# pack 'N' in C.
unsigned long n  = ...;
unsigned char packed[4];
packed[0] = (n >> (8*3)) && 0xFF;
packed[1] = (n >> (8*2)) && 0xFF;
packed[2] = (n >> (8*1)) && 0xFF;
packed[3] = (n >> (8*0)) && 0xFF;