# Clang 12 documentation

## Frequently Asked Questions (FAQ)

- Driver
  - I run `clang -cc1 ...` and get weird errors about missing headers
  - I get errors about some headers being missing (`stddef.h`, `stdarg.h`)

## 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 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.