Yes you wouldn't be able to take advantage of OS specific features, and you'd have to write platform agnostic code on top of what you do have, but that's the price to pay for portability.
Windows does have a libc, but unlike UNIX where it comes with the OS, on Windows it comes with Visual Studio. That's what the "Visual Studio redistributables" are, since Windows has no linker maps and therefore no ABI versioning. Also, Windows has had a POSIX subsystem since the Windows NT days. That's what Windows Services for UNIX ("Interix") runs on.
I'm not saying don't use #ifdefs at all; what I am saying is don't use any OS-specific features if you want your program to be portable, or at least don't use those OS specific features which would immediately preclude usage of your software on other operating systems.
The only exception to this rule is if you are specifically writing software optimized for an operating system, like for example AmigaOS, or illumos/Solaris, or FreeBSD. However, unlike GNU/Linux based operating systems, it is a practical affair to find enough common functionality in illumos/Solaris and the BSD's to write software optimized for all of those operating systems and still be portable between all of them. The same does not hold true for software which has been written using GNU features because they are completely proprietary to GNU and require porting to BSD's and illumos. Sometimes, it's not even feasible to port those features because they are misfeatures or so badly designed that they are broken out of the box.
>I'm not saying don't use #ifdefs at all; what I am saying is don't use any OS-specific features if you want your program to be portable, or at least don't use those OS specific features which would immediately preclude usage of your software on other operating systems.
But this is exactly what #ifdefs solve.
You both have a portable program and can rely on OS-specific features. You can #ifdef GNU behaviour so that you can use the GNU shit on Linux and rely on more portable behaviour if you don't have GNU. Or you use muslc which doesn't pull as much crap. You can #ifdef that too.
I think it seems you want contradictory things. On one hand you claim that portable programs are desirable, on the other you say that you shouldn't rely on OS specific behaviour.
All behaviour is OS specific and outside of very trivial programs I challenge you to find an application that does not rely on #ifdefs, OS specific behaviour or duplicating code while also being portable.
Portability is not what I'd consider the utmost goal of software engineering, that would be solving the problem. Then comes maintainability. Portability is something at the end of a fairly long list.
Portability is what made UNIX the most widespread and successful operating system of all time.
As an engineer yes I want to solve practical problems with a computer, but I also don’t want to dictate (within reason) which OS the user must use. For example, if one of potential users of my software has a really advanced FreeBSD or OpenBSD setup and they can compile link and package my software, it would enable them to keep the advantages of their setup, and my software would make their advanced setup even more useful, like compound return on investment. Another advantage of this approach is that by not dictating the platform, it makes the user more productive and saves their time. People just want to get a task done and solve a problem, and respecting their time should be one of the programmer’s priorities.
We're not dictating what the user is using for their OS, again as I mentioned, there should always be a pure POSIX fallback. But #ifdefs help to take advantage of what the OS can do if the user decides to do it and we should not take that away from the user either.
Additionally I feel like it should be mentioned that even when POSIX and UNIX was a thing, programs had to include #ifdefs because target platforms wouldn't support X or didn't support Y in the same way as another platform. POSIX may have been designed to help against that but people still had to port their software a lot, with lots of #ifdefs and runtime shims.
Windows does have a libc, but unlike UNIX where it comes with the OS, on Windows it comes with Visual Studio. That's what the "Visual Studio redistributables" are, since Windows has no linker maps and therefore no ABI versioning. Also, Windows has had a POSIX subsystem since the Windows NT days. That's what Windows Services for UNIX ("Interix") runs on.
I'm not saying don't use #ifdefs at all; what I am saying is don't use any OS-specific features if you want your program to be portable, or at least don't use those OS specific features which would immediately preclude usage of your software on other operating systems.
The only exception to this rule is if you are specifically writing software optimized for an operating system, like for example AmigaOS, or illumos/Solaris, or FreeBSD. However, unlike GNU/Linux based operating systems, it is a practical affair to find enough common functionality in illumos/Solaris and the BSD's to write software optimized for all of those operating systems and still be portable between all of them. The same does not hold true for software which has been written using GNU features because they are completely proprietary to GNU and require porting to BSD's and illumos. Sometimes, it's not even feasible to port those features because they are misfeatures or so badly designed that they are broken out of the box.