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Matlab Coder and the uint64 size_t

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Greg on 13 Jun 2012
I've been trying to write some Matlab Coder code that needs a bunch of coder.ceval calls to functions in mat.h and mex.h. I've come into a problem, however, because my 64-bit system defines the "size_t" macro as a "long unsigned int", meaning a "uint64" which is not suppoted by codegen. If I attempt to return size_t variables as int32 or uint32, I definitely get the wrong answer (specifically, that sizeof(double)=6, sizeof(char)=4, sizeof(int)=3, etc). Does this mean that the Matlab Coder flat-out does not support coder.ceval calls on 64-bit systems that define size_t as a long int? That seems fairly crippling.

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Walter Roberson
Walter Roberson on 13 Jun 2012
You found a work-around for this, right?

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Accepted Answer

Kaustubha Govind
Kaustubha Govind on 13 Jun 2012
If you have access to the Fixed-Point Toolbox, it looks like you might be able to get around the limitation the declaring the output of the function as a fixed-point type:
y = fi(0, 0, 64, 0);
Alternatively, you could perhaps create a new "helper" C-function to cast size_t to uint32 for you and use coder.opaque to do something like this:
--- C function prototypes ----
size_t myfun(double u);
uint32_t cast_sizet_uint32(size_t);
---- Using them in MATLAB code for code-generation ---
t = coder.opaque('size_t');
t = coder.ceval('myfun', u); %call function that returns size_t
y = uint32(0);
y = coder.ceval('cast_sizet_uint32', t); %call your helper that does the cast


Greg on 14 Jun 2012
Actually, I figured out that you can typecast within a ceval:
t = coder.opaque('size_t');
t = coder.ceval('sizeof(double)');
y = uint32(0);
y = coder.ceval('(unsigned int)',t);
I believe that codegens C that looks something like:
y = (unsigned int)(t);
Which works, even though it's a bit non-standard-looking.
It's still a little annoying, though, when I have an array of size_t values, such as is returned by mxGetDimensions. My attempts at manual pointer-math haven't been very successful, so I guess the C "helper" function would be the way to go.
Kaustubha Govind
Kaustubha Govind on 14 Jun 2012
Thanks for posting your observations! I would also recommend submitting this as an enhancement request to MathWorks Tech Support so that the development team is aware of the need to address this limitation.

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