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## 6 Mixed-Language Programming

This chapter is about mixed-language interoperability, but also applies if you link Fortran code compiled by different compilers. In most cases, use of the C Binding features of the Fortran 2003 and later standards is sufficient.

For example, it is possible to mix Fortran code with C++ code as well as C, if you declare the interface functions as `extern "C"` on the C++ side and `BIND(C)` on the Fortran side, and follow the rules for interoperability with C. Note that you cannot manipulate C++ class objects in Fortran or vice versa except as opaque pointers.

You can use the `gfortran` command to link both Fortran and non-Fortran code into the same program, or you can use `gcc` or `g++` if you also add an explicit `-lgfortran` option to link with the Fortran library. If your main program is written in C or some other language instead of Fortran, see Non-Fortran Main Program, below.