Macro with_c_str 

macro_rules! with_c_str {
    ($ptr:ident) => { ... };
}

Read a nullable C string argument as &str, classifying both ways it can fail.

Expands to a Result, so the usual form is with_c_str!(path)? inside a catch closure. The null message is built from the argument’s own name — with_c_str!(resource_id) reports "resource_id is null", which is the wording the family already uses.

Use this where the entry point has no out-parameter, so that classifying a null argument inside the closure is indistinguishable from classifying it before. Where there is an out-parameter, check for null first and use ffi::c_str_utf8 here instead — see export_bytes_getter!.

Safety

$ptr must be null or point to a NUL-terminated string. This expands to an unsafe read, so it has to appear in an unsafe context.

unsafe fn name_length(path: *const c_char) -> Result<usize, FfiError> {
    let path = unsafe { unparser_shared::with_c_str!(path) }?;
    Ok(path.len())
}

let path = CString::new("a.txt").unwrap();
assert_eq!(unsafe { name_length(path.as_ptr()) }.unwrap(), 5);
assert_eq!(
    unsafe { name_length(std::ptr::null()) }.unwrap_err().1,
    "path is null"
);