NAME    [Toc]    [Back]

     msgsnd -- send a message to a message queue

LIBRARY    [Toc]    [Back]

     Standard C Library (libc, -lc)

SYNOPSIS    [Toc]    [Back]

     #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    [Toc]    [Back]

     The msgsnd() function sends a message to the message queue specified in
     msqid.  The msgp argument points to a structure containing the message.
     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(3)), mtext is an array of bytes, with a size up to that
     of the system limit (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(3)), 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    [Toc]    [Back]

     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    [Toc]    [Back]

     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 message.

			The msgsz argument is less than 0, or 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.

BUGS    [Toc]    [Back]

     NetBSD and FreeBSD do not define the EIDRM error value, which should be
     used in the case of a removed message queue.

HISTORY    [Toc]    [Back]

     Message queues appeared in the first release of AT&T Unix System V.


FreeBSD 5.2.1		       November 24, 1997		 FreeBSD 5.2.1