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

     msgsnd -- send a message to a message queue

     #include <&lt;sys/msg.h>&gt;

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

     The msgsnd() function sends a message to the message queue specified by
     msqid.  msgp points to a structure containing the message.  This struc-
     ture 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 mes-
     sages (see msgrcv(2)); mtext is an array of msgsz bytes, with a size
     between 0 and 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(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

     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.

     Upon successful completion, the value 0 is returned; otherwise the
     value -1 is returned and the global variable errno is set to indicate the

     msgsnd() will fail if:

     [EINVAL]           msqid is not a valid message queue identifier.

                        msgsz is greater than msg_qbytes.

     [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

     [EIDRM]            The message queue was removed while msgsnd() was wait-
                        ing for a resource to become available in order to
                        deliver the message.

     msgctl(2), msgget(2), msgrcv(2)

     Message queues appeared in AT&T System V Release 1 UNIX.

BSD                            December 10, 2014                           BSD