I run clang -cc1 ... and get weird errors about missing headers¶

Given this source file:

#include <stdio.h>

int main() {
  printf("Hello world\n");
}

If you run:

$ clang -cc1 hello.c
hello.c:1:10: fatal error: 'stdio.h' file not found
#include <stdio.h>
         ^
1 error generated.

clang -cc1 is the frontend, clang is the driver. The driver invokes the frontend with options appropriate for your system. To see these options, run:

$ clang -### -c hello.c

Some clang command line options are driver-only options, some are frontend-only options. Frontend-only options are intended to be used only by clang developers. Users should not run clang -cc1 directly, because -cc1 options are not guaranteed to be stable.

If you want to use a frontend-only option (“a -cc1 option”), for example -ast-dump, then you need to take the clang -cc1 line generated by the driver and add the option you need. Alternatively, you can run clang -Xclang <option> ... to force the driver pass <option> to clang -cc1.

I get errors about some headers being missing (stddef.h, stdarg.h)¶

Some header files (stddef.h, stdarg.h, and others) are shipped with Clang — these are called builtin includes. Clang searches for them in a directory relative to the location of the clang binary. If you moved the clang binary, you need to move the builtin headers, too.

More information can be found in the Builtin includes section.