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 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(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:

     •   The condition which caused the call to block does no longer exist.  The message will be
         sent.

     •   The message queue is removed, in which case -1 will be returned, and errno is set to
         EINVAL.

     •   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:

     •   msg_cbytes is incremented by the size of the message.

     •   msg_qnum is incremented by 1.

     •   msg_lspid is set to the pid of the calling process.

     •   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 message.

                        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.