Passing a String from Matlab to a Fortran Mex Function

I am passing a string from a Matlab routine to a Fortran mex function. However, I am not having much luck pulling it out in the Fortran.

 % My Simple Matlab Code:
 %
temp_string = 'This is a place of business';
Call_String_Test(temp_string);

Could someone please help me to get this string pulled out and printed in the Fortran mex function. I have tried mxgetstring, mxcopyptrtocharacter, mxgetchars, and others. I can't seem to get the write syntax. I either get compiler errors, or matlab blows up. TIA, RICK

Answer 1

[Geoff Hayes]

Hi Rick - the revord.F example does a decent job of describing the steps to read the input string. It can be opened in your editor as

 edit([matlabroot '/extern/examples/refbook/revord.F']);

The code that follows is a variation of the this function. Note that I had problems with the call mexErrMsgIdAndTxt ... calls crashing MATLAB, so I just replaced them with a mexPrintf equivalent.

 #include "fintrf.h"

 C     Gateway routine
       subroutine mexFunction(nlhs, plhs, nrhs, prhs)

 C     mexFunction arguments:
       mwPointer plhs(*), prhs(*)
       integer nlhs, nrhs

 C     Function declarations:
       mwPointer mxCreateString
       mwPointer mxGetString 
       mwPointer mxGetM, mxGetN
       integer mxIsChar

 C     Array information:
       mwPointer mrows, ncols

 C     Arguments for computational routine:
       mwSize  maxbuf
       parameter(maxbuf = 100)
       character*120 line
       integer*4 k
       character*100 input_buf 
       mwPointer  strlen

 C     Check for proper number of arguments. 
       if(nrhs .ne. 1) then
          write(line,*) 'error: incorrect number of input arguments'
          k=mexPrintf(line//achar(13)) 
          return

 C     The input must be a string.
       elseif(mxIsChar(prhs(1)) .ne. 1) then
          write(line,*) 'error: input argument is not a string'
          k=mexPrintf(line//achar(13)) 
          return
       endif

 C     Get the size of the input string.
       mrows = mxGetM(prhs(1))
       ncols = mxGetN(prhs(1))

 C     Get the length of the input string and validate.
       strlen = mrows*ncols
       if (strlen .gt. maxbuf) then
          write(line,*) 'error: input is greater than max str size'
          k=mexPrintf(line//achar(13)) 
          return
       endif

 C     Get the string.
       status = mxGetString(prhs(1), input_buf, maxbuf)

       write(line,*) 'str = ', input_buf
       k=mexPrintf(line//achar(13)) 

       return
       end

The code checks to make sure that there is only one string input. Note how the input string can only have a fixed number of characters. I saved the above to a file called readString.F and compiled as mex readString.F. In the Command Window, I called this function as

 >> readString(1)
 error: input argument is not a string    

 >> readString('hello',2)
 error: incorrect number of input arguments   

 >> readString('hello')
 str = hello      

 >> readString('This is a place of business')
 str = This is a place of business

See the attached for a working copy of this code.

[Rick]

Thank YOU Geoff !!!!!!!!!!! I am now GOOD to GO, thanx......... :-)

[Geoff Hayes]

Glad it worked, Rick!