clang/docs/FAQ.md

# Frequently Asked Questions (FAQ)

```{contents}
:local: true
```

## Driver

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

Given this source file:

```c
#include <stdio.h>

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

If you run:

```console
$ 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:

```console
$ 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.