RTEMS C User's Guide
The GET_PACKET component of the user-provided MPCI layer is called when RTEMS must obtain a packet buffer to send or broadcast a message. This component should be adhere to the following prototype:
rtems_mpci_entry user_mpci_get_packet( rtems_packet_prefix **packet );
where packet is the address of a pointer to a packet. This routine always succeeds and, upon return, packet will contain the address of a packet. If for any reason, a packet cannot be successfully obtained, then the fatal error manager should be invoked.
RTEMS has been optimized to avoid the need for obtaining a packet each time a message is sent or broadcast. For example, RTEMS sends response messages (RR) back to the originator in the same packet in which the request message (RQ) arrived.
RTEMS C User's Guide
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