Determine If Linux or Windows in C++

Detect Windows or Linux in C, C++

It's generally done like this (more or less):

#ifdef _WIN32
#include <windows.h>
#include <stdio.h>
#include <tchar.h>

#define DIV 1048576 
#define WIDTH 7
#endif

#ifdef linux
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#endif


int main(int argc, char *argv[]) 
{
#ifdef _WIN32
MEMORYSTATUSEX statex;
    statex.dwLength = sizeof (statex);
    GlobalMemoryStatusEx (&statex);

    _tprintf (TEXT("There is  %*ld %% of memory in use.\n"),
            WIDTH, statex.dwMemoryLoad);
#endif

#ifdef linux
char cmd[30];
int flag = 0;   
FILE *fp;
char line[130];     
int TotalMem, TotalFree, TotalUsed;

flag=0;
memcpy (cmd,"\0",30);
sprintf(cmd,"free -t -m|grep Total");          
fp = popen(cmd, "r");       
while ( fgets( line, sizeof line, fp))
{   
    flag++;
    sscanf(line,"%*s %d %d %d",&TotalMem, &TotalUsed, &TotalFree);
}
pclose(fp); 

if(flag)
    printf("TotalMem:%d -- TotalUsed:%d -- TotalFree:%d\n",TotalMem,TotalUsed,TotalFree);
else 
    printf("not found\n");
#endif

    return 0;
}

This way, only code for linux will be compiled while on a linux platform, and only windows code will be compiled on a windows platform.

How do I check OS with a preprocessor directive?

The Predefined Macros for OS site has a very complete list of checks. Here are a few of them, with links to where they're found:

Windows

_WIN32   Both 32 bit and 64 bit

_WIN64   64 bit only

__CYGWIN__

Unix (Linux, *BSD, but not Mac OS X)

See this related question on some of the pitfalls of using this check.

unix

__unix

__unix__

Mac OS X

__APPLE__ Also used for classic

__MACH__

Both are defined; checking for either should work.

Linux

__linux__

linux Obsolete (not POSIX compliant)

__linux Obsolete (not POSIX compliant)

FreeBSD

__FreeBSD__

Android

__ANDROID__

Determine if Linux or Windows in C++

Use:

#if defined(WIN32) || defined(_WIN32) || defined(__WIN32) && !defined(__CYGWIN__)
static const std::string slash="\\";
#else
static const std::string slash="/";
#endif

BTW, you can still safely use this slash "/" on Windows as windows understands this perfectly. So just sticking with "/" slash would solve problems for all OSes even like OpenVMS where path is foo:[bar.bee]test.ext can be represented as /foo/bar/bee/test.ext.

How to detect reliably Mac OS X, iOS, Linux, Windows in C preprocessor?

There are predefined macros that are used by most compilers, you can find the list here. GCC compiler predefined macros can be found here.
Here is an example for gcc:

#if defined(WIN32) || defined(_WIN32) || defined(__WIN32__) || defined(__NT__)
   //define something for Windows (32-bit and 64-bit, this part is common)
   #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
#elif __ANDROID__
    // Below __linux__ check should be enough to handle Android,
    // but something may be unique to Android.
#elif __linux__
    // linux
#elif __unix__ // all unices not caught above
    // Unix
#elif defined(_POSIX_VERSION)
    // POSIX
#else
#   error "Unknown compiler"
#endif

The defined macros depend on the compiler that you are going to use.

The _WIN64 #ifdef can be nested into the _WIN32 #ifdef because _WIN32 is even defined when targeting the Windows x64 version. This prevents code duplication if some header includes are common to both
(also WIN32 without underscore allows IDE to highlight the right partition of code).

Autodetection of OS in C/C++

I have used below code in past for checking between linux and windows

#include<stdio.h>

int main(int argc, char *argv[])
{

#ifdef _WIN32
   printf("in Windows");
#endif

#ifdef linux
    printf("In Linux");
#endif

    return 0;
}

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Usually all toolchain has their own predefined macros so you need to use those macros to detect which os is this.

List of such predefined macros.

http://sourceforge.net/p/predef/wiki/OperatingSystems/

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Yes as a side note, Here detection of OS is happened using compile time macros. So according to toolchain respective macro's defination will go in build.

To detect OS version in runtime, like in java we do

System.getProperty("os.name")

Same way in C/C++ we do not have any API.

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In POSIX systems using 'uname' we can get OS name.

C++ compiling on Windows and Linux: ifdef switch

use:

#ifdef __linux__ 
    //linux code goes here
#elif _WIN32
    // windows code goes here
#else

#endif

How do I know if a C program will written on Linux will work elsewhere

Well, this is kind of simple: Your program will work on Windows as long as it only uses two things:

• The C standard library, as described in the C standard. See e.g. n1570.pdf, the latest draft version that's very close to the actual standard. Note that a lot of C standard libraries come with all sorts of extensions, of course you can't use these. If in doubt, compile e.g. with the -std=c11 option (gcc or clang) which should hide all declarations for extensions.

  There's an additional pitfall here: Microsoft's implementation of the C standard isn't complete. It depends on your compiler which C standard library is actually linked, in case of MinGW, it's the very old msvcrt.dll that only conforms to C89, so you might want to read an older standard instead for which functions you can use, e.g. here's a description of the standard library in C89 With Microsoft's C compilers, you get different levels of C99 support also in the standard libraries (they link their own runtime DLLs), depending on the version.

• Third party libraries that are available for Windows, like e.g. SDL, GTK+, etc.

By using open, creat, close, read, write etc., you're using extensions specified in POSIX. Some of the POSIX functions are available on Windows, some are slightly different, some are just missing. You better shouldn't use these if you want to stay portable to Windows. The exception to this rule is if you plan to compile your code with Cygwin, which is a complete POSIX compatibility layer for Windows.

As per IanAbbott's comment, it's also crucial to use integer types correctly and not implicitly assume things like "a long has 64 bits" or "an int can hold a pointer", which are wrong in the general case. Always use the <stdint.h> types for fixed widths, uintptr_t for pointer values, size_t for object sizes, and so on.

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Although not directly related to your question: For most non-trivial programs, you will sooner or later need to use some platform-specific functionality. Often, there are libraries abstracting this away, like e.g. if you need to handle I/O events efficiently, instead of using epoll on Linux, kqueue on FreeBSD, IO completion ports on Windows and so on, you could just use libevent. But in case you don't find such a library, here's a tip:

Clearly separate platform-specific code into its own modules (translation units). This way, the work for porting the whole thing to a different platform gets significantly smaller.

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