What standard C library does Clang use? glibc, its own, or some other one?
Things are changing! - LLVM is working on its own libc implementation https://github.com/llvm/llvm-project/tree/main/libc
They haven't talked about it in a couple of versions: https://releases.llvm.org/12.0.0/docs/Proposals/LLVMLibC.html
llvm-libc, [is] an implementation of the C standard library targeting C17 and above, as part of the LLVM project. llvm-libc will also provide platform specific extensions as relevant. For example, on Linux it also provides pthreads, librt and other POSIX extension libraries.
Parts I find most interesting are:
Designed and developed from the start to work with LLVM tooling and testing like fuzz testing and sanitizer-supported testing.
Use source based implementations as far possible rather than assembly. Will try to fix the compiler rather than use assembly language workarounds.
Looks like LLVM 14 or 15 will be released with it. Watch this space for further documentation!
Clang does not come with its own C standard library. Rather, it "supports a wide variety of C standard library implementations".
Unix-like systems including Mac OS ship with their own. Windows does not. On Windows the default arrangement requires Microsoft's Visual C libraries to be installed. It seems that it's also possible to use Clang on Windows with MinGW's libraries though.
When compiling C++ code, libstdc++ from GNU, or LLVM's libc++ can be used as the C++ Standard Library. They are set by compiling with clang++ using flag -stdlib=libstdc++, or -stdlib=libc++, respectively. This information is laid out reasonably well at https://clang.llvm.org/docs/Toolchain.html.
When compiling C code, the C standard library cannot be switched per-compilation. It is necessary to build the entire compiler toolchain for the selected C library. That means either cross-compilation, or, in case of Linux, compiling in a distribution that uses the chosen C library. If I wanted to compile with musl, I'd compile in Alpine Linux, and so on.
But for LLVM/Clang I'm not sure
It's unclear whether you are asking about which libc clang itself is using (as in, the clang compiler binary), or whether you are asking about which libc the binary that clang produced as a result of compilation and linking is using.
In either case, the answer is: whichever libc you told it to use (either at clang configuration time, or at the time of linking the final binary).
It could be GLIBC, uClibc, Musl, Apple libc, or some other libc.
Eventually, LLVM-libc may be ready and could be used as well.
Update:
Are you saying that every time you install Clang the user must decide with libc it will use?
No: a suitable default -- system libc, which is usually GLIBC (but not always) on Linux, and Apple libc on MacOS, is provided.
I have not touched Windows in over 10 years, but I suspect that MSVCRT.DLL is eventually used by default there.
But the end-user can override the default choice by using suitable -I... and -L... flags at compile and link time.
The end user could also change the default by reconfiguring and rebuilding clang.