INtime® 3.1 Software - tenAsys

Chapter 3: About INtime software’s RT kernel
Mailboxes
Mailboxes provide communication between threads in the same process or in different
processes. They can send information and, since a thread may have to wait for
information before executing, they can synchronize thread execution. There are two
mailbox types:
• Object mailboxes: Send and receive object handles. Available only as high
level objects.
• Data mailboxes: Send and receive data. Available as both high- and low-level
objects. High-level data mailboxes have a maximum message size of 128 bytes.
The next figure shows how threads use an object mailbox to send a handle for a
memory address.
Figure 3-2. Threads using an object mailbox
A
B
Thread A allocates a block of memory and creates a shared-memory handle for it. Data is placed
in this shared memory object.
Thread A sends the shared memory handle to a mailbox.
Thread B waits to receive the shared memory handle at the mailbox. You can specify whether
thread B should wait if the handle isn’t in the mailbox.
Thread B obtains the handle and accesses the data in the memory object by mapping the
memory associated with the memory object into its memory address space.
Mailboxes have thread queues, where threads wait for messages, and message queues,
where messages wait threads to receive them. The thread queue may be FIFO- or
priority-based; the message queue is always FIFO-based.
You use the same system calls to create and delete object and data mailboxes. However,
you use different calls to send and receive messages or data.
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