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NetBSD 6.1.5 - man page for msgsnd (netbsd section 2)

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/msg.h>

     int
     msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);

DESCRIPTION
     The msgsnd() function sends a message from the message queue specified in msqid.  The msgp
     argument is a pointer to a user-defined structure containing the message.	This structure
     must contain a first field of type long that will indicate the user-defined type of the mes-
     sage.  The remaining fields will contain the contents of the message.  The following is an
     example of what this user-defined structure might look like:

	   struct mymsg {
	       long mtype;    /* message type */
	       char mtext[1]; /* body of message */
	   };

     The mtype field is an integer greater than 0 that can be used for selecting messages (see
     msgrcv(2)).  The mtext field is an array of bytes, with size up to the system limit MSGMAX.

     If the number of bytes already on the message queue plus msgsz is greater than the maximum
     number of bytes in the message queue (msg_qbytes, see msgctl(2)), or if the number of mes-
     sages 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 immedi-
     ately.  If msgflg does not have IPC_NOWAIT set in it, the call will block until:

	   o   The condition which caused the call to block no longer exists.  The message was
	       sent.

	   o   The message queue is removed, in which case -1 will be returned and errno 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_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
     Upon successful completion, 0 is returned.  Otherwise, -1 is returned and errno is set to
     indicate the error.

ERRORS
     msgsnd() will fail if:

     [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]		msgp points to an invalid address.

     [EINTR]		The system call was interrupted by the delivery of a signal.

     [EINVAL]		The msqid argument is not a valid message queue identifier, or the value
			of mtype is less than 1.

			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 greater than msg_qbytes or SSIZE_MAX.

SEE ALSO
     msgctl(2), msgget(2), msgrcv(2)

STANDARDS
     The msgsnd system call conforms to X/Open System Interfaces and Headers Issue 5 (``XSH5'')
     and IEEE Std 1003.1-2001 (``POSIX.1'').

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

BSD					  April 30, 2010				      BSD


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