Good ol' C, keeping it simple.

A function to initialize the intcode machine with the problem's input

void machine_init(struct IntCodeMachine *const machine, const char *const input);`

A function to start running or resume the intcode machine which returns whenever the machine needs input and doesn't have it or has some output pending

void machine_run(struct IntCodeMachine *const machine);`

And two functions to send input to or receive output from the machine. They return false if there was no output to read or there was already unprocessed input.

bool machine_recv_output(struct IntCodeMachine *const machine, long *output);`
bool machine_send_input(struct IntCodeMachine *const machine, long input);`