Boost C++ Libraries

A wrapper more aligned with modern C++ practice would use an exception, rather than an errorcode, to communicate failure to its caller. This is straightforward to code in terms of write_ec():

void write( AsyncAPI & api, std::string const& data) {
    AsyncAPI::errorcode ec = write_ec( api, data);
    if ( ec) {
        throw make_exception("write", ec);
    }
}

The point is that since each fiber has its own stack, you need not repeat messy boilerplate: normal encapsulation works.