How Can I Detect the Operating System in the C Preprocessor for Cross-Platform Development?

Detecting Operating Systems in the C Preprocessor for Cross-Platform Development

Determining the underlying operating system during the preprocessor stage is crucial for cross-platform C/C code. Fortunately, most compilers define specific macros that allow for reliable detection.

GCC and Clang Macros

GCC and Clang compilers provide a comprehensive list of predefined macros that can be used for OS identification:

• For Windows: WIN32, _WIN32, WIN32__, __NT
• For macOS: APPLE
• For Linux: linux
• For Unix: unix
• For POSIX: _POSIX_VERSION

Example for GCC

Here's an example of how these macros can be utilized in a GCC compilation:

#if defined(WIN32) || defined(_WIN32) || defined(__WIN32__) || defined(__NT__)
  // Define something for Windows (32-bit and 64-bit)
  #ifdef _WIN64
    // Define something for Windows (64-bit only)
  #else
    // Define something for Windows (32-bit only)
  #endif
#elif __APPLE__
  #include <TargetConditionals.h>
  #if TARGET_IPHONE_SIMULATOR
    // iOS, tvOS, or watchOS Simulator
  #elif TARGET_OS_MACCATALYST
    // Mac's Catalyst (ports iOS API into Mac, like UIKit).
  #elif TARGET_OS_IPHONE
    // iOS, tvOS, or watchOS device
  #elif TARGET_OS_MAC
    // Other kinds of Apple platforms
  #else
    #error "Unknown Apple platform"
  #endif
#else
  #error "Unknown compiler"
#endif

Important Considerations

• The availability of predefined macros varies depending on the compiler used.
• It's recommended to employ macro guards to avoid multiple inclusions of headers.
• _WIN32 (with no underscore) allows IDEs to highlight the appropriate code sections for Windows platforms.
• Windows x64 compilers still define _WIN32, so it can be used as a common base and then employ an inner _WIN64 #ifdef for 64-bit-specific definitions.

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