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     msgsnd — send a message to a message queue


     Standard C Library (libc, -lc)


     #include <sys/types.h>
     #include <sys/ipc.h>
     #include <sys/msg.h>

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


     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

     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.


     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


     The msgsnd() function will fail if:

     [EINVAL]           The msqid argument is not a valid message queue

                        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


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


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