External Interfaces/API

A First Example -- Passing a Scalar

Let's look at a simple example of Fortran code and its MEX-file equivalent. Here is a Fortran computational routine that takes a scalar and doubles it.

•       subroutine timestwo(y, x)
        real*8 x, y
  C     
        y = 2.0 * x
        return
        end
  

Below is the same function written in the MEX-file format.

• C==============================================================
  =
  C     timestwo.f
  C     Multiply the input argument by 2.
  C
  C     This is a MEX-file for MATLAB.
  C     Copyright (c) 1984-2000 The MathWorks, Inc.
  C     $Revision: 1.9 $
  C==============================================================
  =
  
  C     Computational subroutine
        subroutine timestwo(y, x)
        real*8 x, y
  
        y = 2.0 * x
        return
        end
  
  
  C     The gateway routine
        subroutine mexFunction(nlhs, plhs, nrhs, prhs)
  C--------------------------------------------------------------
  -
  C     (pointer) Replace integer by integer*8 on the DEC Alpha
  C     64-bit platform.
  
        integer plhs(*), prhs(*)
        integer mxGetPr, mxCreateDoubleMatrix
        integer x_pr, y_pr
  C--------------------------------------------------------------
  -
  
        integer nlhs, nrhs
        integer mxGetM, mxGetN, mxIsNumeric
        integer m, n, size
        real*8 x, y
  
  C     Check for proper number of arguments. 
        if(nrhs .ne. 1) then
           call mexErrMsgTxt('One input required.')
        elseif(nlhs .ne. 1) then
           call mexErrMsgTxt('One output required.')
        endif
  
  C     Get the size of the input array.
        m = mxGetM(prhs(1))
        n = mxGetN(prhs(1))
        size = m*n
  
  C     Check to ensure the input is a number.
        if(mxIsNumeric(prhs(1)) .eq. 0) then
           call mexErrMsgTxt('Input must be a number.')
        endif
  
  C     Create matrix for the return argument.
        plhs(1) = mxCreateDoubleMatrix(m, n, 0)
        x_pr = mxGetPr(prhs(1))
        y_pr = mxGetPr(plhs(1))
        call mxCopyPtrToReal8(x_pr, x, size)
  
  C     Call the computational subroutine.
        call timestwo(y, x)
  
  C     Load the data into y_pr, which is the output to MATLAB.
        call mxCopyReal8ToPtr(y, y_pr, size)     
  
        return
        end
  

To compile and link this example source file, at the MATLAB prompt type

• mex timestwo.f
  

This carries out the necessary steps to create the MEX-file called timestwo with an extension corresponding to the machine type on which you're running. You can now call timestwo as if it were an M-function.

• x = 2;
  y = timestwo(x)
  y =
       4
  

Examples of Fortran MEX-Files

Passing Strings