There's something a bit strange about the approach of this post that suggests a confusion of concepts (though I'm not saying there's anything factually incorrect about it).<p>The standard C library and the POSIX API are two quite different things.<p>The standard C library is a library of functions defined by the various C language standards. These functions should be available on any C language implementation regardless of the platform (Unix/POSIX, Windows, embedded RTOS, micro-controller with no OS, etc.).<p>The POSIX API is a platform specification for Unix like platforms (though the API is sometimes available non-natively on non-Unix like platforms). It is comparable to the win32 API on windows. It has no particular connection to the C language other than that C is the language in which the API is provided.<p>Perhaps one reason for this confusion is that today many people are familiar with higher level programming environments like Java or Python which provide much richer standard libraries than do C and C++. They include things like multithreading and network communications. C programmers need to use platform specific API's like POSIX or win32 to access these services. Also note that pretty much any higher level language running on a modern OS is ultimately using the platform system API's to provide such services because the OS kernel doesn't allow user space programs access to them other than through a system call mechanism (which the platform API's typically wrap).<p>One other possible source of confusion that the post doesn't mention is that on Unix the library one links to for both the C standard library functions and the Unix/POSIX system API's is called libc (it's usually called glibc on Linux). Despite the name this is not the same thing as the C standard library.
Not sure where you got the idea that fork() isn't a system call. It certainly is on my computer.<p>I think the overall sentiment is right on, but I'm not sure I get the last question. posix is a superset, yes, but it doesn't provide replacements for the C library. Perhaps better phrased as "Why limit yourself to only standard C?"?
One example of good restriction to the "C" library is the lua reference implementation - because of that it can compile for lots of platforms (well 32-bit and above) without a problem.<p>Another one is zlib, and lots of more.
I do not wish to be a troll but most of the information in that post is plain wrong. For one fork() is indeed a system call. If you want a good understanding of the true essence of POSIX (UNIX API) I recommend reading: Advanced Programming in the UNIX Environment.