MATLAB: Avoid Copying MEX array to C Struct

mex copy c struct

I am new to MEX. From the "phonebook.c" example, it appears that copying is required from prhs[] to a C struct . . . using either memcpy() or mxDuplicateArray().

In my case, prhs[] will be very large, (megabytes). Copying is not ideal.

Is there a way, using MEX commands, to set a pointer to a C struct to prhs[] and avoid copying??

Thanks in advance.

Best Answer

The array duplication is only required, when an input array is modified and replied as output. In all other cases you can use the pointer to the array data inside the Mex file also without any problems.

Related Solutions

MATLAB: How to copy data to a new string variable in a C-MEX file

Use the mxArray* returned by the mxCreateString function to copy data to a new mxArray* as follows:

 #include "mex.h"
 void mexFunction( int nlhs, mxArray *plhs[],
                   int nrhs, const mxArray *prhs[] )
 {
 mxArray *origString, *userString;
 int arrayLen;
 /* Initialize */
 origString = mxCreateString("This is the primitive string");
 /* Copy values to new string */
 userString = origString;
 /* Determine array length */
 arrayLen = mxGetNumberOfElements(origString);
 printf("The array length is: %d \n", arrayLen);
 /* Remaining code is just for testing */
 plhs[0] = userString;
 return;
 }

To test the above mexFunction after building to a MEX file, for example mexFunction.mexw32, invoke:

y = mexFunction

MATLAB: MxCreateStructArray and mxCreateStructMatrix field name memory management

"Are the field name character arrays copied before being stored internally?"

Yes. Internally in the mxArray, the field names are copied to a "fieldname pool" that is managed by MATLAB behind the pi pointer. Not directly behind this pointer, but buried a bit. Where it is buried behind the pi pointer has changed over the years (and I haven't actually checked the latest versions), but the bottom line is it is copied. The char pointers you pass in are not in any way attached to the mxArray struct variable.

"Do the character array pointers need to point to the first character of Matlab managed memory (e.g. initialized via mxMalloc)?"

No. Since the field names are copied into MATLAB managed memory, the source pointers can point to anything (native C/C++ memory, MATLAB managed memory, in the middle of an allocated memory block, etc). Or even a mix of the above. Source doesn't matter since the field name strings are copied.

The API functions that actually attach pointers are:

mxSetPr, mxSetPi, mxSetData, mxSetImagData, mxSetCell, mxSetField, mxSetFieldByNumber, mxSetIr, mxSetJc

(In addition to attaching the pointers, they remove addresses from the garbage collection list and in the case of mxArrays change variable types from temporary to sub-element)

Best Answer

Related Solutions

MATLAB: How to copy data to a new string variable in a C-MEX file

MATLAB: MxCreateStructArray and mxCreateStructMatrix field name memory management

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