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This section explains the library functions for performing conversions
such as case mappings on characters. For example, toupper
converts any character to upper case if possible. If the character
can't be converted, toupper returns it unchanged.
These functions take one argument of type int, which is the
character to convert, and return the converted character as an
int. If the conversion is not applicable to the argument given,
the argument is returned unchanged.
Compatibility Note: In pre-ISO C dialects, instead of
returning the argument unchanged, these functions may fail when the
argument is not suitable for the conversion. Thus for portability, you
may need to write islower(c) ? toupper(c) : c rather than just
toupper(c).
These functions are declared in the header file ctype.h.
If c is an upper-case letter,
tolowerreturns the corresponding lower-case letter. If c is not an upper-case letter, c is returned unchanged.
If c is a lower-case letter,
toupperreturns the corresponding upper-case letter. Otherwise c is returned unchanged.
This function converts c to a 7-bit
unsigned charvalue that fits into the US/UK ASCII character set, by clearing the high-order bits. This function is a BSD extension and is also an SVID extension.
This is identical to
tolower, and is provided for compatibility with the SVID. See SVID.