I have been doing some work with assembly and low level stuff like that and had some questions. When programs are compiled they are converted into machine language. The problem is that all of the books I read assumes the user uses Linux.

How come windows .exe files can not be run in Linux? I under stand there are some proprietary software that might be required like directx but I'm not talking about that. How come the same C program compiled in a Linux environment and a windows environment cant be interchanged? What is physically different about compiling a program in windows and Linux? Is one way better then the other or are they just different ways to reach the same place? I haven't done any work with apple computers but I assume that its similar to Linux.

I have been looking around the internet and the best answer I have been able to get so far is "because that's how it is" and "because windows uses file extensions". If somebody were able to give me the facts or point me to a website that does I would be very happy.

This is a reasonable (though still rather vague) summary which is consistent with what I think the reason is... Essentially, Linux executables need to use a different set of OS calls to get input/output and system utilities such as memory allocation to Windows executables. Also, I think the way DLLs and Linux libraries work may well be different

Your initial understanding of how executable files work is incomplete.

If an executable file ran alone in memory then it would have to include all of the code to read the keyboard and draw on the screen. That used to be the case. Then operating systems were invented. Operating systems were very simple in the beginning. Then more capabilities were added over time.

Now when you compile a program your code does not have to include all of the machine instructions to read the keyboard and draw on the screen. However different operating systems implement common features in different ways. Also different operating systems each have some unique features.

The way that a compiled program uses the features of an operating system is by making calls to application program interface (API) functions. These are implemented differently on different operating systems so when a Windows program draws a dot on the screen it accomplishes this by calling the Windows API for this. When an UNIX program wants to draw a dot on the screen it does this by calling the UNIX API for this. These API calls are not compatible from one operating system to another.

That is why a Windows program cannot run directly on UNIX or Linux or other operating systems and vice versa.

Edit: Geez Snark. You weren't even reading this thread when I started writing.

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That makes sense.

Thanks for the very quick response.