passing MATLAB structures to C/C++
Jacob Englander
James Tursa
> I am rewriting a function which was previously implemented in MATLAB as a mex function for speed. The original function took a structure as an input and returned a structure as an output. Is there a way to pass a structure directly to a mex file, or perhaps a way to generate a C/C++ struct from a MATLAB structure?
Yes. But you have to do it manually piece by piece with the mxCreateStruct, mxGetField, mxSetField, etc. API functions. There is no magic function that will take an arbitrary C/C++ struct and turn it into an mxArray for you.
James Tursa
Máday Péter
I was wondering if it should be possible to formulate the problem in
such a way so that the parameters to the mex function would be arrays
and write a wrapper function in MATLAB, that takes care of the
transformation between the representations.
Peter
James Tursa
An m-file wrapper function could get at the individual structure variables and pass them along to the mex routine of course. But that doesn't really save you anything. In fact it just creates a bunch of temporary mxArray header info that you really don't need. The only advantage to this approach IMO is that it may be easier to do some error checking in the m-file vs in a mex routine. Regardless, you will still have to construct your C/C++ struct inside the mex routine ... you can't do it at the m-file level. (Is that what you are asking?). Maybe if you have a specific example of a MATLAB struct and your desired C/C++ struct I could suggest C code that would go back & forth between the two.
James Tursa
Jacob Englander
Philip Borghesani
"Jacob Englander" <jenglan3....@illinois.edu> wrote in message news:hojd27$nnk$1...@fred.mathworks.com...
>I am rewriting a function which was previously implemented in MATLAB as a mex function for speed. The original function took a
>structure as an input and returned a structure as an output. Is there a way to pass a structure directly to a mex file, or perhaps
>a way to generate a C/C++ struct from a MATLAB structure?
>
If the structures are kept simple with no arrays of pointers than the LOADLIBARY, CALLLIB and LIBSTRUCT commands can be used to pass
a C structure from MATLAB to a c interfaced function.
Take a look at matlabroot\extern\examples\shrlib\shrlibsample.c and the documentation for more information.
Phil
Stefan Griebel
>> sm.p1 = 476; sm.p2 = -299; sm.p3 = 1000
sm =
p1: 476
p2: -299
p3: 1000
>> sc = libstruct('c_struct', sm)
??? Error using ==> feval
Undefined function or method 'lib.c_struct' for input arguments of type 'struct'.
Error in ==> libstruct at 17
ptr=feval(['lib.' structtype],initialvalue);
>>
Ayush Pandey
Carlos Martinez
"Stefan Griebel" <stefan....@microtune.com> wrote in message <i1ks37$8gu$1...@fred.mathworks.com>...
Karthikeyan Lanka
I am working on a similar problem right now. I have a C code (say test.c) which takes a structure variable X containing two fields A and B. A is a vector of 10 values and B has two values. I have these datasets in matlab. In such a situation, can you please suggest a MEX C code that takes inputs from matlab database and passes it to test.c?
I am happy to provide you any other additional information in this regard.
Thank you.
Karthikeyan.
James Tursa
James Tursa
// Example mex file to copy a MATLAB struct to C, double the values,
// then return the result back to MATLAB
// struct must have field A with 10 doubles, and field B with 2 doubles
#include "mex.h"
typedef struct {
double A[10];
double B[2];
} ABstruct;
void mx2c( ABstruct *, const mxArray * );
void doubleit( ABstruct * );
mxArray *c2mx( ABstruct * );
//------------------------------------------------------------------------
void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
ABstruct c;
if( nlhs > 1 ) {
mexErrMsgTxt("Too many outputs");
}
if( nrhs ) {
mx2c( &c , prhs[0] ); // Convert mxArray to C struct
doubleit( &c ); // Double all of the values in the C struct
plhs[0] = c2mx( &c ); // Convert C struct to mxArray
}
}
//------------------------------------------------------------------------
void mx2c( ABstruct *c, const mxArray *mx )
{
mxArray *field;
double *pr;
int i;
if( !mxIsStruct(mx) || mxGetNumberOfElements(mx) == 0 ) {
mexErrMsgTxt("Input must be a non-empty struct");
}
// add other mx checks here
field = mxGetField( mx, 0, "A");
if( field == NULL ) {
mexErrMsgTxt("Input struct must have field A");
}
// add other field checks here
pr = mxGetPr(field);
for( i=0; i<10; i++ ) {
c->A[i] = *pr++;
}
field = mxGetField( mx, 0, "B");
if( field == NULL ) {
mexErrMsgTxt("Input struct must have field B");
}
// add other field checks here
pr = mxGetPr(field);
for( i=0; i<2; i++ ) {
c->B[i] = *pr++;
}
}
//------------------------------------------------------------------------
mxArray *c2mx( ABstruct *c )
{
char *fieldnames[2] = {"A","B"};
mxArray *mx, *field;
double *pr;
int i;
mx = mxCreateStructMatrix( 1, 1, 2, fieldnames );
field = mxCreateDoubleMatrix( 1, 10, mxREAL );
pr = mxGetPr(field);
for( i=0; i<10; i++ ) {
*pr++ = c->A[i];
}
mxSetField( mx, 0, "A", field );
field = mxCreateDoubleMatrix( 1, 2, mxREAL );
pr = mxGetPr(field);
for( i=0; i<2; i++ ) {
*pr++ = c->B[i];
}
mxSetField( mx, 0, "B", field );
return mx;
}
//------------------------------------------------------------------------
void doubleit( ABstruct *c )
{
int i;
for( i=0; i<10; i++ ) {
c->A[i] *= 2.0;
}
for( i=0; i<2; i++ ) {
c->B[i] *= 2.0;
}
}
Karthikeyan Lanka
Karthikeyan.
"James Tursa" wrote in message <nt43ov$ku5$1...@newscl01ah.mathworks.com>...