toupper, tolower − convert letter to upper or lower case
int tolower(int c);
toupper() converts the letter c to upper case, if possible.
tolower() converts the letter c to lower case, if possible.
If c is not an unsigned char value, or EOF, the behavior of these functions is undefined.
The value returned is that of the converted letter, or c if the conversion was not possible.
For an explanation of the terms used in this section, see attributes(7).
C89, C99, 4.3BSD.
The details of what constitutes an uppercase or lowercase letter depend on the current locale. For example, the default "C" locale does not know about umlauts, so no conversion is done for them.
In some non-English locales, there are lowercase letters with no corresponding uppercase equivalent; the German sharp s is one example.
isalpha(3), setlocale(3), towlower(3), towupper(3), locale(7)
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