Mixing C, C++, and Fortran 77

This page describes how to cross-use C, C++, and Fortran 77 in an Unix environment. The information provided here is known to work on the following architectures:

• Digital Alpha
• HP Unix Workstations
• Ultrix
• x86, running Linux

The basic rules for calling Fortran functions from C and C++:

• Most (almost all) Fortran compilers add, during compilation, an underscore (_) at the end of the Fortran routine names. Our experience is that the f77 compiler in HP Unix environments does not do this.
  • We noticed that if a fortran subroutine has an underscore anywhere in its name, the GNU g77 compiler adds two (2) underscores at the end of the name.
• In the compiled Fortran code all arguments to functions are passed by their address.
• If a Fortran function takes a character string as an argument, the string length must be passed as the last argument (i.e. after the "ordinary" argument list).

Additional rules for manipulating Fortran common blocks and variables within C and C++:

• Variables that is to be shared between Fortran and C/C++ must be global.
• Most (almost all) Fortran compilers add, during compilation, an underscore (_) at the end of the Fortran common block names. (This is probably true for all Fortran variables but we have not tested this since we always did use common blocks to transfer information between Fortran and C/C++ code.)
• C and C++ vectors (arrays) always start with index 0. In multidimensional arrays the order of the indices are reversed as compared to Fortran 77.

Further examples that can help solve problems you might encounter:

• Transferring large chunks of information between Fortran and C/C++ is fairly straight forward using common blocks and structures.
• When linking you must also include the different run-time libraries needed for the different compilers (examples).
• There are a few complications when you want to access C/C++ functions using Fortran 77. You should keep the rules above in mind, and do the following:
  • Add a special interface function (a wrapper written in C/C++) which is callable from Fortran.
  • Never use fortran code as your main program/function, and always link your programs using C++ as the native language at linkage. (This should make sure that a number of global objects, i.e. cout, cin, ..., are correctly constructed and initialized before your main function is invoked. Corresponding things should happen after main returns.)
  • It is easy to overcome the restriction in the last bullet by creating a very small C++ main function which only calls your ordinary fortran main program. See the special interface function section for an example of this.

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