Compilers define macros that can be used to identify a compiler and platform from compiled code, such as C, C++, Fortran, et al. This can be used for platform-specific or compiler-specific code. If a significant amount of code is needed, it may be better to swap in different code files using the build system instead of lengthy #ifdef logic. There are numerous examples for C and C++ so here we focus on macros of Fortran compilers.

Gfortran compiler macros

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

CAUTION: these macros are actually not available in the Gfortran compiled programs as they are in GCC. A workaround is to have the build system define these for the particular compiler, OS, etc.

Clang LLVM compiler macros

LLVM-like compilers (including AMD AOCC) macro definitions are obtained in an OS-agnostic way by:

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

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

__VERSION__ and __clang_version__ contain string version information.

Intel oneAPI LLVM compiler macros

oneAPI macros set:

• __INTEL_LLVM_COMPILER 1

to distinguish from oneAPI Classic compiler macros like __INTEL_COMPILER 1

Cray compiler macros

Detect Cray compiler wrapper with compiler macro __CRAYXT_COMPUTE_LINUX_TARGET instead than non-universal __CRAYXC or __CRAYXE.

or use Cray environment variable PE_ENV and check for CRAY or PrgEnv-.

NVIDIA HPC compiler macros

Print compiler macros like:

touch main.c main.cpp main.f90

nvc -dryrun main.c
nvc++ -dryrun main.cpp
nvfortran -dryrun main.f90

NVIDIA HPC macros include:

• __NVCOMPILER
• __NVCOMPILER_LLVM__

Flang compiler macros

Flang macros include __FLANG 1

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