Function splinter_append 

pub unsafe extern "C" fn splinter_append(
    key: *const c_char,
    data: *const c_void,
    data_len: usize,
    new_len: *mut usize,
) -> c_int

@brief Appends data to an existing key’s value in-place. @param key The null-terminated key string. @param data Pointer to the data to append. @param data_len Number of bytes to append. @param new_len Output: set to the new total value length on success. May be NULL. @return 0 on success, -1 if key not found or overflow, -2 if args invalid.