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:
- Multiple OS sources
- Hardware manufacturers are loath to choose a single-sourced
hardware component because of the risks implied if that source discontinues production.
For the same reason, manufacturers shouldn't be tied to a single-sourced,
proprietary OS simply because their application source code isn't portable to other OSs.
By building applications to the POSIX standards, developers can use OSs from multiple vendors.
Application source code can be readily ported from platform to platform and from OS
to OS, provided that developers avoid using OS-specific extensions.
- Portability of development staff
- Using a common API for embedded development, programmers
experienced with one realtime OS can directly apply their
skill sets to other projects involving other processors and operating systems.
In addition, programmers with UNIX or POSIX experience can easily work on embedded realtime
systems, since the nonrealtime portion of the realtime OS's API is already familiar territory.
- Development environment
- Even in a cross-hosted development environment, the API remains essentially the same as on the embedded system.
Regardless of the particular
host (Linux, Windows,...) or the target (x86,
ARM), the programmer doesn't need to
worry about platform-specific endian, alignment, or I/O issues.