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Complete Example for Passing Array by Reference to C for Processing

조회 수: 26 (최근 30일)
I am having issues getting Matlab Coder to allow me to pass an array into a CRC calculator .m file, then pass that array down to C and have C do the processing.
function crc = crccalculate10(in1) %#codegen
assert(isa(in1,'uint8'));
assert(all(size(in1) <= [inf]));
crc = 0;
% generate an include in the C code
coder.cinclude('crccalculate10.h');
% evaluate the C function
crc = coder.ceval('crccalculate10', coder.rref(in1), uint8(numel(in1)));
end
As far as I can tell, the issue isn't in the code, it's in the codegen params, especially the -args <> argument, where you would normally specify what variables can be passed in. But I cannot find any examples of passing, what I understand needs to be a variable length array. It is documented in several places, for example here, which lacks the codegen command itself to specify the length. And here provides some examples of fixed size matrices being passed in, but no variable sized. And if you read the coder.varsize documentation, every example shows varsize operating on the output variable, never the input variable. This guy seems to have similar questions on the use of coder.varsize and coder.typeof.
My best guess on use use of typeof basically involves building a coder.Type variable and passing that. The procedure would be basically,
x = [uint8(1) uint8(1) uint8(1)]
coder.varsize('x', [1, Inf])
t = coder.typeof(x)
codegen crccalculate10 -args {t} crccalculate10.c
I checked the type and it looks correct to me. But also, I tried using coder.varsize(t, [1 Inf]). Both cases lead to a compiler error, on a line in my .c that doesn't exist (?!),
Error crccalculate10.c: 19 type error in argument 2 to `crccalculate10'; found 'int' expected 'pointer to unsigned char'
Unfortunately I don't have a line 19 in either my C source or my M file, so that compiler output is very incorrect. But, if I had to guess, the value of coder.rref(in1) is not "t" specified above.
At this point I have exhausted basically everything I can think of to make this work. I might just have to try to reimplement it in Matlab, it might have been faster. Any assistance would be appreciated. That said, I think this use case, being incredibly common in C code, deserves a clear and concise example in the documentation.

채택된 답변

David Fink
David Fink 2019년 3월 22일
Thanks for the recommendation!
We're working on improving our documentation and this is valuable feedback.
Let's see if I can help clarify:
  1. "coder.varsize" is meant to be used within the MATLAB function for which you're generating code
  2. "coder.typeof" is meant to be used to create a type to pass to "codegen"
For your example, it sounds like you want the codegen argument (in args):
coder.typeof(uint8(42),[1 Inf],[0 1])
(This can be saved in a variable "t" like in your code.)
The [1 Inf] specify the maximum size in each dimension, and [0 1] indicate that the second dimension (and not the first) are variable-sized.
Based on the error (which comes from the C compiler), it seems like expects an unsigned character pointer as its second argument. However, it is being passed an int, and based on the MATLAB code, it should be passed a uint8.
crccalculate10.c is one of the C files generated by codegen, which is where the C compiler error occurs. Add "-report" (without quotes) as another argument to codegen, which displays the generated files and other information.
  댓글 수: 3
David Fink
David Fink 2019년 3월 25일
Ah, my apologies for not noticing the root cause earlier. Yes, calling into a C file is supported. The root of the issue is:
The C function and the M function have the same name, which leads to a naming conflict (causing the error). This is because the generated C function name is the same as the M function name, causing a conflict in C.
While your end goal is not generating C code, in order to interface MATLAB with the C function, we generate C code in order to compile it into a MEX, which can be called directly in MATLAB.
In general, specific names can be excluded from appearing in the generated code via the config parameter ReservedNameArray, briefly mentioned on this page:
It can be used as follows:
cfg = coder.config;
cfg.ReservedNameArray = 'name1;name2';
codegen -config cfg ...
This will prevent "name1" and "name2" from appearing as identifiers in the generated code.
However, this is not honored for entry-point functions (the M function name you pass to "codegen") as the expectation is that the C function should have the same name as the M function.
Thus, in this case, the solution is to rename either the M function or the C function from "crccalculate10" so they have different names.
Charles Campbell
Charles Campbell 2019년 3월 25일
편집: Charles Campbell 2019년 3월 25일
This worked! That was the issue.
If anyone finds this and is looking for a concise example, see
  1. The Pass by Reference example here
  2. The typeof example provided in David's response here, with typeof docs here.
  3. Ensure that the ceval, prototype and definition of functions are all named the same.
  4. Ensure the matlab function is named something else.
With those considerations it should compile.
Thanks!

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