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@node Character Handling, String and Array Utilities, Memory, Top | |
@c %MENU% Character testing and conversion functions | |
@chapter Character Handling | |
Programs that work with characters and strings often need to classify a | |
character---is it alphabetic, is it a digit, is it whitespace, and so | |
on---and perform case conversion operations on characters. The | |
functions in the header file @file{ctype.h} are provided for this | |
purpose. | |
@pindex ctype.h | |
Since the choice of locale and character set can alter the | |
classifications of particular character codes, all of these functions | |
are affected by the current locale. (More precisely, they are affected | |
by the locale currently selected for character classification---the | |
@code{LC_CTYPE} category; see @ref{Locale Categories}.) | |
The @w{ISO C} standard specifies two different sets of functions. The | |
one set works on @code{char} type characters, the other one on | |
@code{wchar_t} wide characters (@pxref{Extended Char Intro}). | |
@menu | |
* Classification of Characters:: Testing whether characters are | |
letters, digits, punctuation, etc. | |
* Case Conversion:: Case mapping, and the like. | |
* Classification of Wide Characters:: Character class determination for | |
wide characters. | |
* Using Wide Char Classes:: Notes on using the wide character | |
classes. | |
* Wide Character Case Conversion:: Mapping of wide characters. | |
@end menu | |
@node Classification of Characters, Case Conversion, , Character Handling | |
@section Classification of Characters | |
@cindex character testing | |
@cindex classification of characters | |
@cindex predicates on characters | |
@cindex character predicates | |
This section explains the library functions for classifying characters. | |
For example, @code{isalpha} is the function to test for an alphabetic | |
character. It takes one argument, the character to test, and returns a | |
nonzero integer if the character is alphabetic, and zero otherwise. You | |
would use it like this: | |
@smallexample | |
if (isalpha (c)) | |
printf ("The character `%c' is alphabetic.\n", c); | |
@end smallexample | |
Each of the functions in this section tests for membership in a | |
particular class of characters; each has a name starting with @samp{is}. | |
Each of them takes one argument, which is a character to test, and | |
returns an @code{int} which is treated as a boolean value. The | |
character argument is passed as an @code{int}, and it may be the | |
constant value @code{EOF} instead of a real character. | |
The attributes of any given character can vary between locales. | |
@xref{Locales}, for more information on locales.@refill | |
These functions are declared in the header file @file{ctype.h}. | |
@pindex ctype.h | |
@cindex lower-case character | |
@comment ctype.h | |
@comment ISO | |
@deftypefun int islower (int @var{c}) | |
Returns true if @var{c} is a lower-case letter. The letter need not be | |
from the Latin alphabet, any alphabet representable is valid. | |
@end deftypefun | |
@cindex upper-case character | |
@comment ctype.h | |
@comment ISO | |
@deftypefun int isupper (int @var{c}) | |
Returns true if @var{c} is an upper-case letter. The letter need not be | |
from the Latin alphabet, any alphabet representable is valid. | |
@end deftypefun | |
@cindex alphabetic character | |
@comment ctype.h | |
@comment ISO | |
@deftypefun int isalpha (int @var{c}) | |
Returns true if @var{c} is an alphabetic character (a letter). If | |
@code{islower} or @code{isupper} is true of a character, then | |
@code{isalpha} is also true. | |
In some locales, there may be additional characters for which | |
@code{isalpha} is true---letters which are neither upper case nor lower | |
case. But in the standard @code{"C"} locale, there are no such | |
additional characters. | |
@end deftypefun | |
@cindex digit character | |
@cindex decimal digit character | |
@comment ctype.h | |
@comment ISO | |
@deftypefun int isdigit (int @var{c}) | |
Returns true if @var{c} is a decimal digit (@samp{0} through @samp{9}). | |
@end deftypefun | |
@cindex alphanumeric character | |
@comment ctype.h | |
@comment ISO | |
@deftypefun int isalnum (int @var{c}) | |
Returns true if @var{c} is an alphanumeric character (a letter or | |
number); in other words, if either @code{isalpha} or @code{isdigit} is | |
