Detect Operating System in C

Reading time: 30 minutes | Coding time: 5 minutes

To check the operating system of the host in a C or C++ code, we need to check the macros defined by the compiler (GNU GCC or G++). For example, on Windows platform, the compiler has a special macro named _WIN32 defined. So, if the macro _WIN32 is defined, we are on Windows. Similarly, other operating systems have specific macro defined by the compiler.

Example of checking Windows OS:

#ifdef _WIN32
    printf("You have Windows Operating System");
#endif

Following is the list of macros defined based on the Operating System:

Operating System	Macro present	Notes
Windows 32 bit + 64 bit	_WIN32	for all Windows OS
Windows 64 bit	_WIN64	Only for 64 bit Windows
Apple	__APPLE__	for all Apple OS
Apple	__MACH__	alternative to above
iOS embedded	TARGET_OS_EMBEDDED	include TargetConditionals.h
iOS stimulator	TARGET_IPHONE_SIMULATOR	include TargetConditionals.h
iPhone	TARGET_OS_IPHONE	include TargetConditionals.h
MacOS	TARGET_OS_MAC	include TargetConditionals.h
Android	__ANDROID__	subset of linux
Unix based OS	__unix__
Linux	__linux__	subset of unix
POSIX based	_POSIX_VERSION	Windows with Cygwin
Solaris	__sun
HP UX	__hpux
BSD	BSD	all BSD flavors
DragonFly BSD	__DragonFly__
FreeBSD	__FreeBSD__
NetBSD	__NetBSD__
OpenBSD	__OpenBSD__

Note that the macros are valid for GNU GCC and G++ and may vary for other compilers.

We will go through some basic examples and explore the use of this features in real life as well.

Simple example

In the following example, we are focused on detecting the flavor of Windows we are running which can be a 64 bit or 32 bit. For Windows, our table will be:

Operating System	Macro present
Windows OS 32 bit + 64 bit	_WIN32
Windows OS 64 bit	_WIN64

As _WIN32 is present in both 32 bit and 64 bit Windows Operating System, we need to first check the presence of _WIN32 to confirm that it is a Windows Operating System and then, check the presence of _WIN64 to confirm whether it is a 64 bit Windows Operating System or a 32 bit Windows Operating System.

Following is the code to check your Windows OS:

#include <stdio.h>

int main() 
{
	#ifdef _WIN32 // Includes both 32 bit and 64 bit
	    #ifdef _WIN64
	        printf("Windows 64 bit\n");
	    #else 
	        printf("Windows 32 bit\n");
	    #endif
	#else
	    printf("Not a Windows OS\n");
	#endif
	return 0;
}

Output:

Windows 32 bit

Example of Apple OS

In this example, we use the macros of Apple OS to detect which Apple OS like MacOS or iPhone is being used.

#include <stdio.h>

int main() 
{
	#if __APPLE__
        #include "TargetConditionals.h"
        #if TARGET_OS_IPHONE && TARGET_IPHONE_SIMULATOR
            printf("iPhone stimulator\n");
        #elif TARGET_OS_IPHONE
            printf("iPhone\n");
        #elif TARGET_OS_MAC
            printf("MacOS\n");
        #else
            printf("Other Apple OS\n");
        #endif
	#else
	    printf("Not an Apple OS\n");
	#endif
	return 0;
}

Output:

MacOS

General example

In this example, we are detecting if we have a Windows OS or a Linux based OS.

#include <stdio.h>

int main() {
	#ifdef _WIN32
	    printf("Windows\n");
	#elif __linux__
	    printf("Linux\n");
	#elif __unix__
	    printf("Other unix OS\n");
	#else
	    printf("Unidentified OS\n");
	#endif
	return 0;
}

Usage

With the power to detect the operating system in the language (C and C++ in our case_, we can write a single cross platform code which works on all platforms by separating platform dependant code.

Example:

#include <stdio.h>

int main() 
{
	#if __APPLE__
	    // apple specific code
	#elif _WIN32
	    // windows specific code
	#elif __LINUX__
	    // linux specific code
	#elif BSD
	    // BSD specific code
	#else
	    // general code or warning
	#endif
	// general code
	return 0;
}

At the same time, we can write code optimized for specific platforms. For example, a function call may be supported on all platforms but we can optimize it greatly for a specific platform say Linux but this new code will raise errors in other platforms. In this case, we can use the macros to detect if it is Linux and for this case, we can easily use other alternative optimized code.

Example:

#include <stdio.h>

int main() 
{
	#if __linux__
	    // linux optimized code (will fail in other platforms)
	#else
	    // general code for all platforms
	#endif
	// general code
	return 0;
}