How can I make memory persistent between calls to a MEX-file in MATLAB?

I would like to know if memory allocated with mxCalloc remains persistent between calls to a MEX-file.

Answer 1

[MathWorks Support Team]

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

Answer 2

[James Tursa]

So, the accepted answer has bugs. Namely:

• They don't include the header for memcpy
• They don't check the input argument properly

Either of these can lead to a MATLAB crash if the input isn't as expected. Corrected and more robust code that doesn't rely on the messy memcpy and can correctly handle unexpected or missing input argument is:

// test.c

#include "mex.h"

static double *myarray = NULL;

void exitFcn() {
if (myarray != NULL)
    mexPrintf("Free'ing the persistent memory\n");
    mxFree(myarray);
}
    
void mexFunction(int nlhs,mxArray *plhs[],int nrhs, const mxArray *prhs[]) 
{
    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 mexMakeMemoryPersistent 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 value was '%f'.\n",myarray[0]);
        
    if( nrhs ) {
        if (!mxIsNumeric(prhs[0]) || mxGetNumberOfElements(prhs[0]) == 0)
            mexErrMsgTxt("Input must be non-empty numeric");
        myarray[0] = mxGetScalar(prhs[0]);
        mexPrintf("New value is '%f'.\n",myarray[0]);
    }
                
}