Function cfm_keystore_sign 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn cfm_keystore_sign(
    ks: *mut FFIKeystore,
    module_id: *const c_char,
    app_id: *const c_char,
    key_id: *const c_char,
    algorithm: *const c_char,
    message: *const u8,
    message_len: u32,
    out: *mut *mut u8,
    out_len: *mut u32,
) -> u32

Out-buffer convention for byte results: the caller passes a destination pointer plus its capacity; on success out receives a freshly heap-allocated buffer (freed with CFM_FREE semantics by the embedding engine) and out_len its length.