MATLAB: Safe memory management when interfacing code via MEX

external interfacesmexmexlockmxmalloc

I am using MEX to interface to C/C++ code, which includes the creation of objects that will outlive the call to the MEX-function. These objects will eventually be deallocated by subsequent calls to the MEX function. That is, there is persistent memory in the MEX-function.

If I understand things right, I can increase a lock counter using mexLock whenever memory is allocated in my wrappers using either C "malloc" or C++ "new" and then decrease the lock counter using mexUnlock whenever I free objects using C "free" or C++ "delete". That will prevent the MEX function to be released while the function is still in use.

Is this safe usage? I have read that one should always use mxMalloc/mxFree instead of malloc/free, but when interfacing code, this is not an option.

Best Answer

Locking the mex file is only needed to preserve static data. Any memory allocated using malloc or new will persist until deleted or matlab exits even if your mex file is unloaded and reloaded.

Use of mxMalloc is only* need to prevent leaks in c (or c style) code that throws exceptions via mexErrMsgIdAndTxt ether directly or indirectly.

*The other not recommended need is when the pointer will be passed to mxSetData or related functions.

Related Solutions

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: Can I replace mxmalloc() and mxfree() function calls to malloc() and free() in the existing C MEX S functions. If yes, what precautions do I need to take in Simulink 7.6 (R2010b)

In order to switch from the use of existing mxmalloc() and mxfree() function calls to malloc() and free() you need to take sufficient care about the following:

1. It is highly recommend switching the use of mxmalloc()/mxfree() to malloc()/free() respectively in C MEX S function. This is only applicable for the cases that you have directly assigned a mx* pointer in your S function and call it directly into S function itself.

NOTE: In the case of using an indirect call to an mx* pointer in your S function you would have to dereference it with a pointer of type mx* and also free it using mxfree() only. This implementation cannot be changed moving forward as there is no common pointer interface between MATLAB and Simulink.

2. Moving forward you can simply replace the existing mxmalloc() and mxfree() functions with malloc() and free() counterparts for except those indirect cases that are mentioned above.

On a side note, when using a pointer to an mxArray in a MEX file and using it in an S function, you have to force it to be persistent, since MATLAB deallocates the memory automatically. To do so you have to use the 'mexMakeArrayPersistent' function to use the pointer before it is deallocated automatically. The user can destroy it using the 'mxDestroyArray' function.

Best Answer

Related Solutions

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

MATLAB: Can I replace mxmalloc() and mxfree() function calls to malloc() and free() in the existing C MEX S functions. If yes, what precautions do I need to take in Simulink 7.6 (R2010b)

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