Type Alias CleanupCallback 

pub type CleanupCallback = unsafe extern "C" fn(pamh: *mut pam_handle, data: *mut c_void, pam_end_status: c_int);

Alias for the callback to pam_set_data.

use libpam_sys::aliases::CleanupCallback;
use libpam_sys::pam_set_data;
fn cleanup() -> CleanupCallback {
    // ...
}
let name = CString::new("name").unwrap();
unsafe {
    pam_set_data(handle, name.as_ptr().cast_mut(), data_ptr.cast(), cleanup());
}