alt Hacker News> In an ideal world, there would be a header-only C library provided by the Linux kernel; we would include that file and be done with it. As it turns out, there is no such file, and interfacing with syscalls is complicated.
Because Linux is the exception, UNIX public API is the C library as defined later by POSIX.
The goal to create C and rewrite UNIX V4 into C was exactly to move away from this kind of platform details.
Also UNIX can be seen as C's runtime, in a way, thus traditionally the C compiler was the same of the platform vendor, there were not pick and chose among C compilers and standard libraries, that was left for non-UNIX platforms.
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I actually love this about Linux. The syscall API is much better than libc (both the one defined by POSIX and libc as it actually exists on different Unixen). No errno (which requires weird and inefficient TLS bullshit), no hooks like atfork/atexit/etc., no locales, no silly non-reentrant functions using global variables, no dlopen/dlclose. Just the actual OS interface. Languages that aren't as braindead as C can have their own wrappers around this and skip all that nonsense.
Also, there are syscalls which are basically not possible to directly expose as C functions, because they mess with things that the C runtime considers invariant. An example would be `SYS_clone3`. This is an immensely useful syscall, and glibc uses it for spawning threads these days. But it cannot be called directly from C, you need platform-specific assembly code around it.
Even if one would want to use Linux only through libc, that is not always possible.
Linux has evolved beyond POSIX and many newer syscalls, which can enhance performance in certain scenarios, are not available as libc functions.
They may be invoked either using the generic syscall wrappers provided by glibc besides the standard functions, or by using custom wrappers or possibly by using some special libraries, if such libraries are available.
All true, but note that BSD introduced, and both Linux/glibc and Linux/musl support, a syscall(2) wrapper routine that takes a syscall number, a list of arguments (usually as long's), and performs the syscall magic. The syscall numbers are defined as macros beginning with SYS_. The Linux kernel headers export syscall numbers with macros using the prefix __NR_, but to match the BSD interface Linux libc headers usually translate or otherwise define them using a SYS_ prefix. Using the macros is much better because the numbers often vary by architecture for the same syscall.
See https://man7.org/linux/man-pages/man2/syscall.2.html