EL/IX: Unifying APIs for Linux and Post-PC Computing
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POSIX 1003.1

The latest ISO/IEC POSIX 1003.1 standard no longer refers to the old A, B, and C sections, but they are useful to refer to in this discussion. Section A is the basic Unix API, B is realtime extensions, and C POSIX threads.

The initial implementation will support only the following elements of the POSIX standard:

Base Unix API (Section A)
Most file and directory operations:

  • read, write, open, close, lseek, fnctl, mkdir, rmdir, chdir, link, unlink, opendir, readdir, fsync.
  • no dup, chown, or ftruncate
  • debate stat and chmod

Time operations:

  • time, localtime.
  • No timezone support (initially; this is no big loss since the current standardized interface the time handling functions implement is not sophisticated enough for most implementations anyway.)
  • (Not in POSIX, but include gettimeofday.)

Signals will be included in the standard as an optional element.

The initial implementation may well not include support for signals.

_exit, abort, exit will have slightly modified characteristics from the POSIX standard depending under which environment they are executed in (configurable hard stop, system restart, etc. under eCos).

Realtime extensions (Section B)

  • Async I/O
  • Semaphores (only for semaphores used in a single process)
  • Reliable signals (meaning queueable real-time signals, subject to the limit of the maximum number of outstanding real-time signals)
  • Message passing (glibc will be enhanced to support mqueues, which are missing from the current version of glibc)
  • Time related functions: nanosleep.
  • Other timer-related functions (timer_gettime, clock_gettime, etc.) may be included via patches that exist to the Linux kernel today.

We will support SCHED_RR and SCHED_FIFO (disable timeslice) scheduling policies. The default scheduler will be the eCos MLQ scheduler. We could consider implementing a Linux compatible scheduler for SCHED_OTHER, or as a configuration option, but there doesn't seem to be a valid reason to do this in the embedded context. Other schedulers will be configurable as per std eCos functionality - but this is not strictly part of the EL/IX standard as this feature is not available under Linux. Many eCos scheduler implementations should however be compatible with the eCos pthreads implementation.

mmap and shared memory are not supported - amongst other reasons they would either require filesystem and MMU support as standard, or are not sensible in the context of a single process. We may elect to support mlock/munlock - as NOPs, and potentially add mprotect in the future (but would require some basic MMU support being added to the eCos hardware abstraction layer (HAL)).

POSIX threads (Section C)
All of the pthreads functionality is included with the possible exception of pthread_kill and pthread_cancel. Thread cancellation and signals are optional (configurable) elements of EL/IX, and will probably not be included in the initial implementation.

Per-thread data requirements can be huge. The POSIX standard defines a minimum of 128 keys to be available, which may consume 512 bytes per thread in some implementations. We will provide a STRICT_POSIX configuration option that provides this minimum, but the standard default will be 0 - but configurable. We will also follow this type of philosophy for maximum open file limits, etc.

ISO C Library
The full ISO C Library is included with the following additions and modifications:

  • fileno.
  • All the _r reentrant functions (from Section 8 of 1003.1), namely strtok_r, rand_r, asctime_r, ctime_r, gmtime_r, and localtime_r.
  • Only the C locale will be supported (initially; future versions will be able to support additional locales through configuration options).
  • Multiple timezone support will not be included.
  • Debatable, but probably include environment, but will not include any variables as standard. Include putenv.
  • Maths library - C90 for now with the C9x extension of long double. Other C9x may be supported in the future

BSD socket library
Now in draft as POSIX 1003.1g, excluding STREAMS (which are now optional). We will probably include most of it, but make DNS operations such as gethostbyname optional (most likely to be provided by vendors of TCP/IP stacks that can be configured into non-Linux operating systems).

Not part of EL/IX as such, but infrastructure will be provided to enable third party vendors to plug their stacks and file systems into eCos. This will include support for the select functions, read, write, etc. on sockets and files.

I/O
EL/IX includes BSD sockets (as part of 1003.1g), which should take care of many I/O requirements. We also provide POSIX 1003.1 file I/O facilities which can operate on network drives, flash file systems, RAM and ROM disks, and even real hard disks. But beyond these devices, we really want to support some kind of abstraction layer that can be used to describe a wide range of custom hardware devices that will enable embedded applications to be ported across embedded Linux, eCos, and other operating systems that support EL/IX. For now, the specification of that abstraction layer part of the file system hierarchy is currently excluded from the EL/IX proposal. Since I/O is so fundamental to most uses of embedded systems, and since there is so much variety that must be supported, some sort of abstraction layers need to be defined, which is a subject for the next section.
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