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Pass Scalar Values in C MEX File

Pass Scalar as Matrix

This example shows how to write a MEX file that passes scalar values.

Suppose that you have the following C code, timestwo, that takes a scalar input, a 1-by-1 matrix, and doubles it.

void timestwo(double y[], double x[])
{
  y[0] = 2.0*x[0];
  return;
}

C Code Analysis

To see the function written as a MEX file, open timestwo.c in the MATLAB® Editor.

In C/C++, the compiler checks function arguments for number and type. However, in MATLAB, you can pass any number or type of arguments to a function; the function is responsible for argument checking. MEX files also allow variable inputs. Your MEX file must safely handle any number of input or output arguments of any supported type.

This code checks for the proper number of arguments.

if(nrhs != 1) {
  mexErrMsgIdAndTxt( "MATLAB:timestwo:invalidNumInputs",
    "One input required.");
} else if(nlhs>1) {
  mexErrMsgIdAndTxt( "MATLAB:timestwo:maxlhs",
    "Too many output arguments.");
}

This code checks if the input is a real, scalar double value.

mrows = mxGetM(prhs[0]);
ncols = mxGetN(prhs[0]);
if( !mxIsDouble(prhs[0]) || mxIsComplex(prhs[0]) ||
    !(mrows==1 && ncols==1) ) {
  mexErrMsgIdAndTxt( "MATLAB:timestwo:inputNotRealScalarDouble",
    "Input must be a noncomplex scalar double.");
}

Build and Test Example

Build the MEX file.

mex -v -R2018a timestwo.c

Call the function.

x = 2;
y = timestwo(x)

y =
     4

Pass Scalar by Value

This example shows how to write a MEX file that passes a scalar by value.

The mxGetScalar function returns the value of a scalar instead of a pointer to a copy of the scalar variable, x.

The following C code implements the timestwo_alt function.

void timestwo_alt(double *y, double x)
{
  *y = 2.0*x;
}

Compare the timestwo_alt function signature with the timestwo function signature.

void timestwo_alt(double *y, double x)
void timestwo(double y[], double x[])

The input value x is a scalar of type double. In the timestwo function, the input value is a matrix of type double.

To see the function written as a MEX file, open timestwoalt.c in the MATLAB Editor.

Compare the call to timestwo_alt to the call to timestwo.

  /* Get the scalar value of the input x */
  /* note: mxGetScalar returns a value, not a pointer */
  x = mxGetScalar(prhs[0]);

  /* Assign a pointer to the output */
  y = mxGetDoubles(plhs[0]);
  
  /* Call the timestwo_alt subroutine */
  timestwo_alt(y,x);

  /* Assign pointers to each input and output. */
  x = mxGetDoubles(prhs[0]);
  y = mxGetDoubles(plhs[0]);
  
  /* Call the timestwo subroutine. */
  timestwo(y,x);

The value x created by mxGetScalar is a scalar not a pointer.