toupper(3)		    Library Functions Manual		      toupper(3)

NAME
     toupper, tolower, toupper_l, tolower_l - convert uppercase or lowercase

LIBRARY
     Standard C library (libc, -lc)

SYNOPSIS
     #include <ctype.h>

     int toupper(int c);
     int tolower(int c);

     int toupper_l(int c, locale_t locale);
     int tolower_l(int c, locale_t locale);

 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

     toupper_l(), tolower_l():
	 Since glibc 2.10:
	     _XOPEN_SOURCE >= 700
	 Before glibc 2.10:
	     _GNU_SOURCE

DESCRIPTION
     These functions convert lowercase letters to uppercase, and vice versa.

     If	 c is a lowercase letter, toupper() returns its uppercase equivalent, if
     an uppercase representation exists in the current	locale.	  Otherwise,  it
     returns  c.   The toupper_l() function performs the same task, but uses the
     locale referred to by the locale handle locale.

     If c is an uppercase letter, tolower() returns its lowercase equivalent, if
     a lowercase representation exists in the current locale.  Otherwise, it re-
     turns c.  The tolower_l() function performs the same task, but uses the lo-
     cale referred to by the locale handle locale.

     If c is neither an unsigned char value nor EOF, the behavior of these func-
     tions is undefined.

     The behavior of toupper_l() and tolower_l() is undefined if locale	 is  the
     special locale object LC_GLOBAL_LOCALE (see duplocale(3)) or is not a valid
     locale object handle.

RETURN VALUE
     The  value returned is that of the converted letter, or c if the conversion
     was not possible.

ATTRIBUTES
     For an explanation of the terms used in this  section,  see  attributes(7).
     allbox; lbx lb lb l l l.  Interface Attribute Value T{ toupper(),
     tolower(), toupper_l(), tolower_l() T}   Thread safety  MT-Safe

STANDARDS
     toupper()
     tolower()
	    C11, POSIX.1-2008.

     toupper_l()
     tolower_l()
	    POSIX.1-2008.

HISTORY
     toupper()
     tolower()
	    C89, 4.3BSD, POSIX.1-2001.

     toupper_l()
     tolower_l()
	    POSIX.1-2008.

NOTES
     The standards require that the argument c for these functions is either EOF
     or	 a  value that is representable in the type unsigned char.  If the argu-
     ment c is of type char, it must be cast to unsigned char, as in the follow-
     ing example:

	 char c;
	 ...
	 res = toupper((unsigned char) c);

     This is necessary because char may be the equivalent signed char, in  which
     case a byte where the top bit is set would be sign extended when converting
     to int, yielding a value that is outside the range of unsigned char.

     The  details of what constitutes an uppercase or lowercase letter depend on
     the locale.  For example, the default "C" locale does not	know  about  um-
     lauts, so no conversion is done for them.

     In	 some  non-English  locales,  there are lowercase letters with no corre-
     sponding uppercase equivalent; the German sharp s is one example.

SEE ALSO
     isalpha(3), newlocale(3), setlocale(3),  towlower(3),  towupper(3),  uselo-
     cale(3), locale(7)

Linux man-pages 6.18		   2026-02-08			      toupper(3)