A common problem with realtime application development is
that each realtime OS tends to come equipped with its own
proprietary API. In the absence of industry standards, this
isn't an unusual state for a competitive marketplace to
evolve into, since surveys of the realtime marketplace
regularly show heavy use of in-house proprietary operating
systems. POSIX represents a chance to unify this
marketplace.
Among the many POSIX standards, those of most interest to
embedded systems developers are:
- 1003.1—defines the API for process management,
device I/O, filesystem I/O, and basic IPC. This encompasses
what might be described as the base functionality of a UNIX
OS, serving as a useful standard for many applications. From
a C-language programming perspective, ANSI X3J11 C is
assumed as a starting point, and then the various aspects of
managing processes, files, and tty devices are detailed
beyond what ANSI C specifies.
- Realtime Extensions—defines a set
of realtime extensions to the base 1003.1 standard. These
extensions consist of semaphores, prioritized process
scheduling, realtime extensions to signals, high-resolution
timer control, enhanced IPC primitives, synchronous and
asynchronous I/O, and a recommendation for realtime
contiguous file support.
- Threads—further extends the POSIX
environment to include the creation and management of
multiple threads of execution within a given address space.
- Additional Realtime Extensions—defines further extensions to the realtime standard.
Facilities such as attaching interrupt handlers are described.
- Application Environment Profiles—defines several AEPs (Realtime AEP,
Embedded Systems AEP, etc.) of the POSIX
environment to suit different embedded capability sets.
These profiles represent embedded OSs with/without filesystems and other capabilities.
Note:
For information about the many POSIX drafts and standards, see the IEEE website at
http://www.ieee.org/.
Apart from any "bandwagon" motive for adopting
industry standards, there are several specific advantages to
applying the POSIX standard to the embedded realtime marketplace.