Manual Pages  — MSGSND

    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.