Toupper Tolower character translation #include <ctype.h> toupper int c tolower int c _toupper int c _tolower int c The functions toupper and tolower have as their domain the integers in the range -1 to 255. Toupper converts lower-case to upper-case, and tolower converts upper-case to lower-case. All other arguments are returned unchanged. The macros _toupper and _tolower do the same things as the corresponding functions, but they have restricted domains and they are faster. The argument to _toupper must be lower-case, and the argument to _tolower must be upper-case. Arguments that are outside each macros domain, such as passing a lower-case to _tolower, yield garbage results.