The second and third arguments are the method used to acquire more
bytes from the input stream. It's a user supplied function, and one
argument. The interpreter will call this function once for each input
byte, until EOF is returned. "io2022_f" is a typedef to a function
pointer which returns an integer (the next byte to interpret or EOF)
and taks a void pointer as the sole argument. A very simple input
function could be:

int my_input (void *arg) { return fgetc ((FILE *)arg); }

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