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Message passing is the technique used by processes for communicating with each other.
It is based on a few basic functions :
= msg_register =
Registers the current process as the owner of a system channel.
System channels are aliases for PIDs (negative PIDs resolve to corresponding real PID).
They range from -1 to -255. They are allocated as follows :
from -1 to -19 : drivers
-1 = keyboard
-2 = mouse
from -20 to -29 : network services
-20 = networking subsystem (may be in several parts)
from -30 to -39 : system services
-30 = virtual file system
-31 = logger
-32 = user server
from -40 to -49 : UI services
-40 = graphic server (includes VESA driver for now)
-41 = sound server
Modules for HDD drivers, network cards, sound cards, ... do not register as devices but instead
notify of their presence to the corresponding service.
= msg_send =
Sends a message to a process. Arguments:
- Receiver PID or system channel id
- Message data (up to 64k bytes, minimum 4 bytes)
The first 4 bytes of the message data always designate the message code (function).
The kernel may intercept some special messages (as "kill process" or stuff like that).
= msg_info =
Gets first message in the queue for current process.
Function can block waiting for a message if asked by caller and if no other function already does it.
When it is there, return the following:
- If a message has come
- The sender PID
- The message code (first 4 bytes of data)
- The length of the data
= msg_get =
Reads the data of the first message in the queue for current process.
Is passed a pointer where the data will be written. It is supposed that enougth memory is already allocated.
From a kernel point of view, message passing is asynchronious,
but there also are userland functions for making them look synchronious by waiting for an answer.
Usually, message codes | 0x80000000 are answers to a message.
Most processes, most of the time, will be in a loop waiting for messages to come.
The drivers are the principal exception to that, they instead wait for an interrupt or another hardware signal.
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