is an integer greater than 0 that can be used for selecting messages (see is an array of bytes, with a size up to that of the system limit (MSGMAX).
If the number of bytes already on the message queue plus is bigger than the maximum number of bytes on the message queue ( see or the number of messages on all queues system-wide is already equal to the system limit, determines the action of If has IPC_NOWAIT mask set in it, the call will return immediately. If 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 messag queue is removed, in which case -1 will be returned, and is set to EINVAL. The caller catches a signal. The call returns with set to EINTR.
After a successful call, the data structure associated with the message queue is updated in the following way: is incremented by the size of the message. is incremented by 1. is set to the pid of the calling process. is set to the current time.
The message queue was removed while was waiting for a resource to become available in order to deliver the message.
is less than 0, or greater than The calling process does not have write access to the message queue. There was no space for this message either on the queue, or in the whole system, and IPC_NOWAIT was set in points to an invalid address. The system call was interrupted by the delivery of a signal.