Structure as init argument in a mex file - c++

Question

I have a mask in simulink that has an init argument field. The init argument in my case is a structure. Now I want to use this structure in the .ccp (to make a mex file).
void init()
{
mxArray *initarg = GetInitArg();
...
}
The GetInitArg() is :
#ifndef GET_INIT_ARG
#define GET_INIT_ARG
mxArray *GetInitArg() {
return rtsys->initArg;
}
#endif
When the initarg is an int, I can call it this way in the void init():
int arg = (int)mxGetPr(initarg)[0];
Now, how would I do if initarg is a Matlab structure?
EDIT
I tried using #remus answer.
My struct look like this :
typedef struct
{
const char *task;
aaa_type aaa;
bbb_type bbb;
ccc_type ccc;
} arg_t;
The struct aaa_type, bbb_type and ccc_type are defined like this :
typedef struct
{
double p1;
double p2;
double p3;
double p4;
double p5;
double p6;
} aaa_type;
I try to get the init arg like this :
void init()
{
mxArray *initarg = GetInitArg();
arg_t arg* = (arg_t*)mxGetPr(initarg);
...
}
But at the arg_t line i'm getting two compilation errors:
error C2143: syntax error : missing ';' before '*'
error C2059: syntax error : '='

Answer 1

The list of MEX functions related to accessing structures is below:
mxGetField
mxSetField
mxGetNumberOfFields
mxGetFieldNameByNumber
mxGetFieldNumber
mxGetFieldByNumber
mxSetFieldByNumber
If you have a 1x1 structure, here's how you'd get one of the values:
mxArray *field_name = mxGetField(initArg, 0, "field_name");
Note that the result is another mxArray. From there you want the usual mxGetPr() for double arrays, or other mxGet... for other datatypes.
See the MEX documentation section on C/C++ Matrix Library for API details on these functions: http://www.mathworks.com/help/matlab/cc-mx-matrix-library.html

Answer 2

If you have the structure definition then you could cast the parameter pointer to it (I haven't tested but it should work since the Matlab structure is a contiguous memory block). Let's say you define your structure somewhere in a .h file:
typedef struct {
double a;
double b;
} mystruct_t;
Then:
mystruct_t *arg = (mystruct_t*)mxGetPr(initarg);
And you can access its members:
if (arg->a == 1) // or whatever