Interoperability examples between C, C++ and Fortran

Fortran subroutines and functions are easily called from C and C++.

Use the standard C binding to define variable and bind functions/subroutines.

This project is also a way to quickly check if compilers you have are ABI-compatible. For example:

• Clang and Gfortran
• (Windows) MSVC and Intel oneAPI ifx

Demonstrate linking of

• C and C++ program calling Fortran libraries
• Fortran program calling C and C++ libraries

We assume the compilers are C++20 and Fortran 2018 capable.

This repo's examples are also known to work with:

• NVidia HPC SDK (nvc++, nvfortran)
• AOCC AMD Optimizing Compliers
• Cray compilers (cc, ftn)

In general, avoid the FortranCInterface of CMake and mangling function names. Instead, use Fortran 2003 standard bind(C).

Other real-world examples include fortran-filesystem using C++ stdlib filesystem from Fortran and standard sleep implementation as used in blocktran and Fortran Standard Library.

Build

cmake -B build

cmake --build build

ctest --test-dir build

Note the use of CMake target property LINKER_LANGUAGE necessary for CMake with Intel oneAPI or NVIDIA HPC SDK on Linux.

• C main program with Fortran library: LINKER_LANGUAGE C
• C++ main program with Fortran library: LINKER_LANGUAGE CXX
• Fortran main program with C or C++ library: LINKER_LANGUAGE Fortran

add_executable(f_main main.f90 lib.cpp)
set_property(TARGET f_main PROPERTY LINKER_LANGUAGE Fortran)

add_executable(c_main main.c lib.f90)
set_property(TARGET c_main PROPERTY LINKER_LANGUAGE C)

add_executable(cpp_main main.cpp lib.f90)
set_property(TARGET cpp_main PROPERTY LINKER_LANGUAGE CXX)

Examples

While the examples prioritize C++, there are also several companion C examples to go with the C++ examples. There are also some Fortran main programs calling C or C++.

arrays

The examples "array", "malloc", "vector" show distinct ways to send arrays to/from Fortran with C and C++.

bool

nvfortran supports F2018 standard C_BOOL if nvfortran -Munixlogical is used.

• ifort might return an incorrect C_BOOL .true. in iso_c_binding - Intel Community
• Internal representation of LOGICAL variables (The GNU Fortran Compiler
• Solved: Re: Error #8809: An OPTIONAL or EXTERNAL dummy argument to a BIND(C) procedure is not interoperable - Intel Community
• Branchless conditional arithmetic oddities - #18 by sblionel - Language enhancement - Fortran Discourse
• oneAPI -standard-semantics
• https://www.fortran90.org/src/gotchas.html#c-fortran-interoperability-of-logical
• https://info.ornl.gov/sites/publications/Files/Pub158443.pdf Section 3.1.2
• https://shroud.readthedocs.io/_/downloads/en/latest/pdf/ section 11.4

The logical constants .TRUE. and .FALSE. are defined to be the four-byte values -1 and 0 respectively. A logical expression is defined to be .TRUE. if its least significant bit is 1 and .FALSE. otherwise.

Example output, Fortran interfacing with C or C++

• Intel oneAPI 2023 without -fpscomp logicals
• NVHPC 2023.5 without -Munixlogical

 logical_not(T): F
   storage_size()  bits   hex(in)  hex(out)
         C_BOOL:     8         1        FE
 logical_not(true) should be false: 1

 logical_not(F): T
   storage_size()  bits   hex(in)  hex(out)
         C_BOOL:     8         0        FF

• Intel oneAPI 2023 with -fpscomp logicals
• NVHPC 2023.5 with -Munixlogical

 logical_not(T): F
   storage_size()  bits   hex(in)  hex(out)
         C_BOOL:     8         1         0

 logical_not(F): T
  storage_size()  bits   hex(in)  hex(out)
        C_BOOL:     8         0         1
 OK: boolean-logical not

Error handling

Using Fortran statement "stop" or "error stop" with a C/C++ main program works like with a Fortran main program. The "error" examples show this.

ISO_Fortran_binding.h

On 2019-01-12 a GCC commit from Paul Thomas brought ISO_Fortran_binding.h to GCC 9.

This means if using Clang compiler e.g. on macOS, you may need to switch to GCC (till Clang/Flang someday includes ISO_Fortran_binding.h).

Compilers having "ISO_Fortran_binding.h" include:

• GCC ≥ 9
• Intel oneAPI
• Clang / Flang LLVM ≥ 19
• IBM OpenXL
• Cray Fortran

Notes

struct memory alignment

Some examples use scalar C struct. ISO_C_BINDING handles mapping struct to/from Fortran TYPE. For arrays of struct, compiler pragma may be needed.

Valgrind needed suppression to avoid memory alignment (uninitialized memory) warnings for C struct input to nanosleep.

MacOS

For MacOS with Apple's Clang and Homebrew GCC, it MAY be needed to have in ~/.zshrc like the following: (check directory / versions on your Mac)

export LIBRARY_PATH=$LIBRARY_PATH:/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib
export CPLUS_INCLUDE_PATH=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include
export CXXFLAGS=-I$CPLUS_INCLUDE_PATH
export CFLAGS=$CXXFLAGS

References

• StackOverflow