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TOWLOWER(3) Library Functions Manual TOWLOWER(3)

towlower, towlower_l, towupper, towupper_l
wide-character case letter conversion utilities

#include <wctype.h>
towlower(wint_t wc);
towlower_l(wint_t wc, locale_t locale);
towupper(wint_t wc);
towupper_l(wint_t wc, locale_t locale);

The towlower() and towlower_l() functions convert an upper-case wide character to the corresponding lower-case letter. The towupper() and towupper_l() functions convert a lower-case wide character to the corresponding upper-case letter.
The functions towlower_l() and towupper_l() use the specified locale, whereas towlower() and towupper() use the thread-specific locale set with uselocale(3), falling back to the global locale set with setlocale(3).

These functions return the corresponding character, if any. Otherwise, wc is returned unchanged.

iswlower(3), tolower(3), toupper(3), towctrans(3)

The functions towlower() and towupper() conform to ISO/IEC 9899/AMD1:1995 (“ISO C90, Amendment 1”) and ISO/IEC 9899:1999 (“ISO C99”), and towlower_l() and towupper_l() to IEEE Std 1003.1-2008 (“POSIX.1”).

The functions towlower() and towupper() have been available since OpenBSD 3.8, and towlower_l() and towupper_l() since OpenBSD 6.2.
September 5, 2017 OpenBSD-current