Ctype.h

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C++ Standard Library headers
C Standard Library headers
assert.h
ctype.h
errno.h
float.h
limits.h
locale.h
math.h
setjmp.h
signal.h
stdarg.h
stddef.h
stdio.h
string.h
stdlib.h
time.h

The header ctype.h in the ANSI C Standard Library for the C programming language contains declarations for character classification functions.

Contents 1 History 2 Implementation 3 The contents of <ctype.h> 4 Incorrect Usage 5 External link

[edit] History

Early toolsmiths writing in C under Unix began developing idioms at a rapid rate to classify characters into different types. For example, the following test identifies a letter:

if ('A' <= c && c <= 'Z' || 'a' <= c && c <= 'z')

which gives a correct result if the character set is ASCII. Nevertheless, this idiom does not work for other character sets such as EBCDIC.

Pretty soon, programs became thick with tests such as the one above, or worse, tests almost like the one above. A programmer can write the same idiom several different ways, which slows comprehension and increases the chance for errors.

Before long, the idioms were replaced by the functions in <ctype.h>.

[edit] Implementation

Unlike the above example, the character classification routines are not written as comparison tests. In most C libraries, they are written as static table lookups instead of macros or functions.

For example, an array of 256 eight-bit integers, arranged as bitfields, is created, where each bit corresponds to a particular property of the character, e.g., isdigit, isalpha. If the lowest-order bit of the integers corresponds to the isdigit property, the code could be written thusly:

#define isdigit(x) (TABLE[x] & 1)

Early versions of Linux used a potentially faulty method similar to the first code sample:

#define isdigit(x) ((x) >= '0' && (x) <= '9')

This can cause problems if x is changed by being evaluated---for instance, if one calls isdigit(x++) or isdigit(run_some_program()). It would not be immediately evident that the argument to isdigit is being evaluated twice. For this reason, the table-based approach is generally used.

The difference between these two methods became a point of interest during the SCO v. IBM case. (See Linus Torvalds's kernel mailing list post below.)

[edit] The contents of <ctype.h>

The <ctype.h> contains prototypes for a dozen character classification functions. All of these functions except isdigit are locale-specific; their behavior may change if the locale changes.

Tests	In the form int isfunc(int); Return a nonzero number for true and zero for false.
isalnum	test for alphanumeric character
isalpha	test for alphabetic character
isblank	test for blank character (new in C99)
iscntrl	test for control character
isdigit	test for digit. Not locale-specific.
isgraph	test for graphic character
islower	test for lowercase character
isprint	test for printable character
ispunct	test for punctuation character
isspace	test for space character
isupper	test for uppercase character
isxdigit	test for hexadecimal digit
Character conversion	in the form int tofunc(int); Return the converted character unless it is not alphabetic.
tolower	convert character to lowercase
toupper	convert character to uppercase

The Single Unix Specification Version 3 adds functions similar to the above:

isascii	return if the parameter is between 0 and 127
toascii	converts a character to ASCII

[edit] Incorrect Usage

The standard clearly states (§7.4-1):

In all cases the argument is an int, the value of which shall be representable as an unsigned char or shall equal the value of the macro EOF. If the argument has any other value, the behavior is undefined.

Unfortunately many programmers forget that a char type may be either signed or unsigned, depending on the implementation. If the char types are signed, the implicit conversion from char to int may generate negative values, resulting in undefined behavior. That usually means that if the argument is used as an index to a lookup table, it will access an area outside of the correct table, and may even crash the program.

The correct way to use char arguments is to first cast them to unsigned char.

[edit] External link

• Explanation of the table method by Linus Torvalds