Function callback_logger_handler 

pub unsafe extern "C" fn callback_logger_handler(
    componentEnvironment: fmi2ComponentEnvironment,
    instanceName: fmi2String,
    status: fmi2Status,
    category: fmi2String,
    message: fmi2String,
    ...
)

This function is implemented in logger.c Note: This can be re-implemented in pure Rust once the c_variadics feature stabilizes. See: https://doc.rust-lang.org/beta/unstable-book/language-features/c-variadic.html