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Full Discussion: Problem with msgget()
Top Forums Programming Problem with msgget() Post 302088868 by blowtorch on Friday 15th of September 2006 12:13:22 AM
Old 09-15-2006
test1.c:
Code:
#include<stdio.h>
#include<sys/msg.h>

int main() {
        int msqid;

        msqid=msgget(938,666|IPC_CREAT);
        fprintf(stdout,"msqid from program 1: %d\n",msqid);
        fprintf(stdout,"now sleeping for 30 seconds\n");
        sleep(30);
        msgctl(msqid,IPC_RMID,NULL);
}

test2.c:
Code:
#include<stdio.h>
#include<sys/msg.h>

int main() {
        int msqid;

        msqid=msgget(938,666|IPC_CREAT);
        fprintf(stdout,"msqid from program 2: %d\n",msqid);
        msgctl(msqid,IPC_RMID,NULL);
}

I used the above bits of test code to check if at any point the message queue id changed. It didn't
In fact, the man page of msgget makes it quite clear:
Quote:
The msgget() argument returns the message queue identifier
associated with key.

A message queue identifier and associated message queue and
data structure (see intro(3)) are created for key if one of
the following are true:

o key is IPC_PRIVATE.

o key does not already have a message queue identifier
associated with it, and (msgflg&IPC_CREAT) is true.
Remember that msgget will always return a queue id that is associated with the key. This means that if there is already a queueid that is associated with the key, thats what you'll get. A new queue id is only created under the circumstances specified (key is IPC_PRIVATE or queueid does not already exist).

So if you have a program that created a message queue with a certain key, then another program that tries to create a message queue with the same key will *always* get the existing message queue id ,unless one of the programs removes the queue from the system.

In your case, if you have two programs, is one of the programs removing the message queue before the second one can access it? If this happens, then the queue id associated with your key will no longer exist and you will get a new queue id.

And don't worry about the size of the queue id. It makes no difference to your program.
 

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LEARN ABOUT SUNOS

msgget

msgget(2)							   System Calls 							 msgget(2)

NAME

       msgget - get message queue

SYNOPSIS

       #include <sys/msg.h>

       int msgget(key_t key, int msgflg);

DESCRIPTION

       The msgget() argument returns the message queue identifier associated with key.

       A  message  queue identifier and associated message queue and data structure (see intro(2)) are created for key if one of the following are
       true:

	 o  key is IPC_PRIVATE.

	 o  key does not already have a message queue identifier associated with it, and (msgflg&IPC_CREAT) is true.

       On creation, the data structure associated with the new message queue identifier is initialized as follows:

	 o  msg_perm.cuid, msg_perm.uid, msg_perm.cgid, and msg_perm.gid are set to the effective user ID and effective group ID, respectively, of
	    the calling process.

	 o  The low-order 9 bits of msg_perm.mode are set to the low-order 9 bits of msgflg.

	 o  msg_qnum, msg_lspid, msg_lrpid, msg_stime, and msg_rtime are set to 0.

	 o  msg_ctime is set to the current time.

	 o  msg_qbytes is set to the system limit. See NOTES.

RETURN VALUES

       Upon successful completion, a non-negative integer representing a message queue identifier is returned. Otherwise, -1 is returned and errno
       is set to indicate the error.

ERRORS

       The msgget() function will fail if:

       EACCES	       A message queue identifier exists for key, but operation permission (see intro(2)) as specified by the low-order 9 bits	of
		       msgflg would not be granted.

       EEXIST	       A message queue identifier exists for key but (msgflg&IPC_CREAT) and (msgflg&IPC_EXCL) are both true.

       ENOENT	       A message queue identifier does not exist for key and (msgflg&IPC_CREAT) is false.

       ENOSPC	       A  message  queue  identifier  is to be created but the system-imposed limit on the maximum number of allowed message queue
		       identifiers system wide would be exceeded. See NOTES.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Standard			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       rctladm(1M), intro(2), msgctl(2), msgrcv(2), msgsnd(2), setrctl(2), ftok(3C), attributes(5), standards(5)

NOTES

       The system-defined limit used to initialize msg_qbytes is the minimum  enforced	value  of  the	calling  process's  process.max-msg-qbytes
       resource control.

       The  system-imposed  limit  on  the  number of message queue identifiers is maintained on a per-project basis using the project.max-msg-ids
       resource control.

	See rctladm(1M) and setrctl(2) for information about using resource controls.

SunOS 5.10							    11 Feb 2003 							 msgget(2)
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