Platform C Preprocessor Definitions

Question

I'm writing a small library in C++ that I need to be able to build on quite a few different platforms, including iPhone, Windows, Linux, Mac and Symbian S60. I've written most of the code so that it is platform-agnostic but there are some portions that must be written on a per-platform basis.

Currently I accomplish this by including a different header depending on the current platform but I'm having trouble fleshing this out because I'm not sure what preprocessor definitions are defined for all platforms. For windows I can generally rely on seeing WIN32 or _WIN32. For Linux I can rely on seeing _UNIX_ but I am less certain about the other platforms or their 64-bit variants. Does anyone have a list of the different definitions found on platforms or will I have to resort to a config file or gcc parameter?

Answer 1

I have this sourceforge pre-compiler page in my bookmarks.

• rubenvb: I edited you link to point to the new wiki-ized version, which is much more up to date, although Google always points to the old version.

Answer 2

The definitions are going to be purely up to your compiler vendor. If you are using the same compiler (say, gcc) on all your platforms then you will have a little bit easier time of it.

You might also want to try to instead organize your project such that most of the .h files are not platform dependent. Split your implementation (cpp files) into separate files; one for the nonspecific stuff and one for each platform. The platform specific ones can include 'private' headers that only make sense for that platform. You may have to make adapter functions to get something like this to work 100% (when the system libs take slightly differed arguments) but I have found it to be really helpful in the end, and bringing on a new platform is a whole lot easier in the future.

Answer 3

Neither the C nor the C++ standards define such symbols, so you are going to be at the mercy of specific C or C++ implementations. A list of commonly used symbols would be a useful thing to have, but unfortunately I haven't got one.

Answer 4

I don't think there exists a universal list of platform defines judging by the fact that every cross-platform library I have seen has an ad-hoc config.h full of these stuff. But you can consider looking at the ones used by fairly portable libraries like libpng, zlib etc.

Here's the one used by libpng

Answer 5

If you want to look through the default preprocessor symbols for a given system on which you have GCC (e.g. Mac OS X, iOS, Linux), you can get a complete list from the command-line thus:

echo 'main(){}' | cpp -dM

These are often of limited use however, as at the stage of the compilation at which the preprocessor operates, most of the symbols identify the operating system and CPU type of only the system hosting the compiler, rather than the system being targeted (e.g. when cross-compiling for iOS). On Mac OS X and iOS, the right way to determine the compile-time characteristics of the system being targeted is

#include <TargetConditionals.h>

This will pick up TargetConditionals.h from the Platform and SDK currently in use, and then you can determine (e.g.) endianness and some other characteristics from some of the Macros. (Look through TargetConditionals.h to see what kinds of info you can glean.)

• rubenvb: Although checking -dM output might help, it can't guarantee as much information as the predef sourceforge wiki page.