[SOLVED] Why is 'size_t' in external C library interpreted wrong?

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Tyson Lawrence
Tyson Lawrence on 22 May 2020
Edited: Tyson Lawrence on 26 Jun 2020
I'm running into odd behavior while interfacing with an external C library. For a function like the following,
int foo(size_t sz, uint8_t buf[8])
{
memset(buf, 0, 8);
memcpy(buf, &sz, sizeof(size_t));
return sizeof(size_t);
}
I get the error "Warning: The data type 'error' used by function foo does not exist" when loading the library. And, when I run foo, the contents of buf don't make sense to me. If I change the size_t to uint64_t (or any other suitable integral type), then I get what I expect out in buf.
How is Matlab treating size_t? Why does it appear to be treating it as an 'error' type?
SOLUTION: Added appropriate include (stddef.h) in library header.

Answers (1)

James Tursa
James Tursa on 22 May 2020
Edited: James Tursa on 22 May 2020
I am unaware of MATLAB treating size_t differently, but there is a potential error in your code snippet:
int foo(size_t sz, uint8_t buf[8])
:
return sizeof(size_t);
The sizeof( ) operator returns a size_t type, but your function returns an int. So on a 64-bit system these will probably not match since size_t is likely a 64-bit integer and int is likely a 32-bit integer. You should change the signature to:
size_t foo(size_t sz, uint8_t buf[8])
What version of MATLAB are you running? 64-bit? Are you using the loadlibrary function? Are you loading a third party library or a library you wrote? Is there a reference to 'error' in the header that you are using? Maybe a typedef?
As for your problem, I don't see a reference to an 'error' data type in your code, so there is nothing for us to analyze.
  3 Comments
Tyson Lawrence
Tyson Lawrence on 26 Jun 2020
This is an insanely slow response... I got pulled away from this work for a stretch. I want to add the answer in case others have similar issues.
I resolved the problem by adding the appropriate includes in my library interface (stddef.h in this case). The problem didn't show up in my unit tests or Python API testing, so I missed it. Usually Matlab warns when something is missing.

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