MATLAB: In mex file, I can’t overwrite scalar value through the pointer in matlab2016a, although I can overwrite the scalar value through the pointer in matlab2013a and I can also overwrite the array through the pointer in matlab2016a.

MATLABmexpointer

[environment]
OS: OSX10.11 and maxOS Sierra(10.12)
MATLAB: matlab2013a and matlab2016a
Xcode: xcode7 and xcode8
——————————————————
In my work, I must use the following mex file in the old package.
 //destructiveMatrixWriteAtIndices.c
 //------------------------------------------------------
#include <matrix.h>  /* Matlab matrices */
#include <mex.h>
#include <stddef.h>  /* NULL */
#define notDblMtx(it) (!mxIsNumeric(it) || !mxIsDouble(it) || mxIsSparse(it) || mxIsComplex(it))
void mexFunction(int nlhs,       /* Num return vals on lhs */
     mxArray *plhs[],    /* Matrices on lhs      */
     int nrhs,       /* Num args on rhs    */
     const mxArray *prhs[]     /* Matrices on rhs */
     )
  {
  double *mtx;
  double *newValues;
  double *doubleStartIndex;
  int i, startIndex, size; 
  mxArray *arg;
if (nrhs != 3) mexErrMsgTxt("requires 3 arguments.");
/* ARG 1: MATRIX  */
arg = prhs[0];
if notDblMtx(arg) mexErrMsgTxt("MTX arg must be a real non-sparse matrix.");
mtx = mxGetPr(arg);
arg = prhs[1];
if notDblMtx(arg) mexErrMsgTxt("MTX arg must be a real non-sparse matrix.");
newValues = mxGetPr(arg);
size = (int) mxGetM(arg) * mxGetN(arg);
arg = prhs[2];
if notDblMtx(arg) mexErrMsgTxt("MTX arg must be a real non-sparse matrix.");
doubleStartIndex = mxGetPr(arg);
startIndex = (int) doubleStartIndex[0];
for (i=0; i<size; i++)
  {  
    mtx[i+startIndex] = newValues[i];        
  }
return;
}
//------------------------------------------------------
This mex file is the function to overwrite the scalar and the part of matrix through the pointer.
e.g. in matlab2013a command window (scalar in matlab2013a)
 a = 1;
 destructiveMatrixWriteAtIndices(a, 3, 0);
and the variable "a" becomes "3".
e.g. in matlab2013a and matlab2016a command window (matrix in matlab2013a and matlab2016a)
 a = [1, 2];
 destructiveMatrixWriteAtIndices(a, 3, 0);
and the variable "a" becomes "[3, 2]".
e.g. in matlab2016a command window (scalar in matlab2016a)
 a = 1;
 destructiveMatrixWriteAtIndices(a, 3, 0);
and the variable "a" becomes "1"! Why?
I also used the lldb, and revealed the strange behavior of this code.
In matlab2013a and matlab2016a, when I run the following snippet.
 a = 1;
 destructiveMatrixWriteAtIndices(a, 3, 0);
The lldb revealed "*mtx = 3" at the end of the mex function in both matlab, but the mex function couldn't pass the result(*mtx = 3, or prhs[0] = 3) through the pointer in the only matlab2016a.
It's very strange behavior!
※I have understood that this mex function is very danger, but this mex function was used at some points in the package that I must use. Therefore, I must fix this mex file and make the package run in matlab2016a.
Please help me.

Best Answer

Scalar values no longer (as of R2015b) create mxarrays by default. They are created as needed when a mex or buit-in function is called. Run the following code as a script (or function) after calling format debug.
 %script t1
  a=5;
  a
  a
 % end script
 >> t1
 a =
Structure address = 61963510
m = 1
n = 1
pr = aa4a51c0
pi = 0
     5
a =
Structure address = 61963510
m = 1
n = 1
pr = f93815a0
pi = 0
     5
Note that the pr values are different. The mex file is modifying a copy of the scalar value that is used by the MATLAB code.
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