true of a character, then @code{isalnum} is also true. | |
@end deftypefun | |
@cindex hexadecimal digit character | |
@comment ctype.h | |
@comment ISO | |
@deftypefun int isxdigit (int @var{c}) | |
Returns true if @var{c} is a hexadecimal digit. | |
Hexadecimal digits include the normal decimal digits @samp{0} through | |
@samp{9} and the letters @samp{A} through @samp{F} and | |
@samp{a} through @samp{f}. | |
@end deftypefun | |
@cindex punctuation character | |
@comment ctype.h | |
@comment ISO | |
@deftypefun int ispunct (int @var{c}) | |
Returns true if @var{c} is a punctuation character. | |
This means any printing character that is not alphanumeric or a space | |
character. | |
@end deftypefun | |
@cindex whitespace character | |
@comment ctype.h | |
@comment ISO | |
@deftypefun int isspace (int @var{c}) | |
Returns true if @var{c} is a @dfn{whitespace} character. In the standard | |
@code{"C"} locale, @code{isspace} returns true for only the standard | |
whitespace characters: | |
@table @code | |
@item ' ' | |
space | |
@item '\f' | |
formfeed | |
@item '\n' | |
newline | |
@item '\r' | |
carriage return | |
@item '\t' | |
horizontal tab | |
@item '\v' | |
vertical tab | |
@end table | |
@end deftypefun | |
@cindex blank character | |
@comment ctype.h | |
@comment GNU | |
@deftypefun int isblank (int @var{c}) | |
Returns true if @var{c} is a blank character; that is, a space or a tab. | |
This function is a GNU extension. | |
@end deftypefun | |
@cindex graphic character | |
@comment ctype.h | |
@comment ISO | |
@deftypefun int isgraph (int @var{c}) | |
Returns true if @var{c} is a graphic character; that is, a character | |
that has a glyph associated with it. The whitespace characters are not | |
considered graphic. | |
@end deftypefun | |
@cindex printing character | |
@comment ctype.h | |
@comment ISO | |
@deftypefun int isprint (int @var{c}) | |
Returns true if @var{c} is a printing character. Printing characters | |
include all the graphic characters, plus the space (@samp{ }) character. | |
@end deftypefun | |
@cindex control character | |
@comment ctype.h | |
@comment ISO | |
@deftypefun int iscntrl (int @var{c}) | |
Returns true if @var{c} is a control character (that is, a character that | |
is not a printing character). | |
@end deftypefun | |
@cindex ASCII character | |
@comment ctype.h | |
@comment SVID, BSD | |
@deftypefun int isascii (int @var{c}) | |
Returns true if @var{c} is a 7-bit @code{unsigned char} value that fits | |
into the US/UK ASCII character set. This function is a BSD extension | |
and is also an SVID extension. | |
@end deftypefun | |
@node Case Conversion, Classification of Wide Characters, Classification of Characters, Character Handling | |
@section Case Conversion | |
@cindex character case conversion | |
@cindex case conversion of characters | |
@cindex converting case of characters | |
This section explains the library functions for performing conversions | |
such as case mappings on characters. For example, @code{toupper} | |
converts any character to upper case if possible. If the character | |
can't be converted, @code{toupper} returns it unchanged. | |
These functions take one argument of type @code{int}, which is the | |
character to convert, and return the converted character as an | |
@code{int}. If the conversion is not applicable to the argument given, | |
the argument is returned unchanged. | |
@strong{Compatibility Note:} In pre-@w{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 @code{islower(c) ? toupper(c) : c} rather than just | |
@code{toupper(c)}. | |
These functions are declared in the header file @file{ctype.h}. | |
@pindex ctype.h | |
@comment ctype.h | |
@comment ISO | |
@deftypefun int tolower (int @var{c}) | |
If @var{c} is an upper-case letter, @code{tolower} returns the corresponding | |
lower-case letter. If @var{c} is not an upper-case letter, | |
@var{c} is returned unchanged. | |
@end deftypefun | |
@comment ctype.h | |
@comment ISO | |
@deftypefun int toupper (int @var{c}) | |
If @var{c} is a lower-case letter, @code{toupper} returns the corresponding | |
upper-case letter. Otherwise @var{c} is returned unchanged. | |
@end deftypefun | |
@comment ctype.h | |
@comment SVID, BSD | |
@deftypefun int toascii (int @var{c}) | |
This function converts @var{c} to a 7-bit @code{unsigned char} value | |
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. | |
@end deftypefun | |
@comment ctype.h | |
