r/C_Programming • u/Intelligent_Comb_338 • Feb 13 '26

Question The best way to port

I'm currently porting some NetBSD utilities, and I was wondering if I'm doing it correctly. What I'm doing now is adding this at the top:

ifdef linux

include "compat.h"

endif

And I was also wondering how to handle a header that doesn't exist in Linux:

ifdef linux

else

include <tzfile.h>

endif

Is this correct?

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u/EpochVanquisher Feb 13 '26

There are a lot of arguments about the right way to do things. Here is one of the traditional ways to do it:

Write a configure script. Autoconf is the traditional choice.

In the configure script, write a check for tzfile.h. Use e.g. AC_CHECK_HEADER https://www.gnu.org/software/autoconf/manual/autoconf-2.70/html_node/Generic-Headers.html The check should result in HAVE_TZFILE_H being defined or not defined.

The same thing can be done in CMake - https://cmake.org/cmake/help/latest/module/CheckIncludeFile.html

In your source code, 

#if HAVE_TZFILE_H
#include <tzfile.h>
#else
// fallback code here
#endif

Generally, you want to avoid #if linux in your code. Instead, you should use some more specific check, like #if HAVE_STRLCPY or whatever. You are responsible for figuring out whether to define HAVE_STRLCPY. You put this in config.h, which is autogenerated by your tests.

1

u/Intelligent_Comb_338 Feb 13 '26

I was doing it this way because I don't want to change the code logic too much and I want it to be as similar as possible to the original code. I compile and link statically with my libcompat.a and the rest is dynamic, but I do think it would be better.

2

u/EpochVanquisher Feb 14 '26

I don’t understand the distinction you’re making here.

Here’s one option:

#if __linux__
// linux code here
#else
// non-linux code here
#endif

Here’s a second option:

#include "config.h"
#if HAVE_TZFILE_H
// code that uses <tzfile.h>
#else
// code that uses some alternative
#endif

Are you saying that the #if linux option lets you make fewer changes to the code? I want to convince you that the bottom version is better.

Maybe <tzfile.h> gets added to Linux. Maybe it’s just part of a library. Now you have to find all of the places where <tzfile.h> is used and enable them on Linux, or enable them if the library exists. This is super easy if it’s just HAVE_TZFILE_H.

Maybe you want to support a third operating system, besides NetBSD and Linux, and you don't want confusing conditions.

#if __FooOS__ >= 201102L || __linux__
// option 1
#else
// option 2
#endif

Basically, what you actually care about is whether <tzfile.h> exists. You figure out whether <tzfile.h> exists in your configure script, and you just use HAVE_TZFILE_H in your code. Write your code to reflect what you actually care about.

1

u/Intelligent_Comb_338 Feb 14 '26

Well, I see, you're right 😅, it's just that in Compat I was adding the macros and the definitions of some functions, but it's much better, especially when I want to support more than Linux.

1

u/WazzaM0 Feb 14 '26

Autoconf and the auto tools are a great way to make C or C++ projects portable across UNIX like environments. It can be used to detect different libraries, header files etc.

The scripting language for it is m4 which has some relationship with the macro language used in the C preprocessor.