Mixing C++ and Fortran

Questions

  • Can we use CMake to build mixed-language projects?

Objectives

  • Learn how the built-in FortranCInterface module can help you work with projects mixing Fortran and C/C++.

CMake has native support for many programming languages. At the time of writing, C, C++, Fortran, CUDA, Objective-C, ISPC, and ASM are officially supported. When programming applications and libraries in a scientific context, it is often required to mix components written in different languages. This is mostly true because legacy, tried-and-true components are extremely hard to replace by non-professional programmers. In this episode, we will show how to mix Fortran and C/C++.

In order to use multiple languages in your project, you can declare it within the project command:

project(my-project LANGUAGES CXX Fortran)

Languages can also be declared later on in your CMakeLists.txt with invocations to the enable_language command. You can specify sources in multiple languages for any given executable or library target. CMake will resolve which compiler to use for each based on their extension: for example .f90 will use the Fortran compiler, without preprocessor. Linking of mixed-language targets will be performed through the compiler of the language with the highest priority. In C/Fortran projects, the Fortran compiler will call the linker; in C++/Fortran projects, the C++ compiler will do the honors.

The workhorse module for mixing C/C++ and Fortran is the built-in FortranCInterface module. Whether you are working in Fortran and linking a C/C++ library or viceversa, you should always check that the compilers for the two languages are able to talk to each other. That is where the FortranCInterface_VERIFY function comes into play:

include(FortranCInterface)

# if you are working with C and Fortran
FortranCInterface_VERIFY()

# if you are working with C++ and Fortran
FortranCInterface_VERIFY(CXX)

Fortran using C/C++

If you are using Fortran2003 (and beyond), it is fairly straightforward to employ C/C++ libraries. The iso_c_binding built-in module was indeed mandated by the standards’ committee starting from the 2003 edition, and provides a standardized interface between C, the de facto lingua franca of programming, and Fortran. We will not delve into the details of iso_c_binding, suffice it so say that interoperability between basic datatypes, pointers, and function call conventions is nowadays well-established. 1

Exercise 24: A Fortran executable using a C/C++ library

In this exercise, you will build a Fortran executable linking to libraries written in C++ and the system library backtrace, written in C.

The final executable, bt-randomgen-example, will print a few random integers, produced by the C++ library, and a backtrace, obtained from the C library. This is a sample output:

$ bt-randomgen-example

 Get a random number    20
 Get a random number    13
 Get a random number    30
 Get a random number    24
 Get a random number    40
 Get a random number    31
 Get a random number    33
 Get a random number    28
 Get a random number    33
 Get a random number    13
 Get a random number    11
 Get a random number    40
 Get a random number     7
 Get a random number    28
 Get a random number     5
 Get a random number    27
 Get a random number     4
 Get a random number    39
 Get a random number    38
 Get a random number    39
Printing backtrace
./build/src/bt-randomgen-example[0x401316]
./build/src/bt-randomgen-example[0x401369]
/nix/store/a3syww9igm49zdzq3ibzw9m8ccvsgxla-glibc-2.32/lib/libc.so.6(__libc_start_main+0xed)[0x7f87aa2b1dbd]
./build/src/bt-randomgen-example[0x40110a]

The scaffold project is in content/code/day-2/24_fortran-cxx. The project has the following source tree:

fortran-cxx/
└── src
    ├── bt-randomgen-example.f90
    ├── interfaces
    │   ├── interface_backtrace.f90
    │   ├── interface_randomgen.f90
    │   └── randomgen.cpp
    └── utils
        └── util_strings.f90

  1. Add CMakeLists.txt files where necessary. You can either declare Fortran, C++, and C as project languages, or enable C++ and C in the interfaces folder.

  2. In the src folder, create an executable from the bt-randomgen-example.f90 file. This executable will have to be linked to the libraries created in the utils and interfaces folders.

  3. Modify the scaffold CMakeLists.txt in the interfaces folder to build a shared library from the C++ and Fortran sources. Beware, for CMake to resolve Fortran modules dependencies, you need to specify the corresponding sources with PUBLIC visibility level.

  4. Do not forget to verify that the C/C++ and Fortran compilers are compatible!

  5. Try out the executable and remember that the build tree mirrors the source tree.

A working solution is in the solution subfolder.

C/C++ using Fortran

Whenever a mix of C/C++ and Fortran is necessary, one needs to be aware of some fundamental differences between the languages:

  • Fortran arrays are column-major.

  • All function arguments are passed by-reference.

  • Fortran compilers mangle function names. Usually by adding an underscore at the end.

  • Fortran is case-insensitive.

Fortran90 introduced a number of modern features: modules, function overloading, and user-defined types. These features further complicate interoperability: they require compilers to perform more extensive name mangling. As the mangling is not standard-mandated, each vendor can decide how to perform it.

The FortranCInterface module fortunately comes to the rescue! The function FortranCInterface_HEADER will generate a header file with all the macros needed to mangle names as appropriate for the compiler in use:

Exercise 25: A C/C++ executable using a Fortran library

Your goal is to link a C++ executable to a BLAS/LAPACK library. The final executable will be named linear-algebra: it scales a vector with DSCAL and performs a linear solve with DGESV. We assume the BLAS/LAPACK library to be written in Fortran. This means that the symbols for DSCAL and DGESV are mangled in a compiler-dependent way. The linear-algebra executable will accept the dimension of the square matrix and vector as command-line input, for example:

$ linear-algebra 1000

C_DSCAL done
C_DGESV done
info is 0
check is 4.80085e-12

The scaffold project is in content/code/day-2/25_cxx-fortran. The project has the following source tree:

cxx-fortran/
├── README.md
└── src
    ├── linear-algebra.cpp
    └── math
        ├── CxxBLAS.cpp
        ├── CxxBLAS.hpp
        ├── CxxLAPACK.cpp
        └── CxxLAPACK.hpp

  1. Inspect the contents of the C++ sources in the math subfolder. They refer to a fc_mangle.h header file, which is not part of the project, as it will be automatically generated.

  2. Create an executable from the linear-algebra.cpp source file.

  3. Complete the scaffold CMakeLists.txt in the math subfolder. In particular, you want to check compatibility of compilers and generate the fc_mangle.h header. Hint: you will have to use the SYMBOLS option to the FortranCInterface_HEADER.

  4. Try out the executable and remember that the build tree mirrors the source tree.

A working solution is in the solution subfolder.

Keypoints

  • Always check whether the Fortran and C/C++ compilers you are using are interoperable.

  • Fortran name-mangling header files for C/C++ can be conveniently autogenerated by CMake.

Footnotes

1

You can find out more about iso_c_binding and Fortran/C interoperability in the GNU Fortran manual.