problem in msgctl() system call

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msgsnd

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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