05-28-2008
Semaphore problem....
I'm having an issue implementing my semaphores....
The following is how I'm setting up the semaphore.
First: I get the semkey (which I've wrapped in an IF statement using perror() but in an attempt to keep the code clutter free I've removed it here)
semkey = ftok("./request", 'S' )
Second: I get the semid (again wrapped in an IF statement )
semid = semget ( semkey, 1, 0666 | IPC_CREAT )
Third: I initialise semaphore #0 to 1
arg.val = 1
semctl ( semid, 0, SETVAL, arg )
The following is where the problem is occuring (it seems), this is where I'm actually using the semaphore - semop() - to allow access to a shared memory segment (critical code)
while ( allocate.sem_op == -1 )
{
//printf ( "waiting for semaphore to release\n" );
}
allocate.sem_op = -1;
if ( semop( semid, &allocate, 1 ) == -1 )
{
perror ("Semaphore error3: semop()");
exit(1);
}
//CRITICAL CODE HERE
allocate.sem_op = 1;
if ( semop( semid, &allocate, 1 ) == -1 )
{
perror ("Semaphore error4: semop()");
exit(1);
}
I've left a fair bit of the code out, such as struct sembuf and union semun stuff, but that appears to be working properly.....
The error I'm getting is this:
Semaphore error4: semop(): File too large
Where should I look or what is it that I'm doing wrong?
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LEARN ABOUT OSX
thread::semaphore
Thread::Semaphore(3pm) Perl Programmers Reference Guide Thread::Semaphore(3pm)
NAME
Thread::Semaphore - Thread-safe semaphores
VERSION
This document describes Thread::Semaphore version 2.12
SYNOPSIS
use Thread::Semaphore;
my $s = Thread::Semaphore->new();
$s->down(); # Also known as the semaphore P operation.
# The guarded section is here
$s->up(); # Also known as the semaphore V operation.
# Decrement the semaphore only if it would immediately succeed.
if ($s->down_nb()) {
# The guarded section is here
$s->up();
}
# Forcefully decrement the semaphore even if its count goes below 0.
$s->down_force();
# The default value for semaphore operations is 1
my $s = Thread::Semaphore->new($initial_value);
$s->down($down_value);
$s->up($up_value);
if ($s->down_nb($down_value)) {
...
$s->up($up_value);
}
$s->down_force($down_value);
DESCRIPTION
Semaphores provide a mechanism to regulate access to resources. Unlike locks, semaphores aren't tied to particular scalars, and so may be
used to control access to anything you care to use them for.
Semaphores don't limit their values to zero and one, so they can be used to control access to some resource that there may be more than one
of (e.g., filehandles). Increment and decrement amounts aren't fixed at one either, so threads can reserve or return multiple resources at
once.
METHODS
->new()
->new(NUMBER)
"new" creates a new semaphore, and initializes its count to the specified number (which must be an integer). If no number is
specified, the semaphore's count defaults to 1.
->down()
->down(NUMBER)
The "down" method decreases the semaphore's count by the specified number (which must be an integer >= 1), or by one if no number
is specified.
If the semaphore's count would drop below zero, this method will block until such time as the semaphore's count is greater than or
equal to the amount you're "down"ing the semaphore's count by.
This is the semaphore "P operation" (the name derives from the Dutch word "pak", which means "capture" -- the semaphore operations
were named by the late Dijkstra, who was Dutch).
->down_nb()
->down_nb(NUMBER)
The "down_nb" method attempts to decrease the semaphore's count by the specified number (which must be an integer >= 1), or by one
if no number is specified.
If the semaphore's count would drop below zero, this method will return false, and the semaphore's count remains unchanged.
Otherwise, the semaphore's count is decremented and this method returns true.
->down_force()
->down_force(NUMBER)
The "down_force" method decreases the semaphore's count by the specified number (which must be an integer >= 1), or by one if no
number is specified. This method does not block, and may cause the semaphore's count to drop below zero.
->up()
->up(NUMBER)
The "up" method increases the semaphore's count by the number specified (which must be an integer >= 1), or by one if no number is
specified.
This will unblock any thread that is blocked trying to "down" the semaphore if the "up" raises the semaphore's count above the
amount that the "down" is trying to decrement it by. For example, if three threads are blocked trying to "down" a semaphore by
one, and another thread "up"s the semaphore by two, then two of the blocked threads (which two is indeterminate) will become
unblocked.
This is the semaphore "V operation" (the name derives from the Dutch word "vrij", which means "release").
NOTES
Semaphores created by Thread::Semaphore can be used in both threaded and non-threaded applications. This allows you to write modules and
packages that potentially make use of semaphores, and that will function in either environment.
SEE ALSO
Thread::Semaphore Discussion Forum on CPAN: http://www.cpanforum.com/dist/Thread-Semaphore <http://www.cpanforum.com/dist/Thread-Semaphore>
threads, threads::shared
MAINTAINER
Jerry D. Hedden, <jdhedden AT cpan DOT org>
LICENSE
This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
perl v5.16.2 2012-10-11 Thread::Semaphore(3pm)