This project is using the phrase "system call" idiosyncratically / incorrectly: by it they mean "calls to the system() function", not "low-level OS facilities". POCO and ACE seem to be about actual system calls (network programming, threads, pipes, etc.) They're for very different purposes.

libcox is for when your existing code would do something like system("uname") or system("ls") (... except not even with system(), with popen() or something), and you want it portable to Windows.

As it explains quite clearly, this is "portable system calls". Lightweight indeed, it's the whole point!

So if you ask this question, you might as well ask why system calls are not more expansive in their capabilities.

I think there's a reason, system calls are much simpler. They are building bricks. If you want to layer something complex on top, you're free to do so.

About your suggestions, ACE's documentation is particularly bad. I wouldn't recommend it to anyone.

Not sure if climbing this high on HN on a U.S. holiday indicates a real need or clever marketing for this new project, but congratulations to them for getting the word out.

Thanks also to you for sharing a couple well-established alternatives.

A few more (particularly nspr and apr): http://www.shlomifish.org/open-source/portability-libs/