Packet generator in C++ with memory and latency constraints

I have to send data over a network (for some definition of network) stream, consisting of:

1. A preamble;
2. a payload; and
3. a checksum.

I have two constraints: little memory available and need to keep latency low, so buffering the packet or payload is out of the question.

Given a free choice of implementation language I would have used something with co-routine support and write a pair of functions, let's call them transmitPacket() and generatePayload(), of which the former:

1. Sends the preamble;
2. reads the payload one byte at a time from generatePayload();
3. sends the payload byte;
4. updates the checksum;
5. when the payload is done, sends the checksum.

This would have been trivial and expressed the intent clearly to future maintainers.

Sadly, I don't have a free choice of language. I am stuck with C++11.

I cannot think of a clear and clean way of approaching this problem given the aforementioned constraints, so if you have experience with similar problems in the past, any implementation suggestions would be welcome.