# XO1104: C header cannot be bound

`use c "header.h"` reads the header with clang's AST dump and generates an Xo declaration file and a C adapter (into `.xo/cbind/` or `$XO_CBIND_DIR`). This failed: the header is neither a file relative to the module directory nor on the C include path, clang reported an error in it, a `link` file does not exist, or the line's `uses` names an unknown effect. The message carries the reason. `link "x.c"` (or `.a`, `.o`) is a path relative to the module directory; `link "z"` (no extension or `/`) links the library -lz. `xo bind --c <header>` runs the binding step alone. See core 4.4 and decision 0116.

## Example

```xo
use c "missing.h" link "missing.c"

fn main(os: Os) uses ffi {
  os.stdio.println("hi")
}
```

## Fix

```xo
use c "zlib.h" link "z"

fn main(os: Os) uses ffi {
  os.stdio.println(zlib.zlib_version())
}
```

Run `xo explain XO1104` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).