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uctypes.bytearray_at(addr, size)
Capture memory at the given address and size as
bytearray
object. Unlike
bytes_at()
function above, memory is captured by reference, so it can be both written too, and you will
access current value at the given memory address.
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 following sources:
Predefined address, when accessing hardware registers on a baremetal system.
Lookup these addresses in datasheet for a particular MCU/SoC.
As a return value from a call to some FFI (Foreign Function Interface) function.
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 a file or network
socket).
Structure objects
Structure objects allow accessing individual fields using standard dot 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 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 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
dereference, 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:
5.3.2 uctypes
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