Comment by pjmlp
4 hours ago
> 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.
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
Except with BSDs you are on your own if you go down that route, because there are no stability guarantees.
It is more of an implementation detail for the rest of the C APIs than anything else.
Indeed. Another reason to use the system's macros rather than hardcoding integer literals--the numbers can change between releases. Though that doesn't guarantee the syscall works the same way between releases wrt parameters and return value semantics, if it still exists at all. And I believe OpenBSD removed the syscall wrapper altogether after implementing the pinsyscalls feature.
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.
> But it cannot be called directly from C
No system call can, you need a wrapper like syscall() provided by glibc. glibc also provides a dedicated wrapper for the clone system call which properly sets up the return address for the child thread. No idea what you're angry about
Sure, you need a tiny bit of asm to do the actual syscall. That's not what I'm talking about. Most syscalls are easy to wrap, clone is slightly harder but doable (as evidenced by glibc). clone3 is for all intents and purposes impossible to write a general C wrapper for. It allows you to create situations such as threads that share virtual memory but not file descriptors, or vice-versa. That is, it can leave the caller in a situation that violates core assumptions by libc.
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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.
That isn't a valid reason, given the existence of Solaris, HP-UX, DG/UX, Tru64, NeXTSTEP, and so many other UNIXes that grew beyond AT&T UNIX System V.
All of them provide C APIs to their additional features not covered by POSIX.
What Linux has is that due to the way syscalls are exposed there is a certain laziness to cover everything on glibc, or its replacements like musl.
I think rather than "laziness" I would say it's an instance of the widespread phenomenon of "shipping the orgchart". Because for Linux the kernel developers and the libc developers are separate communities, the boundary between those components becomes more meaningful, more visible to the end-user, and more likely to have lag where one side supports something and the other doesn't yet. (That goes both ways, incidentally -- the handling of POSIX threads was driven more from the libc side of the fence and it took a while before the kernel provided enough facilities to make it cleanly doable, and there are still corners like setuid() where there is a mismatch between what the standard wants and the primitives the kernel provides). Where an OS has a more tightly integrated development team the kernel/libc boundary is more likely to stay an internal one.
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IIRC it’s not just laziness, there are things glibc explicitly doesn’t want to expose for various reasons, and since the two projects are essentially unrelated you get the intersection of what both sides are happy with.
Traditional unices develop the kernel and the libc together, as a system, so any kernel feature they want to expose they can just do so.
You are right, but I did not comment on what might be desirable, but on what is the current status.
Because I do not like certain decisions in the design of glibc, I am skeptical about their ability do define good standard APIs for the more recent syscalls, so perhaps it is better that they did not attempt to do this.