Commit 343b5c10 authored by Paul Sokolovsky's avatar Paul Sokolovsky
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docs/uctypes: Improve documentation.

Seealso and Limitations sectiosn added, better formatting and grammar.
parent 79b40d11
:mod:`uctypes` -- access C structures
=====================================
:mod:`uctypes` -- access binary data in a structured way
========================================================
.. module:: uctypes
:synopsis: access C structures
:synopsis: access binary data in a structured way
This module implements "foreign data interface" for MicroPython. The idea
behind it is similar to CPython's ``ctypes`` modules, but actual API is
different, streamlined and optimized for small size.
behind it is similar to CPython's ``ctypes`` modules, but the actual API is
different, streamlined and optimized for small size. The basic idea of the
module is to define data structure layout with about the same power as the
C language allows, and the access it using familiar dot-syntax to reference
sub-fields.
.. seealso::
Module :mod:`ustruct`
Standard Python way to access binary data structures (doesn't scale
well to large and complex structures).
Defining structure layout
-------------------------
Structure layout is defined by a "descriptor" - a Python dictionary which
encodes field names as keys and other properties required to access them as
an associated values. Currently, uctypes requires explicit specification of
offsets for each field. Offset are given in bytes from structure start.
associated values. Currently, uctypes requires explicit specification of
offsets for each field. Offset are given in bytes from a structure start.
Following are encoding examples for various field types:
Scalar types::
* Scalar types::
"field_name": uctypes.UINT32 | 0
in other words, value is scalar type identifier ORed with field offset
(in bytes) from the start of the structure.
Recursive structures::
* Recursive structures::
"sub": (2, {
"b0": uctypes.UINT8 | 0,
......@@ -36,7 +45,7 @@ Following are encoding examples for various field types:
a structure descriptor dictionary (note: offsets in recursive descriptors
are relative to a structure it defines).
Arrays of primitive types::
* Arrays of primitive types::
"arr": (uctypes.ARRAY | 0, uctypes.UINT8 | 2),
......@@ -44,7 +53,7 @@ Following are encoding examples for various field types:
with offset, and second is scalar element type ORed number of elements
in array.
Arrays of aggregate types::
* Arrays of aggregate types::
"arr2": (uctypes.ARRAY | 0, 2, {"b": uctypes.UINT8 | 0}),
......@@ -52,21 +61,21 @@ Following are encoding examples for various field types:
with offset, second is a number of elements in array, and third is
descriptor of element type.
Pointer to a primitive type::
* Pointer to a primitive type::
"ptr": (uctypes.PTR | 0, uctypes.UINT8),
i.e. value is a 2-tuple, first element of which is PTR flag ORed
with offset, and second is scalar element type.
Pointer to aggregate type::
* Pointer to an aggregate type::
"ptr2": (uctypes.PTR | 0, {"b": uctypes.UINT8 | 0}),
i.e. value is a 2-tuple, first element of which is PTR flag ORed
with offset, second is descriptor of type pointed to.
Bitfields::
* Bitfields::
"bitf0": uctypes.BFUINT16 | 0 | 0 << uctypes.BF_POS | 8 << uctypes.BF_LEN,
......@@ -103,17 +112,18 @@ Module contents
.. data:: LITTLE_ENDIAN
Little-endian packed structure. (Packed means that every field occupies
exactly as many bytes as defined in the descriptor, i.e. alignment is 1).
Layout type for a little-endian packed structure. (Packed means that every
field occupies exactly as many bytes as defined in the descriptor, i.e.
the alignment is 1).
.. data:: BIG_ENDIAN
Big-endian packed structure.
Layour type for a big-endian packed structure.
.. data:: NATIVE
Native structure - with data endianness and alignment conforming to
the ABI of the system on which MicroPython runs.
Layout type for a native structure - with data endianness and alignment
conforming to the ABI of the system on which MicroPython runs.
.. function:: sizeof(struct)
......@@ -145,33 +155,55 @@ Structure descriptors and instantiating structure objects
Given a structure descriptor dictionary and its layout type, you can
instantiate a specific structure instance at a given memory address
using uctypes.struct() constructor. Memory address usually comes from
using :class:`uctypes.struct()` constructor. Memory address usually comes from
following sources:
* Predefined address, when accessing hardware registers on a baremetal
system. Lookup these addresses in datasheet for a particular MCU/SoC.
* As return value from a call to some FFI (Foreign Function Interface)
* As a return value from a call to some FFI (Foreign Function Interface)
function.
* From uctypes.addressof(), when you want to pass arguments to FFI
* From uctypes.addressof(), when you want to pass arguments to an FFI
function, or alternatively, to access some data for I/O (for example,
data read from file or network socket).
data read from a file or network socket).
Structure objects
-----------------
Structure objects allow accessing individual fields using standard dot
notation: ``my_struct.field1``. If a field is of scalar type, getting
it will produce primitive value (Python integer or float) corresponding
to value contained in a field. Scalar field can also be assigned to.
notation: ``my_struct.substruct1.field1``. If a field is of scalar type,
getting it will produce a primitive value (Python integer or float)
corresponding to the value contained in a field. A scalar field can also
be assigned to.
If a field is an array, its individual elements can be accessed with
standard subscript operator - both read and assigned to.
the standard subscript operator ``[]`` - both read and assigned to.
If a field is a pointer, it can be dereferenced using ``[0]`` syntax
(corresponding to C ``*`` operator, though ``[0]`` works in C too).
Subscripting pointer with other integer values but 0 are supported too,
Subscripting a pointer with other integer values but 0 are supported too,
with the same semantics as in C.
Summing up, accessing structure fields generally follows C syntax,
except for pointer derefence, you need to use ``[0]`` operator instead
of ``*``.
except for pointer derefence, when you need to use ``[0]`` operator
instead of ``*``.
Limitations
-----------
Accessing non-scalar fields leads to allocation of intermediate objects
to represent them. This means that special care should be taken to
layout a structure which needs to be accessed when memory allocation
is disabled (e.g. from an interrupt). The recommendations are:
* Avoid nested structures. For example, instead of
``mcu_registers.peripheral_a.register1``, define separate layout
descriptors for each peripheral, to be accessed as
``peripheral_a.register1``.
* Avoid other non-scalar data, like array. For example, instead of
``peripheral_a.register[0]`` use ``peripheral_a.register0``.
Note that these recommendations will lead to decreased readability
and conciseness of layouts, so they should be used only if the need
to access structure fields without allocation is anticipated (it's
even possible to define 2 parallel layouts - one for normal usage,
and a restricted one to use when memory allocation is prohibited).
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