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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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toupper(3) Library Functions Manual toupper(3)
toupper, tolower, toupper_l, tolower_l - convert uppercase or
lowercase
Standard C library (libc, -lc)
#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
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(), │ Thread safety │ MT-Safe │
│ tolower_l() │ │ │
└──────────────────────────────────────┴───────────────┴─────────┘
toupper()
tolower()
C11, POSIX.1-2008.
toupper_l()
tolower_l()
POSIX.1-2008.
toupper()
tolower()
C89, 4.3BSD, POSIX.1-2001.
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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 toupper(3)
Pages that refer to this page: duplocale(3), isalpha(3), toascii(3), towlower(3), towupper(3), wctrans(3), locale(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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