User Tasks <-> Mach Message <-> Kernel Services Task <-> Msg <-> Port <-> Msg <-> Kernel
- send: Ordered
- send-once: unordered
Table of Contents
------------ header ------------ optional descriptors ------------ body ------------
- header (fixed-size) destination, size(header+body)
- body (variable-sized) containing kernel and user data
- contains descriptors of additional port rights to transmitted
- descriptors of ‘out-of-line’ memory regions to be sent and a variable amount of user data
- trailer (variable-sized) kernel appended message attributes
lsmp list mach ports
- kernel maintained message queue
- multiple sender, single receiver
In userspace, mach port names name rights a process has over a particular message queue.
- send right: enqueue an unlimited number of messages to a particular message queue
- send-once right: enqueue a single message
- receive right: dequeue an unlimited number of messages
- portset right: dequeue an unlimited number of messages from multiple message queues
- dead-name right: do nothing (no longer has a receiver)
for kernel owned ports for the kernel MIG apis, the messages never get queued, there’s a fast path which turns them into synchronous syscalls (
- Port rights can be transferred between tasks via messages
- Tasks can receive messages from ports and port sets
- Tasks manipulate port sets with a port set name
- A port may not belong to more than one port set
- If a port is a member of a port set, the holder of the receive right cannot receive directly from the port
- Port rights are a secure, location-independent way of naming ports
- Port rights do not carry any location information
- Each task has its own space of port rights.
- Port rights are named with unsigned integers.
sizeof(mach_port_t) = sizeof(mach_port_name_t) = sizeof(void*)
Three basic rights
- receive rights
- send rights
- send-once rights
- A port name can name any of these types of rights, or name a port-set, be a dead name, or name nothing.
- Dead names act as place-holders to prevent a name from being otherwise used.
- A port is destroyed, or dies, when its receive right is de-allocated.