is... Functions
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The routines in the is family test a given integer value. They
return a nonzero value if the integer satisfies the test condition
and 0 if it does not. An ASCII-character-set environment is
assumed. These routines are implemented both as functions and as
macros.
The isascii routine produces meaningful results for all integer
values. However, the remaining routines produce a defined result
only for integer values corresponding to the ASCII character set
(that is, only where isascii holds true) or for the non-ASCII
value EOF (defined in STDIO.H).
Return Value
All of these functions return a nonzero value if the tested
character is in the right category and 0 if not.
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