clang: 'stdio.h' file not found

If you get the error:

$ clang ,,x.c
,,x.c:1:10: fatal error: 'stdio.h' file not found
#include <stdio.h>
         ^~~~~~~~~
1 error generated.

you might think you're missing headers or the Xcode-provided/related clang toolchain and so need to xcode-select --install a toolchain or find a .pkg file containing headers or something. Another cause you might find is that you've installed a different clang toolchain (in my case Espressif's) and put it ahead of the system one in PATH. So check which clang you're invoking if it gives weird results.