@comment SVID | |
@deftypefun int _tolower (int @var{c}) | |
This is identical to @code{tolower}, and is provided for compatibility | |
with the SVID. @xref{SVID}.@refill | |
@end deftypefun | |
@comment ctype.h | |
@comment SVID | |
@deftypefun int _toupper (int @var{c}) | |
This is identical to @code{toupper}, and is provided for compatibility | |
with the SVID. | |
@end deftypefun | |
@node Classification of Wide Characters, Using Wide Char Classes, Case Conversion, Character Handling | |
@section Character class determination for wide characters | |
@w{Amendment 1} to @w{ISO C90} defines functions to classify wide | |
characters. Although the original @w{ISO C90} standard already defined | |
the type @code{wchar_t}, no functions operating on them were defined. | |
The general design of the classification functions for wide characters | |
is more general. It allows extensions to the set of available | |
classifications, beyond those which are always available. The POSIX | |
standard specifies how extensions can be made, and this is already | |
implemented in the GNU C library implementation of the @code{localedef} | |
program. | |
The character class functions are normally implemented with bitsets, | |
with a bitset per character. For a given character, the appropriate | |
bitset is read from a table and a test is performed as to whether a | |
certain bit is set. Which bit is tested for is determined by the | |
class. | |
For the wide character classification functions this is made visible. | |
There is a type classification type defined, a function to retrieve this | |
value for a given class, and a function to test whether a given | |
character is in this class, using the classification value. On top of | |
this the normal character classification functions as used for | |
@code{char} objects can be defined. | |
@comment wctype.h | |
@comment ISO | |
@deftp {Data type} wctype_t | |
The @code{wctype_t} can hold a value which represents a character class. | |
The only defined way to generate such a value is by using the | |
@code{wctype} function. | |
@pindex wctype.h | |
This type is defined in @file{wctype.h}. | |
@end deftp | |
@comment wctype.h | |
@comment ISO | |
@deftypefun wctype_t wctype (const char *@var{property}) | |
The @code{wctype} returns a value representing a class of wide | |
characters which is identified by the string @var{property}. Beside | |
some standard properties each locale can define its own ones. In case | |
no property with the given name is known for the current locale | |
selected for the @code{LC_CTYPE} category, the function returns zero. | |
@noindent | |
The properties known in every locale are: | |
@multitable @columnfractions .25 .25 .25 .25 | |
@item | |
@code{"alnum"} @tab @code{"alpha"} @tab @code{"cntrl"} @tab @code{"digit"} | |
@item | |
@code{"graph"} @tab @code{"lower"} @tab @code{"print"} @tab @code{"punct"} | |
@item | |
@code{"space"} @tab @code{"upper"} @tab @code{"xdigit"} | |
@end multitable | |
@pindex wctype.h | |
This function is declared in @file{wctype.h}. | |
@end deftypefun | |
To test the membership of a character to one of the non-standard classes | |
the @w{ISO C} standard defines a completely new function. | |
@comment wctype.h | |
@comment ISO | |
@deftypefun int iswctype (wint_t @var{wc}, wctype_t @var{desc}) | |
This function returns a nonzero value if @var{wc} is in the character | |
class specified by @var{desc}. @var{desc} must previously be returned | |
by a successful call to @code{wctype}. | |
@pindex wctype.h | |
This function is declared in @file{wctype.h}. | |
@end deftypefun | |
To make it easier to use the commonly-used classification functions, | |
they are defined in the C library. There is no need to use | |
@code{wctype} if the property string is one of the known character | |
classes. In some situations it is desirable to construct the property | |
strings, and then it is important that @code{wctype} can also handle the | |
standard classes. | |
@cindex alphanumeric character | |
@comment wctype.h | |
@comment ISO | |
@deftypefun int iswalnum (wint_t @var{wc}) | |
This function returns a nonzero value if @var{wc} is an alphanumeric | |
character (a letter or number); in other words, if either @code{iswalpha} | |
or @code{iswdigit} is true of a character, then @code{iswalnum} is also | |
true. | |
@noindent | |
This function can be implemented using | |
@smallexample | |
