How to Check Which Operating System with C++

Many times we are developing portable programs with C++ that we need to create a single code that works on different platforms.

And for that we need to use PRE PROCESSING DIRECTIVES: #ifdef, #if defined(), …

In this case there are specific MACROS for each operating system, for example:

If you want to detect if the system is Linux, just use the macro: __linux__;
If you want to check if it’s Windows, you can use: _WIN32 .

Checking if it’s Linux or Windows

Below is an example of C++ code that runs on both operating systems and emits a “Hello, World!” depending on the operating system:

#include <iostream>

int main(){

#if defined(__linux__) // Or #if __linux__
  std::cout << "Hello, GNU/Linux!" << '\n';
#elif _WIN32
  std::cout << "Hello, Windows!" << '\n';
#else
  std::cout << "Hello, Other!" << '\n';
#endif

}

In case of Windows, you can still create another #if inside to detect if:

_WIN64 , this Windows is 64-bit only;
_WIN32 , both 64-bit and 32-bit
__CYGWIN__ another more modern alternative.

Example:

#include <iostream>

int main(){

#if __linux__
  std::cout << "Hello, GNU/Linux!" << '\n';
#elif _WIN32
  #if defined(_WIN64)
    std::cout << "Hello, Windows 64-bit!" << '\n';
  #else
    std::cout << "Hello, Windows!" << '\n';
  #endif
#else
  std::cout << "Hello, Other!" << '\n';
#endif

}

Checking various operating systems

You can still detect all operating systems. Below is another example that not only detects Linux and Windows, but also checks if it is: macOS, FreeBSD, Android:

#include <iostream>

int main(){

#if defined(__linux__) // Or #if __linux__
  std::cout << "Hello, GNU/Linux!" << '\n';
#elif __FreeBSD__
  std::cout << "Hello, FreeBSD!" << '\n';
#elif __ANDROID__
  std::cout << "Hello, Android!" << '\n';
#elif __APPLE__
  std::cout << "Hello, macOS!" << '\n';
#elif _WIN32
  std::cout << "Hello, Windows!" << '\n';
#else
  std::cout << "Hello, Other!" << '\n';
#endif

}

You can still check if it is UNIX type (Linux or BSD, macOS NOT): unix, __unix or __unix__ .

For a complete list of all macros for operating systems you can see here.

Using gcc to detect macro names

To have a list of macros you can detect, use gcc or g++:

It may be that clang and clang++ also have these options, but I haven’t reinstalled them for testing yet.

On GNU/Linux:

g++ -dM -E - </dev/null
# Or
gcc -dM -E - </dev/null

On Windows

g++ -dM -E - <NUL:
# Or
gcc -dM -E - <NUL:

The output will be extensive:

Small snippet below.

#define __SSP_STRONG__ 3
#define __DBL_MIN_EXP__ (-1021)
#define __UINT_LEAST16_MAX__ 0xffff
#define __FLT16_HAS_QUIET_NAN__ 1
#define __ATOMIC_ACQUIRE 2
#define __FLT128_MAX_10_EXP__ 4932
#define __FLT_MIN__ 1.17549435082228750796873653722224568e-38F
#define __GCC_IEC_559_COMPLEX 2
#define __UINT_LEAST8_TYPE__ unsigned char
#define __SIZEOF_FLOAT80__ 16
#define __INTMAX_C(c) c ## L
#define __CHAR_BIT__ 8
#define __UINT8_MAX__ 0xff
#define __SCHAR_WIDTH__ 8
#define __WINT_MAX__ 0xffffffffU
#define __FLT32_MIN_EXP__ (-125)
#define __ORDER_LITTLE_ENDIAN__ 1234
#define __SIZE_MAX__ 0xffffffffffffffffUL
#define __WCHAR_MAX__ 0x7fffffff
#define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2 1
...