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Full Discussion: IPC Message Queue. msgrcv doesnt work..
Top Forums UNIX for Dummies Questions & Answers IPC Message Queue. msgrcv doesnt work.. Post 302385846 by daredevil82m on Sunday 10th of January 2010 09:45:57 AM
Old 01-10-2010
Error IPC Message Queue. msgrcv doesnt work..
Hi everybody,

this is the situation. there is a programm XYZ which opens a message queue with the key 47110815 and waits for a SIGUSR1. After receiving this signal it sends a message with type 100 and a number (as ASCII) in the message-body.

I have to write a prog which frist sends the SIGUSR1 to that process XYZ and receives the message which XYZ loaded in the message queue. After that it has to send a message (with type = the number in the messagebody). After receiving my message XYZ will modify it and load it again in the message queue. I have to read that modified message again. I've written the code but it hangs up at the line where it should receive the message (msgrcv) for the first time.

executing XYZ will display its pid. we will start my prog with that pid as parameter and will use it to send SIGUSR1 to XYZ.

I cant view XYZ but can ensure that it works, cuz its from our proffessor.. It'd be great if onyone could help. Here is my code:

#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <string.h>
#include <signal.h>
#include <unistd.h>
#define KEY 47110815
#define MSIZE 100

struct message{
long type;
char data[MSIZE];
};

struct message m;

main(int i, char *p[]){
int id;
long number; char s[100];

kill(atoi(p[1]),10); //sends SIGUSR1 to process XYZ
printf("\nSIGUSR1 sent!\n");
id=msgget(KEY,0666 );

printf("Waiting for 1. Message from XYZ...\n");
msgrcv(id,&m,MSIZE,(long)100,0);
printf("Message received!\n");

number = m.type = (long) atoi(m.data);
strcpy(&m.data[0],"abcde");
msgsnd(id,&m,MSIZE,0);
printf("Message sent with Typ=%d Id=%d\n",m.type,id);

printf("Waiting for changed Message...\n");
sleep(0.5);
msgrcv(id,&m,MSIZE,(long)number,0);
printf("Changed Message received!:");
printf(">>%d>>%s\n",m.type,m.data);
}
 

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LEARN ABOUT REDHAT

ipc::msg

IPC::Msg(3pm)						 Perl Programmers Reference Guide					     IPC::Msg(3pm)

NAME

       IPC::Msg - SysV Msg IPC object class

SYNOPSIS

	   use IPC::SysV qw(IPC_PRIVATE S_IRWXU);
	   use IPC::Msg;

	   $msg = new IPC::Msg(IPC_PRIVATE, S_IRWXU);

	   $msg->snd(pack("l! a*",$msgtype,$msg));

	   $msg->rcv($buf,256);

	   $ds = $msg->stat;

	   $msg->remove;

DESCRIPTION

       A class providing an object based interface to SysV IPC message queues.

METHODS

       new ( KEY , FLAGS )
	   Creates a new message queue associated with "KEY". A new queue is created if

	   o   "KEY" is equal to "IPC_PRIVATE"

	   o   "KEY" does not already  have  a	message queue associated with it, and "FLAGS & IPC_CREAT" is true.

	   On creation of a new message queue "FLAGS" is used to set the permissions.

       id  Returns the system message queue identifier.

       rcv ( BUF, LEN [, TYPE [, FLAGS ]] )
	   Read a message from the queue. Returns the type of the message read.  See msgrcv.  The  BUF becomes tainted.

       remove
	   Remove and destroy the message queue from the system.

       set ( STAT )
       set ( NAME => VALUE [, NAME => VALUE ...] )
	   "set" will set the following values of the "stat" structure associated with the message queue.

	       uid
	       gid
	       mode (oly the permission bits)
	       qbytes

	   "set" accepts either a stat object, as returned by the "stat" method, or a list of name-value pairs.

       snd ( TYPE, MSG [, FLAGS ] )
	   Place a message on the queue with the data from "MSG" and with type "TYPE".	See msgsnd.

       stat
	   Returns an object of type "IPC::Msg::stat" which is a sub-class of "Class::Struct". It provides the following fields. For a description
	   of these fields see you system documentation.

	       uid
	       gid
	       cuid
	       cgid
	       mode
	       qnum
	       qbytes
	       lspid
	       lrpid
	       stime
	       rtime
	       ctime

SEE ALSO

       IPC::SysV Class::Struct

AUTHOR

       Graham Barr <gbarr@pobox.com>

COPYRIGHT

       Copyright (c) 1997 Graham Barr. All rights reserved.  This program is free software; you can redistribute it and/or modify it under the
       same terms as Perl itself.

perl v5.8.0							    2002-06-01							     IPC::Msg(3pm)
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