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TOLOWER(3) Library Functions Manual TOLOWER(3)

tolower, tolower_l, _tolower
upper case to lower case letter conversion

#include <ctype.h>
int
tolower(int c);
int
tolower_l(int c, locale_t locale);
int
_tolower(int c);

The tolower() and tolower_l() functions convert an upper-case letter to the corresponding lower-case letter. The _tolower() function is identical to tolower() except that c must be an upper-case letter.
OpenBSD always uses the C locale for these functions, ignoring the global locale, the thread-specific locale, and the locale argument.

If the argument to the tolower() or tolower_l() function is an upper-case letter, the corresponding lower-case letter is returned if there is one; otherwise the argument is returned unchanged. If the argument to the _tolower() function is an upper-case letter, the corresponding lower-case letter is returned; otherwise the output is undefined.

On systems supporting non-ASCII single-byte character encodings, the results of tolower() and _tolower() may depend on the LC_CTYPE locale(1).

isalnum(3), isalpha(3), isascii(3), isblank(3), iscntrl(3), isdigit(3), isgraph(3), islower(3), isprint(3), ispunct(3), isspace(3), isupper(3), isxdigit(3), stdio(3), toascii(3), toupper(3), towlower(3), ascii(7)

The tolower() and _tolower() functions conform to ANSI X3.159-1989 (“ANSI C89”), and tolower_l() to IEEE Std 1003.1-2008 (“POSIX.1”).

The tolower() function first appeared in Version 7 AT&T UNIX and acquired the current semantics in AT&T System III UNIX, where _tolower() first appeared.
The tolower_l() function has been available since OpenBSD 6.2.

The argument c must be EOF or representable as an unsigned char; otherwise, the result is undefined.
September 5, 2017 OpenBSD-current