Passing structures through mex

Ok. I have a structure I want to have access to in my mex file (which I'm writing in C). The values are located in a field within a field (fieldception). Here's an explicit example:

I want to access:

sys.gen.window_size

Where window_size contains an array with the x and y size of a picture (i.e. sys.gen.window_size = size(picture)).

I know you're supposed to use mxGetField. Matlab help says I need to write the field name when sending a structure as an argument, but since the field is a subfield, would I just enter "gen.window_size?"

Or... should I just make life easier and rewrite the structure contain no subfields?

Thanks

Answer 1

[James Tursa]

E.g., the following code snippet w/o any argument checking, assuming you pass in the structure sys as the first argument:

mxArray *gen, *window_size;
double *pr;
   :
gen = mxGetField(prhs[0],0,"gen");
window_size = mxGetField(gen,0,"window_size");
pr = mxGetPr(window_size);

pr[0] and pr[1] are your window size values.

NOTE: gen and window_size are pointers to the original fields of your workspace variable. They are not copies, so be sure that you do not call mxDestroyArray on them when you are done.

[Justin]

I will try this out. Thanks for the help!

[Justin]

Hey James, if window_size is a pointer to the array stored in sys.gen.window_size, can't you just do *window_size to access the first value pointed to by window_size and not have to define the variable pr?

[Justin]

Epic triple post.

Anyhow, I tried what you suggested and it worked. Thanks! Had minor issues involving the fact that you must declare pr as a double, even though functions such as size in matlab return integers since matlab makes all variables double unless specified. Thanks again.

-Justin

[Walter Roberson]

size() returns double precision values that happen to be integral numbers. It is not a matter of how variables are declared, it is a matter of what datatype is returned from size().

[James Tursa]

@Justin: To answer your question, No you cannot do *window_size to access the first value. window_size points to an mxArray variable structure, not to the data within the structure. You have to use mxGetPr (or its cousins) to get at the actual data.

[Justin]

Thanks again!

[Jan]

@Justin: Is the problem solved now? Then please accept the answer.