Answer by Lundin for Trigraph translator

There is absolutely no reason why WRITE_BUFFER should be a macro and not a function.

Don't do exotic stuff like --argc; ++argv;. C programmers expect argument 0 to be the name of the executable. Similarly, argc % 2 is weird, why would you check for an even amount of arguments? Check for an exact amount of arguments, no more, no less.

Similarly, you should be able to keep all error handling out of the loops and just do it once. Keep it simple.

I'd expect the various valid trigraph sequences to be stored in some manner of table, rather than a big switch. For example, you could sacrifice 128 bytes data for a fast lookup table based on 7 bit ASCII:

const char trigraph [128] = {  ['#'] = '=',  ['['] = '(',  ...};char in  = ... // input from filechar out = trigraph[in];if(out != 0) // was it a candidate for trigraph replacement?{    // printf("??%c", out); etc