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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 LINUX

msgget

MSGGET(2)						     Linux Programmer's Manual							 MSGGET(2)

NAME

       msgget - get a message queue identifier

SYNOPSIS

       #include <sys/types.h>
       #include <sys/ipc.h>
       #include <sys/msg.h>

       int msgget(key_t key, int msgflg);

DESCRIPTION

       The  msgget()  system call returns the message queue identifier associated with the value of the key argument.  A new message queue is cre-
       ated if key has the value IPC_PRIVATE or key isn't IPC_PRIVATE, no message queue with the given key key exists, and IPC_CREAT is  specified
       in msgflg.

       If  msgflg  specifies both IPC_CREAT and IPC_EXCL and a message queue already exists for key, then msgget() fails with errno set to EEXIST.
       (This is analogous to the effect of the combination O_CREAT | O_EXCL for open(2).)

       Upon creation, the least significant bits of the argument msgflg define the permissions of the message queue.  These permission	bits  have
       the same format and semantics as the permissions specified for the mode argument of open(2).  (The execute permissions are not used.)

       If a new message queue is created, then its associated data structure msqid_ds (see msgctl(2)) is initialized as follows:

	      msg_perm.cuid and msg_perm.uid are set to the effective user ID of the calling process.

	      msg_perm.cgid and msg_perm.gid are set to the effective group ID of the calling process.

	      The least significant 9 bits of msg_perm.mode are set to the least significant 9 bits of msgflg.

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

	      msg_ctime is set to the current time.

	      msg_qbytes is set to the system limit MSGMNB.

       If the message queue already exists the permissions are verified, and a check is made to see if it is marked for destruction.

RETURN VALUE

       If successful, the return value will be the message queue identifier (a nonnegative integer), otherwise -1 with errno indicating the error.

ERRORS

       On failure, errno is set to one of the following values:

       EACCES A  message  queue  exists  for  key,  but  the  calling  process does not have permission to access the queue, and does not have the
	      CAP_IPC_OWNER capability.

       EEXIST A message queue exists for key and msgflg specified both IPC_CREAT and IPC_EXCL.

       ENOENT No message queue exists for key and msgflg did not specify IPC_CREAT.

       ENOMEM A message queue has to be created but the system does not have enough memory for the new data structure.

       ENOSPC A message queue has to be created but the system limit for the maximum number of message queues (MSGMNI) would be exceeded.

CONFORMING TO

       SVr4, POSIX.1-2001.

NOTES

       IPC_PRIVATE isn't a flag field but a key_t type.  If this special value is used for key, the system call ignores everything but	the  least
       significant 9 bits of msgflg and creates a new message queue (on success).

       The following is a system limit on message queue resources affecting a msgget() call:

       MSGMNI System  wide  maximum  number  of message queues: policy dependent (on Linux, this limit can be read and modified via /proc/sys/ker-
	      nel/msgmni).

   Linux Notes
       Until version 2.3.20 Linux would return EIDRM for a msgget() on a message queue scheduled for deletion.

BUGS

       The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its function.

SEE ALSO

       msgctl(2), msgrcv(2), msgsnd(2), ftok(3), capabilities(7), mq_overview(7), svipc(7)

COLOPHON

       This page is part of release 3.27 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2004-05-27								 MSGGET(2)
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