codegen: can I use a Matlab global structure in a mex function created with codegen?

I have a global structure S defined in my Matlab function and I use it in another function I want to compile with coder. Is it possible? How can I tell the coder that S is a Matlab structure? Example:

File function_1.m:

function_1

global S
S = struct ('a', 0);

File function_2.m:

function_2 %#codegen

global S
x = S.a + 2;

If I compile function_2.m the coder notify an error; how can I do?

Answer 1

[Fred Smith]

MATLAB globals are tricky.. There is a single global workspace containing all of your MATLAB global variables. You bind a name to the global workspace using:

global g;

This accesses an existing global g or creates a new global g containing an empty matrix.

The codegen command will use the value of your MATLAB global to guess a type for the global variable. By default generated MEX-functions pick up the global values from MATLAB so that it behaves just like your MATLAB code. In standalone code generation, the global will start with the value it had when you issued the codegen command.

You can define a global in MATLAB at the command line as follows:

>> global g;
>> g = struct('a',0);

After this codegen should infer the type from this definition and will start with the same initial value as MATLAB.

Alternatively, you can use the -global option to the codegen command to override the type, and optionally the initial value of the global. You need to specify the type if you want the global to be variable-sized. This is done in the same way as the -args specification for inputs.

Hope that helps,

Fred

Answer 2

[Kaustubha Govind]

I wonder if you should do something like:

function_2 %#codegen

global S
if isempty(S)
 S = struct ('a', 0);
end
x = S.a + 2;

[Giuela]

Doesn't work, the error message for the line #4 ("if isempty(S)") is:

Could not find initial value for global variable 'S'

[Kaustubha Govind]

Is 'S' defined in the MATLAB workspace at this point? It looks like your original definition should have worked according to Generate C Code from Code Containing Global Data