The %val Construct :: Creating Fortran MEX-Files (External Interfaces)

External Interfaces

The %val Construct

The %val construct is supported by most, but not all, Fortran compilers. DIGITAL Visual Fortran does support the construct. %val causes the value of the variable, rather than the address of the variable, to be passed to the subroutine. If you are using a Fortran compiler that does not support the %val construct, you must copy the array values into a temporary true Fortran array using special routines. For example, consider a gateway routine that calls its computational routine, yprime, by

call yprime(%val(yp), %val(t), %val(y))

If your Fortran compiler does not support the %val construct, you would replace the call to the computational subroutine with

C Copy array pointers to local arrays.
       call mxCopyPtrToReal8(t, tr, 1)
       call mxCopyPtrToReal8(y, yr, 4)
C
C Call the computational subroutine.
       call yprime(ypr, tr, yr)
C
C Copy local array to output array pointer.
       call mxCopyReal8ToPtr(ypr, yp, 4)

You must also add the following declaration line to the top of the gateway routine.

```
real*8 ypr(4), tr, yr(4)
```

Note that if you use mxCopyPtrToReal8 or any of the other mxCopy__ routines, the size of the arrays declared in the Fortran gateway routine must be greater than or equal to the size of the inputs to the MEX-file coming in from MATLAB. Otherwise mxCopyPtrToReal8 will not work correctly.

Fortran MEX-Files Examples of Fortran MEX-Files