How can I use a coder.opaque value in a Matlab expression with codegen?

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I need to read in some parameter files for my system. I am using the example in the help files for coder.opaque to declare the file ID value. I would like to check the ID to see if the fopen call completed successfully, but I keep getting an error saying it will not accept a coder.opaque data type for the comparison:
% Declare 'fid' as an opaque type 'FILE *'
fid = coder.opaque('FILE *', 'NULL');
fid = coder.ceval('fopen', cmName, ['rb' 0]); % 0 term strings for c
if 0 == fid
coder.ceval('exit', '-1');
The error occurs at the line where I compare the fid to 0. I have tried several alternatives (assigning fid to another var, casting fid to a double, making a function to do the test) but they all fail with a variation of not allowing a comparison with a coder.opaque type.
I imagine that others have run into this problem, but I did not see any entries for it in the database.
Any help would be appreciated.
Thanks Sean

Accepted Answer

Ryan Livingston
Ryan Livingston on 21 Sep 2016
Note that as of MATLAB R2013a fopen, fclose, and fprintf are supported for codegen. Other file I/O functions have been added more recently so you may just be able to use those directly in MATLAB without using coder.ceval at all.
If you decide to use coder.ceval, you can do this:
fp = coder.opaque('FILE *', 'NULL');
fp = coder.ceval('fopen',...);
NULL = coder.opaque('FILE *', 'NULL');
if fp == NULL
% Something went wrong
% All good

More Answers (2)

Nils Lande
Nils Lande on 21 Sep 2016
Edited: Nils Lande on 21 Sep 2016
coder.eval('(int)', fid) can be used to return the value as integer.
fp_int = int32(0);
fp = coder.opaque('FILE *', 'NULL'); % Declare the FILE pointer
fp = coder.ceval('fopen', szFileName, szMode);
fp_int = coder.ceval('(int)', fp);
if(fp_int == 0)
coder.ceval('printf', ['fopen returned NULL' char(10) 0]);
coder.ceval('printf', ['File opened' char(10) 0]);
  1 Comment
Sean on 21 Sep 2016
Thank you for your help. This is very similar to the following solution.

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Kaustubha Govind
Kaustubha Govind on 5 Nov 2012
I don't think coder.opaque type variables can participate in MATLAB expressions. You might need to convert that expression to another coder.ceval call.


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