iswctype (wc, wctype ("alnum")) | |
@end smallexample | |
@pindex wctype.h | |
It is declared in @file{wctype.h}. | |
@end deftypefun | |
@cindex alphabetic character | |
@comment wctype.h | |
@comment ISO | |
@deftypefun int iswalpha (wint_t @var{wc}) | |
Returns true if @var{wc} is an alphabetic character (a letter). If | |
@code{iswlower} or @code{iswupper} is true of a character, then | |
@code{iswalpha} is also true. | |
In some locales, there may be additional characters for which | |
@code{iswalpha} is true---letters which are neither upper case nor lower | |
case. But in the standard @code{"C"} locale, there are no such | |
additional characters. | |
@noindent | |
This function can be implemented using | |
@smallexample | |
iswctype (wc, wctype ("alpha")) | |
@end smallexample | |
@pindex wctype.h | |
It is declared in @file{wctype.h}. | |
@end deftypefun | |
@cindex control character | |
@comment wctype.h | |
@comment ISO | |
@deftypefun int iswcntrl (wint_t @var{wc}) | |
Returns true if @var{wc} is a control character (that is, a character that | |
is not a printing character). | |
@noindent | |
This function can be implemented using | |
@smallexample | |
iswctype (wc, wctype ("cntrl")) | |
@end smallexample | |
@pindex wctype.h | |
It is declared in @file{wctype.h}. | |
@end deftypefun | |
@cindex digit character | |
@comment wctype.h | |
@comment ISO | |
@deftypefun int iswdigit (wint_t @var{wc}) | |
Returns true if @var{wc} is a digit (e.g., @samp{0} through @samp{9}). | |
Please note that this function does not only return a nonzero value for | |
@emph{decimal} digits, but for all kinds of digits. A consequence is | |
that code like the following will @strong{not} work unconditionally for | |
wide characters: | |
@smallexample | |
n = 0; | |
while (iswdigit (*wc)) | |
@{ | |
n *= 10; | |
n += *wc++ - L'0'; | |
@} | |
@end smallexample | |
@noindent | |
This function can be implemented using | |
@smallexample | |
iswctype (wc, wctype ("digit")) | |
@end smallexample | |
@pindex wctype.h | |
It is declared in @file{wctype.h}. | |
@end deftypefun | |
@cindex graphic character | |
@comment wctype.h | |
@comment ISO | |
@deftypefun int iswgraph (wint_t @var{wc}) | |
Returns true if @var{wc} is a graphic character; that is, a character | |
that has a glyph associated with it. The whitespace characters are not | |
considered graphic. | |
@noindent | |
This function can be implemented using | |
@smallexample | |
iswctype (wc, wctype ("graph")) | |
@end smallexample | |
@pindex wctype.h | |
It is declared in @file{wctype.h}. | |
@end deftypefun | |
@cindex lower-case character | |
@comment ctype.h | |
@comment ISO | |
@deftypefun int iswlower (wint_t @var{wc}) | |
Returns true if @var{wc} is a lower-case letter. The letter need not be | |
from the Latin alphabet, any alphabet representable is valid. | |
@noindent | |
This function can be implemented using | |
@smallexample | |
iswctype (wc, wctype ("lower")) | |
@end smallexample | |
@pindex wctype.h | |
It is declared in @file{wctype.h}. | |
@end deftypefun | |
@cindex printing character | |
@comment wctype.h | |
@comment ISO | |
@deftypefun int iswprint (wint_t @var{wc}) | |
Returns true if @var{wc} is a printing character. Printing characters | |
include all the graphic characters, plus the space (@samp{ }) character. | |
@noindent | |
This function can be implemented using | |
@smallexample | |
iswctype (wc, wctype ("print")) | |
@end smallexample | |
@pindex wctype.h | |
It is declared in @file{wctype.h}. | |
@end deftypefun | |
@cindex punctuation character | |
@comment wctype.h | |
@comment ISO | |
@deftypefun int iswpunct (wint_t @var{wc}) | |
Returns true if @var{wc} is a punctuation character. | |
This means any printing character that is not alphanumeric or a space | |
character. | |
@noindent | |
This function can be implemented using | |
@smallexample | |
iswctype (wc, wctype ("punct")) | |
@end smallexample | |
@pindex wctype.h | |
It is declared in @file{wctype.h}. | |
@end deftypefun | |
@cindex whitespace character | |
@comment wctype.h | |
@comment ISO | |
@deftypefun int iswspace (wint_t @var{wc}) | |
Returns true if @var{wc} is a @dfn{whitespace} character. In the standard | |
@code{"C"} locale, @code{iswspace} returns true for only the standard | |
whitespace characters: | |
@table @code | |
@item L' ' | |
space | |
@item L'\f' | |
formfeed | |
@item L'\n' | |
newline | |
@item L'\r' | |
