MATLAB: Building Mex file with Fortan

MATLABmex

I am trying to understand how to build mex files. I made following test code It compiles using the Visual Fortran Compiler 2011 but when i try to call it matlab shuts down with the message segmentation violation detected. Does anyone sees what goes wrong?

if true
  #include <fintrf.h>
C
C Gateway routine
C
    SUBROUTINE MEXFUNCTION(NLHS, PLHS, NRHS, PRHS)
C mexFunction arguments:
    MWPOINTER PLHS(*), PRHS(*)
    MWPOINTER NLHS, NRHS
   mwPointer mxGetPr,mxGetPi
   mwPointer mxCreateDoubleMatrix,mxCreateNumericArray
   mwSize mxGetM, mxGetN
       MWPOINTER YPP 
       REAL *8 RYPP(1)
       YPP = MXGETPR(PLHS(1))
       RYPP(1) = 10
       CALL MXCOPYREAL8TOPTR(RYPP, YPP, 1)
      RETURN
      END
end

Best Answer

Your main problem is that you need to create the output array PLHS(1) before you access it. Also your NLHS and NRHS types are wrong. Try this:

#include <fintrf.h>
C
C Gateway routine
C
    SUBROUTINE MEXFUNCTION(NLHS, PLHS, NRHS, PRHS)
    implicit none
C mexFunction arguments:
    MWPOINTER PLHS(*), PRHS(*)
    INTEGER NLHS, NRHS
    mwPointer, external :: mxGetPr, mxGetPi
    mwPointer, external :: mxCreateDoubleMatrix, mxCreateNumericArray
    mwSize, external :: mxGetM, mxGetN
    MWPOINTER YPP 
    mwSize M, N
    integer*4 ComplexFlag
    REAL *8 RYPP(1)
    RYPP(1) = 10
    M = 1
    N = 1
    ComplexFlag = 0
    PLHS(1) = mxCreateDoubleMatrix( M, N, ComplexFlag )
    YPP = MXGETPR(PLHS(1))
    CALL MXCOPYREAL8TOPTR(RYPP, YPP, M*N)
    RETURN
    END

Related Solutions

MATLAB: Mex files crashes when run

The main problem is that you, the programmer, need to create the output mxArray plhs(1). When you run the mex routine the only mxArray variables you start with are the prhs(etc) variables. At that point in the code, prhs(1) is unallocated. MATLAB does not create the plhs(etc) outputs for you since it does not know what the output is supposed to be ... that is your job as the programmer. Also, you should check that the inputs are int64 type before you use them that way. E.g.,

C     Gateway routine
      subroutine mexFunction(nlhs, plhs, nrhs, prhs)
C     Declarations
      implicit none
C     mexFunction arguments:
      mwPointer plhs(*), prhs(*)
      integer nlhs, nrhs
C     Pointers to input/output mxArrays:
      mwPointer a, b
      mwPointer c
C     Function Declerations
      mwPointer mxGetData
          integer*4, external :: mxClassIDFromClassName
          integer*4, external :: mxIsInt64
          mwPointer, external :: mxGetNumberOfElements
          mwPointer, external :: mxCreateNumericMatrix
          mwSize :: one = 1
          integer*4 :: mxREAL = 0
          integer*4 classid
          if( nrhs /= 2 .or. nlhs > 1 ) then
            call mexErrMsgTxt("Need 2 inputs and at most 1 output")
          endif
          if( mxIsInt64(prhs(1))==0 .or. mxIsInt64(prhs(2))==0 ) then
            call mexErrMsgTxt("Inputs must be int64 class")
          endif
          if( mxGetNumberOfElements(prhs(1)) /= 1 .or.
       &      mxGetNumberOfElements(prhs(2)) /= 1 ) then
            call mexErrMsgTxt("Inputs must be scalar")
          endif
          classid = mxClassIDFromClassName("int64")
          plhs(1) = mxCreateNumericMatrix(one,one,classid,mxREAL)
          a = mxGetData(prhs(1))
          b = mxGetData(prhs(2))
          c = mxGetData(plhs(1))
C     Call the computational subroutine.
      call add(%val(a), %val(b), %val(c))
          return
          end
    C-----------------------------------------------------------------------
    C     Computational subroutine
          subroutine add(a, b, c)
          integer*8 a, b, c
          c = a + b
          return
          end

MATLAB: Segmentation violation error when I run a mex file

You should use "implicit none" in your code ... it is one of the few friends you have in Fortran.

You should declare all of your functions to make sure the compiler knows the type that is returned.

You should never use constants for arguments to the API functions ... always use variables to ensure that the argument types match.

E.g.,

The documentation shows the following signature for mexFunction:

subroutine mexFunction(nlhs, plhs, nrhs, prhs)
integer*4 nlhs, nrhs
mwPointer plhs(*), prhs(*)

So use integer*4 for the nlhs and nrhs, not integer. Probably not a source of the error since I would expect integer to be integer*4 even on a 64-bit system, but why not use the published signature?

Then consider mxGetM and mxGetN. Since you don't declare them they will get an integer return type by default (probably integer*4 on your system), but it is entirely possible that the actual return type is integer*8 on your 64-bit system. So declare them properly to avoid this potential system crashing mistake:

mwPointer, external :: mxGetM, mxGetN

Then consider the mxCopyPtrToReal8 calls you make using a constant in the last argument. That constant will likely be an integer*4, but the signature is:

subroutine mxCopyPtrToReal8(px, y, n)
mwPointer px
real*8 y(n)
mwSize n

That mwSize might be an integer*8 on a 64-bit system. So this call:

call mxCopyPtrToReal8(np,nr,1)

should look like this instead:

mwSize numel
    :
numel = 1
call mxCopyPtrToReal8(np,nr,numel)

I can't comment on anything else you have posted until you format it correctly with the "{ } Code" button. It is unreadable until you do this.

Best Answer

Related Solutions

MATLAB: Mex files crashes when run

MATLAB: Segmentation violation error when I run a mex file

Related Question