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Full Discussion: Unix message Queue
Top Forums UNIX for Advanced & Expert Users Unix message Queue Post 38919 by Perderabo on Wednesday 30th of July 2003 11:12:50 AM
Old 07-30-2003
Hmmm....you do have a point. I was getting CBYTES mixed up with QBYTES.

I agree, for the most part, QNUM = 0 and CBYTES != 0 is not consistent. But I can think of one exception where it might make sense. When you do your msgsnd, you are setting msgflg to zero. That means that IPC_NOWAIT is turned off. The msgsnd system call checks to see if this message would exceed some limits. This includes a check of the message queue you are using as well as some system wide limits. If a msgsnd would exceed a limit and it did not set IPC_NOWAIT, the msgsnd system call will suspend until it is safe to send the message. I don't know what state the message queue is in during this suspension. It could well be that CBYTES has been increased but QNUM has not yet been incremented.

This would explain why it works for a few hours and then locks up. Reproduce the error and then run "ipcs -qa" and compare the total messages queue resources in use against your kernel parameters. If this is the problem, you will need to increase the kernel parameters. Or you may have a message queue leak somewhere.

That is my only idea. If it's not that, I would next suspect a broken kernel. You might check for any appropiate patches.
 

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MSGSND(2)						      BSD System Calls Manual							 MSGSND(2)

NAME
msgsnd -- send a message to a message queue LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h> int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg); DESCRIPTION
The msgsnd() function sends a message to the message queue specified in msqid. The msgp argument points to a structure containing the mes- sage. This structure should consist of the following members: long mtype; /* message type */ char mtext[1]; /* body of message */ mtype is an integer greater than 0 that can be used for selecting messages (see msgrcv(2)), mtext is an array of msgsz bytes. The argument msgsz can range from 0 to a system-imposed maximum, MSGMAX. If the number of bytes already on the message queue plus msgsz is bigger than the maximum number of bytes on the message queue (msg_qbytes, see msgctl(2)), or the number of messages on all queues system-wide is already equal to the system limit, msgflg determines the action of msgsnd(). If msgflg has IPC_NOWAIT mask set in it, the call will return immediately. If msgflg does not have IPC_NOWAIT set in it, the call will block until: o The condition which caused the call to block does no longer exist. The message will be sent. o The message queue is removed, in which case -1 will be returned, and errno is set to EINVAL. o The caller catches a signal. The call returns with errno set to EINTR. After a successful call, the data structure associated with the message queue is updated in the following way: o msg_cbytes is incremented by the size of the message. o msg_qnum is incremented by 1. o msg_lspid is set to the pid of the calling process. o msg_stime is set to the current time. RETURN VALUES
The msgsnd() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error. ERRORS
The msgsnd() function will fail if: [EINVAL] The msqid argument is not a valid message queue identifier The message queue was removed while msgsnd() was waiting for a resource to become available in order to deliver the mes- sage. The msgsz argument is greater than msg_qbytes. The mtype argument is not greater than 0. [EACCES] The calling process does not have write access to the message queue. [EAGAIN] There was no space for this message either on the queue, or in the whole system, and IPC_NOWAIT was set in msgflg. [EFAULT] The msgp argument points to an invalid address. [EINTR] The system call was interrupted by the delivery of a signal. HISTORY
Message queues appeared in the first release of AT&T Unix System V. BUGS
NetBSD and FreeBSD do not define the EIDRM error value, which should be used in the case of a removed message queue. BSD
July 9, 2009 BSD
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