Frequently Asked Questions (FAQ)

Driver

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 :doc:`driver <DriverInternals>`. 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 :ref:`libtooling_builtin_includes` section.