The Fortran Interface

The Fortran interface is essentially a wrapper of the C interface discussed in Section 3.4. The way to hook up IPOPT in a Fortran program is very similar to how it is done for the C interface, and the functions of the Fortran interface correspond one-to-one to the those of the C and C++ interface, including their arguments. You can find an implementation of the example problem (4)-(7) in $IPOPTDIR/Ipopt/examples/hs071_f.

The only special things to consider are:

• The return value of the function IPCREATE is of an INTEGER type that must be large enough to capture a pointer on the particular machine. This means, that you have to declare the ``handle'' for the IpoptProblem as INTEGER*8 if your program is compiled in 64-bit mode. All other INTEGER-type variables must be of the regular type.
• For the call of IPSOLVE (which is the function that is to be called to run IPOPT), all arrays, including those for the dual variables, must be given (in contrast to the C interface). The return value IERR of this function indicates the outcome of the optimization (see the include file IpReturnCodes.inc in the IPOPT include directory).
• The return IERR value of the remaining functions has to be set to zero, unless there was a problem during execution of the function call.
• The callback functions (EV_* in the example) include the arguments IDAT and DAT, which are INTEGER and DOUBLE PRECISION arrays that are passed unmodified between the main program calling IPSOLVE and the evaluation subroutines EV_* (similarly to UserDataPtr arguments in the C interface). These arrays can be used to pass ``private'' data between the main program and the user-provided Fortran subroutines.

  The last argument of the EV_* subroutines, IERR, is to be set to 0 by the user on return, unless there was a problem during the evaluation of the optimization problem function/derivative for the given point X (then it should return a non-zero value).