toupper, tolower, toupper_l, tolower_l — convert uppercase or lowercase


#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);
[Note] Note
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:


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 returns c. The tolower_l() function performs the same task, but uses the locale referred to by the locale handle locale.

If c is neither an unsigned char value nor EOF, the behavior of these functions 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.


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).

Interface Attribute Value
toupper(), tolower(), toupper_l(), tolower_l() Thread safety MT-Safe


toupper(), tolower(): C89, C99, 4.3BSD, POSIX.1-2001, POSIX.1-2008.

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


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 argument c is of type char, it must be cast to unsigned char, as in the following 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 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), newlocale(3), setlocale(3), towlower(3), towupper(3), uselocale(3), locale(7)


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