mirror of
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f6ecf80ebb
The documentation grew one page at a time, so the tree follows the
history of who wrote what and not the shape of NuttX. Scheduling is
spread over three places, a driver page can sit above the subsystem
that owns it, and the front page lists everything at the same level.
That is a lot to face when all you want to know is where the scheduler
lives.
This change files every page under the code it describes. It is a move,
not a rewrite: outside the ten pages named below, every page keeps the
text that is already in master, and no page's text is deleted.
What it does:
* Groups the table of contents into nine chapters.
* Moves the OS subsystems under os/: scheduling, memory, drivers,
filesystem, networking, IPC, interrupts, libs, time.
* Renames the platform pages to the names the source tree uses, and
derives their tags from the tree instead of by hand.
* Splits guides/ by subject.
* Adds Documentation/redirects.py, with a rule for every page that left
its old path, so old URLs keep working. The redirect page also carries
a link's #anchor across to the new page.
Ten pages have text that is new or rewritten. Nine of them are the
landing page of a chapter, which has to exist for the new structure:
index the front page
os/index OS Design
os/scheduling/index Scheduling
os/interrupts/index Interrupts
os/ipc/index IPC
os/time/index Time and timers
about/index About
developing/index Developing NuttX
ReleaseNotes/index Release notes
The tenth is os/libs/libbuiltin, the only page here with technical
content: libs/libbuiltin/ had no page at all. Five SVG diagrams come
with these pages, hand-written XML with no editor metadata.
Nothing outside Documentation/ is touched.
How it was checked:
* Sphinx builds with -W: no warnings, and no document left outside a
toctree.
* A script, offered in the PR, proves the narrow claim this rests on.
For every page outside the ten named above it erases what a move
touches -- link target, path, tag line, toctree block, table border --
from the whole old text and the whole new text, and requires the two
to be byte for byte identical. It also requires every sentence of a
deleted page to turn up somewhere, and every page that left its old
path to have a redirect, from a URL that existed, to where its content
went. It exits non-zero and names the page if any of that is not true,
and it tests added pages too, so forgetting to declare one cannot make
it pass.
* An independent audit checked 133 factual claims on these ten pages
against the tree, one shell command per claim: 130 confirmed, 1
refuted and fixed here, 2 not checkable.
* tools/checkpatch.sh is clean over the range.
The diff is large because moving a page changes every link that points
to it. Most of it is pure renames, and board pages that gained one tag
line.
Assisted-by: Claude:claude-opus-5
96 lines
4.6 KiB
ReStructuredText
96 lines
4.6 KiB
ReStructuredText
=========================================
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Application OS vs. Internal OS Interfaces
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=========================================
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NuttX provides a standard, portable OS interface for use by
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applications. This standard interface is controlled by the
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specifications proved at `OpenGroup.org <http://opengroup.org>`__.
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These application interfaces, in general, should not be used
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directly by logic executing within the OS. The reason for this is
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that there are certain properties of the standard application
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interfaces that make them unsuitable for use within the OS These
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properties include:
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#. **Use of the per-thread** ``errno`` **variable**: Handling of
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return values, particularly, in the case of returned error
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indications. Most legacy POSIX OS interface return information
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via a *per-thread* ``errno``. There must be no alteration of
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the ``errno`` value that must be stable from the point of view
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of the application. So, as a general rule, internal OS logic
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must never modify the ``errno`` and particularly not by the
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inappropriate use of application OS interfaces within OS
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itself.
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Within the OS, functions do not return error information via
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the ``errno`` variable. Instead, the majority of internal OS
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function return error information as an integer value: Returned
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values greater than or equal to zero are success values;
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returned values less than zero indicate failures. Failures are
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reported by returning a negated ``errno`` value from
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``include/errno.h``,
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#. **Cancellation Points**: Many of the application OS interfaces
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are *cancellation points*, i.e., when the task is operating in
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*deferred cancellation* state, it cannot be deleted or
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cancelled until it calls an application OS interface that is a
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cancellation point.
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The POSIX specification is very specific about this, specific
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both in identifying which application OS interfaces are
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cancellation points and specific in the fact that it is
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prohibited for any OS operation other than those listed in the
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specification to generate cancellation points. If internal OS
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logic were to reuse application OS interfaces directly then it
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could very easily violate this POSIX requirement by incorrectly
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generating cancellation points on inappropriate OS operations
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and could result in very difficult to analyze application
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failures.
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#. **Use of per-task Resources**: Many resources are only valid in
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the task group context in which a thread operates. Above we
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mentioned one: ``errno`` is only valid for the thread that is
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currently executing. So, for example, the ``errno`` at the time
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of a call is a completely different variable than, say, the
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``errno`` while running in a work queue task.
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File descriptors are an even better example: An open file on
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file descriptor 5 on task A is *not* the same open file as
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might be used on file descriptor 5 on task B.
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As a result, internal OS logic may not use application OS
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interfaces that use file descriptors or any other *per-task*
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resource.
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Within NuttX, this is handled by supporting equivalent internal OS
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interfaces that do not break the above rules. These internal
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interfaces are intended for use *only* within the OS and should
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not be used by application logic. Some examples include:
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- ``nxsem_wait()``: functionally
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equivalent to the standard application interface
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``sem_wait()``. However, ``nxsem_wait()`` will not modify the
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errno value and will not cause a cancellation point. (see
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``include/nuttx/semaphore.h`` for other internal OS interfaces
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for semaphores).
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- ``nxmq_send()``: functionally equivalent
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to the standard application interface ``mq_send()``. However,
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``nxmq_send()`` will not modify the errno value and will not
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cause a cancellation point (see ``include/nuttx/mqueue.h`` for
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other internal OS interfaces for POSIX message queues).
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- ``file_read()``: functionally equivalent
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to the standard application interface ``read()``. However,
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``file_read()`` will not modify the errno value, will not cause
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a cancellation point, and uses a special internal data
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structure in place of the file descriptor (see
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``include/nuttx/fs/fs.h`` for other internal OS interfaces for
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VFS functions).
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- ``psock_recvfrom()``: functionally
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equivalent to the standard application interface
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``recvfrom()``. However, ``psock_recvfrom()`` will not modify
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the errno value, will not cause a cancellation point, and uses
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a special internal data structure in place of the socket
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descriptor (see ``include/nuttx/net/net.h`` for other internal
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OS interfaces for sockets).
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