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

       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 │
       └─────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

       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 unsignedchar 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.

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

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

       Standard C library (libc, -lc)

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

       The standards require that the argument c  for  these  functions  is  either  EOF  or  a  value  that  is
       representable  in the type unsignedchar.  If the argument c is of type char, it must be cast to unsignedchar, as in the following example:

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

       This is necessary because char may be the equivalent signedchar, 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
       unsignedchar.

       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.

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

isalpha(3), newlocale(3), setlocale(3), towlower(3), towupper(3), uselocale(3), locale(7)

Linux man-pages 6.9.1                              2024-05-02                                         toupper(3)

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

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

#include<ctype.h>inttoupper(intc);inttolower(intc);inttoupper_l(intc,locale_tlocale);inttolower_l(intc,locale_tlocale);

   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