flocking files is like putting up a stoplight at an intersection. It works only if people pay attention to whether the light is red or
green  - or yellow in the case of a shared lock. The red light doesn't stop traffic; it merely signals that traffic should stop. A
desperate, ignorant, or rude person will still go flying through the intersection no matter what the light says. Likewise, flock
only blocks out other flockers  - not processes trying to do I/O. Unless everyone is polite, accidents can (and will) happen.

ABOUT if ($lock){

Checks if a lock has already been set (ie $lock contains a value greater tha zero)

Hope that helps
LVL 10

Expert Comment

ID: 2669518

well put maneshr.. good analogy

Author Comment

ID: 2671483
Thanks for you help!

(And I don't know how to give the points to more than one people,so thanks for teraplane.

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