Mex File. Too many output arguments

Hello

I have created a mex file which works fine when is called with only an output argument, but it gives me the next output when called with more than one:

??? Error using ==> mtimes Too many output arguments.

I manage the output arguments in the mex function like shown next:

void mexFunction( int nlhs, mxArray *plhs[],int nrhs, const mxArray *prhs[] ) { mxArray *Result0,*Result1,*Result2,....

....

    Result0                         = mxCreateDoubleMatrix(N,M,mxCOMPLEX);
    ResultValuesRe0                 = mxGetPr(Result0);
    ResultValuesIm0                 = mxGetPi(Result0);

    Result1                         = mxCreateDoubleMatrix(N,M,mxCOMPLEX);
    ResultValuesRe1                 = mxGetPr(Result1);
    ResultValuesIm1                 = mxGetPi(Result1);

    Result2                         = mxCreateDoubleMatrix(N,M,mxCOMPLEX);
    ResultValuesRe2                 = mxGetPr(Result2);
    ResultValuesIm2                 = mxGetPi(Result2);
....
....

plhs[0] =Result0; plhs[1] =Result1; plhs[2] =Result2;

}

I just do not get it. This is the way I created some mex files some time ago, and they worked out.

Why this error when calling with, for example, two output arguments?

The way I call it: [x,y] = function_mex(input_arguments)

[Walter Roberson]

How does the "mtimes" figure in to this? Does the code call the MATLAB mtimes, or did you call your code mtimes ?

[Christian]

I do not have a clue idea why this message on "mtimes". I do not call mtimes from the mex function. When I call the function with two outputs, it gives that message.

With only one output argument, the output is given by plhs[0]=Result0. However, if I change the code such as plhs[0]=Result1, the output changes accordingly (no error).

I have opted to divide the code and create 3 functions, each one with only one output argument, since I think I am far from knowing what is going on.

Thank you

Answer 1

[Jan]

A save method to create the outputs in the Mex-file is checking the value of nlhs:

switch (nlhs) {
  case 3: plhs[2] = Result2;   // fall through!
  case 2: plhs[1] = Result1;   // fall through!
  case 1:                      // fall through!
  case 0: plhs[0] = Result0;
  default:
    // Error message: Too many outputs...
}

For nlhs==0 the created output is copied to the variable ans implicitely. Therefore a distinction between 0 and 1 output is not required.

But: This does not explain the original problem, because even with 1 output the original code creates too many outputs.

[Christian]

I remember I added the next line:

mexPrintf("number output arguments %d \n",nlhs);

The result was always that nlhs is equal to 1, independently of the way I called the function. Another thing I do not understand why.

[Jan]

Under Matlab 2009a, Windows7/64, MSVC 2008 I get nlhs==0, if the Mex-function is called without inputs.