Am I coding for an OS or the Processor?

Question

It is said that by using C/C++, one can write 'native' programs - that run on the platform. I am confused about what is considered native - the processor architecture or the OS version?

For example:

I have a 32 bit processor and Windows 7 ( 32 bit ), and I compile and generate and .exe file. Is it guaranteed to run on any Windows 7 32 Bit? ( Win 7 32 bit on 32/64 Bit machines )

Edit1 : I did not intend only Windows OS here. My example can be extended to Linux also. For example, generating an executable ( by default a.out ) on a 32 bit Linux OS running on 32 bit processor, and then running it on a 32bit Linux on a 64 bit processor.

Edit2 : Thanks for the responses, but I also intended that I am using the standard libraries and functions - nothing OS Specific. Just the once specified by the ANSI or ISO C++ Standard. No references to OS specific windowing systems or other libraries. Thanks

Answer 1

Both; kind of.

The actual instructions don't really differ across Windows and Linux as they are compiled down for a single CPU architecture (x86).

However, a binary is more than just code that runs on bare hardware. For instance, it also contains information that tells the operating system how to load the executable and its dependencies. The binary is packaged in a specific format. This format can be different in different operating systems.

Besides that, the operating system provides some services to the applications (through system calls and APIs). The services that operating systems provide, and the way they can be used varies from an operating system to another.

These reasons contribute to the fact that most of the time a native binary depends on both OS and CPU architecture it's compiled for.

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Answer to the updated question:

C++ Standard doesn't require anything about the nature of the compiled target. It just specifies compatibility requirements at the source level. Consequently, if you stick to the standard libraries, you'll be able to use the same source code to compile on platforms that offer a conforming C++ implementation. The standard doesn't say anything about binary portability. As I mentioned above, the primitive system calls that operating systems provide can vary and the actual implementation of the standard library depends on the way those system calls are provided by the OS.

In order to run a Windows binary on Linux, you need to use some sort of emulation like Wine which understands Windows binary format and simulates Windows API for applications.

Answer 2

1) The processor architecture (plus the targeted libraries static or dynamic)

2) Yes

A 32bit windows application will run on a Windows 64bit platform as WOW .

Answer 3

如果你的（windows）编译器的目标体系结构是x86（32位），那么它可以在任何32位和64位Windows 7上运行。但是如果它的x86-64，它只能在64位Windows 7上运行。

Answer 4

To answer the title specifically, you code for both.

The executable contains machine code, which is specific to the processor, and a lot of metadata for the OS on how to load/execute the program, which is specific to the OS.

The code may also (and typically does) contain calls into functions defined by the OS. And so, while it is just perfectly ordinary machine code that any compatible CPU will understand, it attempts to call code that only exists on Windows.

So "native" really means both. You code for the specific OS (and all compatible OS'es) and that specific CPU (and all compatible CPUs).

In the case of Windows, you typically target a specific version of Windows, and the program will then work on that, and future versions of Windows.

for the processor on which Windows (and your program) runs, the executable contains x86 machine code, which can be executed on any x86 CPU, whether it is from Intel, AMD, Via or whoever else have made compatible processors over the years.

Answer 5

Without being able to see your code, only you can tell us whether you're coding for 32-bit or 64-bit platform - for example, if you reinterpret_cast a pointer into a 32 bit int then back to a pointer, you are coding for 32-bit, whereas if you use a type such as int_ptr you are safe whether your code is compiled for 32 or 64 bit machines. Similarly, coding for Windows desktops your coding can assume the machine's endianess.

If, as in your example, you compile that code for 32-bit Windows 7, then it will also run on 64 bit Windows 7. If you use Windows 7 features, it won't run on earlier versions. Microsoft are very good at backward compatibility, so it probably will run on later versions.

Answer 6

Short answer: No.

Longer: When you compile "native code", you compile for a specific processor architecture; MIPS, ARM, x86, 68k, Sparc and so on. These architectures can have a wordlength of 8, 16, 32 and 64 (there are exceptions). Also these architectures can have extensions from generation to generation like MMX, SSE, SSE2, Neon and such.

Also you need to consider the operating system and what libraries you can use, and different calling conventions.

So, there's no guarantee. But if you compile with MSVC on Windows 7 it's almost guaranteed to run on Windows 7. I think it only exist for x86 at the moment.