Trying to return array from Fortran to Matlab with mex, getting empty array instead - matlab

Question

So, I'm trying to return a an array of numbers from 1-n.
#include "fintrf.h"
C Gateway routine
subroutine mexFunction(nlhs, plhs, nrhs, prhs)
C Declarations
implicit none
C mexFunction arguments:
mwPointer plhs(*), prhs(*)
integer nlhs, nrhs
mwPointer mxGetPr
mwPointer mxCreateDoubleMatrix
mwPointer mxGetM, mxGetN
mwPointer mrows, ncols
mwSize size
mwPointer x_ptr, y_ptr
integer x_input,i
real*8, allocatable :: vec(:)
x_ptr = mxGetPr(prhs(1))
mrows = mxGetM(prhs(1))
ncols = mxGetN(prhs(1))
size = mrows*ncols
x_ptr=mxGetPr(prhs(1))
call mxCopyPtrToReal8(x_ptr,x_input,size)
allocate (vec(x_input))
do i=1,x_input
vec(i)=i
end do
plhs(1) = mxCreateDoubleMatrix(1, x_input, 0)
y_ptr = mxGetPr(plhs(1))
call mxCopyReal8ToPtr(vec,y_ptr,x_input)
deallocate ( vec )
return
end
I then call the mex file in fortran here
mex testingvec.F
Building with 'gfortran'.
MEX completed successfully.
a=testingvec(10);
and then find
a=[]
Can someone give me some help on this? If someone can give me some example code how to return a matrix as well, that would be sweet.
Thanks guys.
edit: new installment of the code. Still trying to get some help.
#include "fintrf.h"
C Gateway routine
subroutine mexFunction(nlhs, plhs, nrhs, prhs)
C Declarations
implicit none
C mexFunction arguments:
mwPointer plhs(*), prhs(*)
integer nlhs, nrhs
mwPointer mxGetPr
mwPointer mxCreateDoubleMatrix
mwPointer mxGetM, mxGetN
mwPointer mrows, ncols
mwSize size
mwPointer x_ptr, y_ptr
integer i
mwSize sizeone, x_input
integer*4 izero
real*8, allocatable :: vec(:)
x_ptr = mxGetPr(prhs(1))
mrows = mxGetM(prhs(1))
ncols = mxGetN(prhs(1))
size = mrows*ncols
sizeone=1
izero=0
x_ptr=mxGetPr(prhs(1))
call mxCopyPtrToReal8(x_ptr,x_input,size)
allocate (vec(x_input))
do i=1,x_input
vec(i)=i
end do
plhs(1) = mxCreateDoubleMatrix(sizeone,x_input,izero)
call mxCopyReal8ToPtr(vec,mxGetPr(plhs(1)),x_input)
deallocate ( vec )
return
end

Answer 1

There were both declaration problems and issues with calls to mex functions. Here's a solution, which assumes that the input is an integer-valued double giving you the length of the output vector (assuming that this is what you wanted).
#include "fintrf.h"
C Gateway routine
subroutine mexFunction(nlhs, plhs, nrhs, prhs)
C Declarations
implicit none
C mexFunction arguments:
mwPointer plhs(*), prhs(*)
integer*4 nlhs, nrhs
mwPointer mxGetPr
mwPointer mxCreateDoubleMatrix
mwSize mxGetM, mxGetN
mwSignedIndex mrows, ncols
mwSize size, x_input, sizeone
mwPointer x_ptr, y_ptr
integer*4 i, izero, x_int
real*8, allocatable :: vec(:)
real*8 :: x_dbl
sizeone = 1
izero = 0
!check input/output syntax
if (nrhs /= 1) then
call mexErrMsgIdAndTxt("MATLAB:testingvec:rhs",
> "Exactly 1 input variable required.")
end if
if (nlhs /= 1) then
call mexErrMsgIdAndTxt("MATLAB:testingvec:lhs",
> "Exactly 1 output matrix required.")
end if
x_ptr = mxGetPr(prhs(1))
mrows = mxGetM(prhs(1))
ncols = mxGetN(prhs(1))
size = mrows*ncols
call mxCopyPtrToReal8(x_ptr,x_dbl,sizeone)
x_input = int(x_dbl)
allocate (vec(x_input))
do i=1,x_input
vec(i)=i
end do
plhs(1) = mxCreateDoubleMatrix(sizeone, x_input, izero)
y_ptr = mxGetPr(plhs(1))
call mxCopyReal8ToPtr(vec,y_ptr,x_input)
deallocate ( vec )
return
end
I introduced a check for the number of input/output variables (to be updated in the actual program). And I introduced an auxiliary x_dbl which might or might not be necessary. This version reads the double input given to your function, and truncates it to get x_input.