Platform Check Macros

C and C++ compilers automatically define some macros that can be used to:

  • Identify platforms (architecture, endianness, OS, compiler, C++ standard, library)
  • Detect compiler configurations (debug mode, exception handling, etc.)

Collectively, these macros are known as pre-defined compiler macros, and are used in Abseil to conditionally compile code for various purposes like portability, performance, functionality, etc. This document lists the compiler macros Abseil uses to write preprocessor conditionals and build configuration macros.

This document is designed solely to document Abseil’s usage of these macros, and assist developers who need to add architecture-specific code to Abseil.

Note: Feature Check Macros, which developers can use to check for the presence of certain language features, are documented within their own, separate guide.

The macros that Abseil uses are listed below, in tables that list the macro, what it identifies, and what standard defines the macro.

If you wish to check what macros your platform has defined, see Pre-defined Compiler Macros.

Architecture

| Macro | Architecture | Compiler | Notes |
| —————– | —————- | ———————- | ———– |
| __x86_64__ | AMD64 | GNU C, Clang/LLVM | |
| _M_X64 | AMD64 | Visual Studio | |
| __arm__ | ARM | | 32-bit only |
| _M_ARM | ARM | Visual Studio | |
| __aarch64__ | ARM64 | | |
| _M_ARM64 | ARM64 | Visual Studio | |
| __i386__ | Intel x86 | | |
| _M_IX86 | Intel x86 | Visual Studio* | |
| __ia64__ | Intel Itanum | | |
: (IA-64) : : : | _M_IA64 | Intel Itanum | Visual Studio | |
: (IA-64) : : : | __ppc__
| PowerPC | | |
__PPC__
: : : :
__ppc64__
: : : :
__PPC64__ : : : : | _M_PPC | PowerPC | Visual Studio | | | __myriad2__ | Myriad2 | Myriad Development Kit | |
: : (Intel Movidius) : : | __mips__ | MIPS | GNU C | |

* Only defined for 32-bits architectures.

References:

Endianness

No standard portable pre-defined macro exists that can be used to determine endianness. Instead, Abseil defines the following macros in absl/base/config.h:

  • ABSL_IS_LITTLE_ENDIAN
  • ABSL_IS_BIG_ENDIAN

References:

Operating Systems

Macro Operating System Compiler
__ANDROID__ Android Android NDK
__APPLE__ macOS, iOS GNU C, Intel C++, and Apple LLVM
_WIN32 Windows For both 32-bit and 64-bit environments
__linux__ Linux Android NDK, GNU C, Clang/LLVM
__ros__ Akaros  
__Fuchsia__ Fuchsia  

References:

Compilers

Macro Compiler Version Macro Notes
__GNUC__ GCC __GNUC__
__GNUC_MINOR__
__GNUC_PATCHLEVEL__		 Clang also defines __GNUC__ for compatibility with GCC. If you want GCC-only, write defined(__GNUC__) && !defined(__clang__)
__clang__ Clang/LLVM __clang_major__
__clang_minor__
__clang_patchlevel__		  
_MSC_VER MSVC _MSC_VER
_MSC_FULL_VER		  
__EMSCRIPTEN__ Emscripten __EMSCRIPTEN_major__
__EMSCRIPTEN_minor__
__EMSCRIPTEN_tiny__		  
__asmjs__ asm.js    
__wasm__ WebAssembly    
__NVCC__ NVCC    

References:

  • Compilers within the Pre-defined Compiler Macros guide.

Libraries

Library headers need to be included before checking the macros. For example, base/config.h includes <cstddef> for __GLIBCXX__, _LIBCPP_VERSION.

Note that these macros are not compiler-dependent as they are defined within header files.

Macro Library
__GLIBCXX__ libstdc++
_LIBCPP_VERSION libc++
_MSC_VER MSVC
__GLIBC__ GNU glibc
__BIONIC__ Bionic libc

References:

  • Libraries within the Pre-defined Compiler Macros guide.