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Old 03-05-2006, 07:12 AM   #1
sachitha
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System V message queues shared among processes...


i'm using a system V msg queue. which i created in the main program..
and then there are 4 processes-R a,c,b (child processes of one process)
R sends messages to the msg queue..
i want to make the a, c, b to read the messages..

sending part works, but not the reading
the 'a' process reads mostly junk values..(??)

when i'm sharing a msg queue like that do i have to attach it to a "shared memory segment" or something???
 
Old 03-05-2006, 08:57 AM   #2
paulsm4
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Reading and writing from a message queue is conceptually no different from reading or writing from a disk file, a pipe, or a network socket. It's just a stream of bytes: what they mean is entirely up to you.

No, you do not need to set up a shared memory segment. Message Queues ("msgget()") are one type of SysV IPC ("InterProcess Communication" primitive), Shared Memory ("shmget()") is another, Semaphores ("semget()") is yet a third kind of SysV IPC. Using one does not require any of the others; but they can all be used together, in any combination you choose.

Do you have a system where you can experiment with these IPCs yourself (Any *nix system - including Linux - should work fine)?

Otherwise, if you have a Windows system, you might wish to learn about and experiment with the corresponding Windows IPCs.
 
Old 03-05-2006, 10:21 AM   #3
sachitha
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yes i'm working with Linux Redhat... on this.
i do not understand why i get junk values for the reading process...
when i type ipcs -q , i can see that the messages have gone to the msq queue ,(sent by process R).
//the coding for my reading processes..
void readmsqQ(int qid,struct msgbuf *qbuf)
{
//to read message from the message queue...
struct msgbuf msg;
printf("%d %d \n", qid, sizeof(msg));
msgrcv(qid,(struct msgbuf *)qbuf,sizeof(msg),1, 0);

printf("Type: %d Text:%s %d\n", qbuf->cusNo,qbuf->cus_type ,qbuf->servT);
//this above line prints junk values.. when i manually delete the message queue through the terminal
}
 
Old 03-05-2006, 10:50 AM   #4
paulsm4
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Many things *could* be going wrong.

I guess the first step would be to modify your code to check for the return value from "msgrcv()":
Code:
  int iret;
  iret = msgrcv (...);
PS: Please use "[code]" blocks - it definitely helps!

PPS:
Here's a good tutorial (with sample code):
http://www.ecst.csuchico.edu/~beej/guide/ipc/mq.html

My book on SysV IPC's is undoubtedly out of print, but here's the reference anyway (it's a great book; it's served me well over the years):
Unix System Programming, Keith Haviland & Ben Salama, ISBN 0 201 129191

Last edited by paulsm4; 03-05-2006 at 11:24 AM.
 
  


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