MATLAB: Doesn’t the MEX function call from MATLAB match the signature in the MEX file

MATLABmex function signatures c

After completing the "Create A C Source MEX File" tutorial https://www.mathworks.com/help/matlab/matlab_external/standalone-example.html, I'm not clear on how the mapping of function signatures works from MATLAB to C.

In the example (arrayProduct.c), there are two functions in the example MEX file. One is (void) arrayProduct(double, double*,double*,int). The second is the required entry function which has a void return type and arguments (int, mxArray*, int, const mxArray*). When calling the function from MATLAB however, the signature doesn't match either of the functions in the MEX file: [returns matrix] arrayProduct(number,Matrix).

Intuitively, I would expect the function arrayProduct in the .c file to have the same signature as what would be called from MATLAB.

The question is – given a random MEX (*.c) file, how can I tell what the function signature needs to be when called from MATLAB?

Best Answer

Normal mex functions need to have parameters for the number of output arguments, the pointers to the output arguments, the number of input arguments, and the pointers to the input arguments.

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MATLAB: How to make memory persistent between calls to a MEX-file in MATLAB

By default, in a MEX-file, mxCalloc generates non-persistent mxCalloc data. In other words, the memory management facility automatically de-allocates the memory as soon as the MEX-file ends. If you want the memory to persist after the MEX-file completes, call mexMakeMemoryPersistent after calling mxCalloc. If you write a MEX-file with persistent memory, be sure to register a mexAtExit function to free allocated memory in the event your MEX-file is cleared.

For example:

 // test.c
 #include <stdlib.h>
 #include "mex.h"
 static double *myarray = NULL;
 double *pr;
 void exitFcn() {
   if (myarray != NULL)
 mxFree(myarray);
 }
 void mexFunction(int nlhs,mxArray *plhs[],int nrhs, const mxArray *prhs[])
 {
   if (nrhs<1 | mxIsChar(prhs[0]))
 mexErrMsgTxt("Must have one non-string input");
   if (myarray==NULL) {  
 /* since myarray is initialized to NULL, we know
       this is the first call of the MEX-function 
   after it was loaded.  Therefore, we should
   set up myarray and the exit function. */
 /* Allocate array. Use mexMackMemoryPersistent to make the allocated memory persistent in subsequent calls*/
 mexPrintf("First call to MEX-file\n");
         myarray = mxCalloc(1,8);
         mexMakeMemoryPersistent(myarray);
         mexAtExit(exitFcn);
   }
   mexPrintf("Old string was '%f'.\n",myarray[0]);
   pr = mxGetPr(prhs[0]);
   mexPrintf("New string is '%f'.\n",pr[0]);
   memcpy((char*)myarray,(char*)mxGetPr(prhs[0]), sizeof(double));
 }

You can compile this MEX-file with the following command:

mex test.c

To run the MEX-file, pass in a single number. For example:

test(3)

Repetitive calls to the test MEX-file will show that the previous input is stored between calls. To clear the MEX-file from memory, use the command:

clear mex

For more information on persistent variables in MEX-files, refer to the relevent documentation by entering the following at the MATLAB prompt:

doc mexMakeArrayPersistent

and

doc mexMakeMemoryPersistent

MATLAB: C-Mex Error: expected “…” or “,”

mxArray*plhs[] needs a comma after it

Best Answer

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MATLAB: How to make memory persistent between calls to a MEX-file in MATLAB

MATLAB: C-Mex Error: expected “…” or “,”

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