libc.info: tolower

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`tolower'--translate characters to lower case

*Synopsis*
     #include <ctype.h>
     int tolower(int C);
     int _tolower(int C);
   *Description*
`tolower' is a macro which converts upper-case characters to lower
case, leaving all other characters unchanged.  It is only defined when
C is an integer in the range `EOF' to `255'.
   You can use a compiled subroutine instead of the macro definition by
undefining this macro using ``#undef tolower''.
   `_tolower' performs the same conversion as `tolower', but should
only be used when C is known to be an uppercase character (`A'-`Z').
*Returns*
`tolower' returns the lower-case equivalent of C when it is a character
between `A' and `Z', and C otherwise.
   `_tolower' returns the lower-case equivalent of C when it is a
character between `A' and `Z'.  If C is not one of these characters,
the behaviour of `_tolower' is undefined.
*Portability*
`tolower' is ANSI C.  `_tolower' is not recommended for portable
programs.
   No supporting OS subroutines are required.