Compilers define macros that can be used to identify a compiler and platform from within C, C++ and Fortran code. This can be useful for many purposes where short bits of platform-specific or compiler-specific code is needed. If a significant amount of code is needed, it may be better to swap in different code files using the build system instead of lengthly #if defined(foo) logic. There are numerous examples for C and C++ so here we will focus on macros of Fortran compilers.

Gfortran compiler macro definitions are obtained in an OS-agnostic way by:

echo "" | gfortran -dM -E - > macros.txt

that creates a file “macros.txt” containing all the compiler macros.

commonly used macros to detect operating system / compiler configuration include:

• _WIN32 1
• __linux__ 1
• __unix__ 1
• __APPLE__ 1

Intel oneAPI classic compiler macros include the Gfortran macros noted above and additionally:

• __INTEL_COMPILER 1

Intel oneAPI Next Gen LLVM compiler macros set:

• __INTEL_LLVM_COMPILER 1

to distinguish from oneAPI Classic compilers.

Nvidia HPC Fortran compiler macros are printed by:

nvfortran -dM

the Nvidia macros include the Gfortran macros above as well as:

• __PGI 1

Flang macros include

• __FLANG 1

Other Fortran compiler macros that identify the compiler and platform can be found in CMake source code.