carriage return | |
@item L'\t' | |
horizontal tab | |
@item L'\v' | |
vertical tab | |
@end table | |
@noindent | |
This function can be implemented using | |
@smallexample | |
iswctype (wc, wctype ("space")) | |
@end smallexample | |
@pindex wctype.h | |
It is declared in @file{wctype.h}. | |
@end deftypefun | |
@cindex upper-case character | |
@comment wctype.h | |
@comment ISO | |
@deftypefun int iswupper (wint_t @var{wc}) | |
Returns true if @var{wc} is an upper-case letter. The letter need not be | |
from the Latin alphabet, any alphabet representable is valid. | |
@noindent | |
This function can be implemented using | |
@smallexample | |
iswctype (wc, wctype ("upper")) | |
@end smallexample | |
@pindex wctype.h | |
It is declared in @file{wctype.h}. | |
@end deftypefun | |
@cindex hexadecimal digit character | |
@comment wctype.h | |
@comment ISO | |
@deftypefun int iswxdigit (wint_t @var{wc}) | |
Returns true if @var{wc} is a hexadecimal digit. | |
Hexadecimal digits include the normal decimal digits @samp{0} through | |
@samp{9} and the letters @samp{A} through @samp{F} and | |
@samp{a} through @samp{f}. | |
@noindent | |
This function can be implemented using | |
@smallexample | |
iswctype (wc, wctype ("xdigit")) | |
@end smallexample | |
@pindex wctype.h | |
It is declared in @file{wctype.h}. | |
@end deftypefun | |
The GNU C library also provides a function which is not defined in the | |
@w{ISO C} standard but which is available as a version for single byte | |
characters as well. | |
@cindex blank character | |
@comment wctype.h | |
@comment GNU | |
@deftypefun int iswblank (wint_t @var{wc}) | |
Returns true if @var{wc} is a blank character; that is, a space or a tab. | |
This function is a GNU extension. It is declared in @file{wchar.h}. | |
@end deftypefun | |
@node Using Wide Char Classes, Wide Character Case Conversion, Classification of Wide Characters, Character Handling | |
@section Notes on using the wide character classes | |
The first note is probably not astonishing but still occasionally a | |
cause of problems. The @code{isw@var{XXX}} functions can be implemented | |
using macros and in fact, the GNU C library does this. They are still | |
available as real functions but when the @file{wctype.h} header is | |
included the macros will be used. This is the same as the | |
@code{char} type versions of these functions. | |
The second note covers something new. It can be best illustrated by a | |
(real-world) example. The first piece of code is an excerpt from the | |
original code. It is truncated a bit but the intention should be clear. | |
@smallexample | |
int | |
is_in_class (int c, const char *class) | |
@{ | |
if (strcmp (class, "alnum") == 0) | |
return isalnum (c); | |
if (strcmp (class, "alpha") == 0) | |
return isalpha (c); | |
if (strcmp (class, "cntrl") == 0) | |
return iscntrl (c); | |
@dots{} | |
return 0; | |
@} | |
@end smallexample | |
Now, with the @code{wctype} and @code{iswctype} you can avoid the | |
@code{if} cascades, but rewriting the code as follows is wrong: | |
@smallexample | |
int | |
is_in_class (int c, const char *class) | |
@{ | |
wctype_t desc = wctype (class); | |
return desc ? iswctype ((wint_t) c, desc) : 0; | |
@} | |
@end smallexample | |
The problem is that it is not guaranteed that the wide character | |
representation of a single-byte character can be found using casting. | |
In fact, usually this fails miserably. The correct solution to this | |
problem is to write the code as follows: | |
@smallexample | |
int | |
is_in_class (int c, const char *class) | |
@{ | |
wctype_t desc = wctype (class); | |
return desc ? iswctype (btowc (c), desc) : 0; | |
@} | |
@end smallexample | |
@xref{Converting a Character}, for more information on @code{btowc}. | |
Note that this change probably does not improve the performance | |
of the program a lot since the @code{wctype} function still has to make | |
the string comparisons. It gets really interesting if the | |
@code{is_in_class} function is called more than once for the | |
same class name. In this case the variable @var{desc} could be computed | |
once and reused for all the calls. Therefore the above form of the | |
function is probably not the final one. | |
@node Wide Character Case Conversion, , Using Wide Char Classes, Character Handling | |
@section Mapping of wide characters. | |
The classification functions are also generalized by the @w{ISO C} | |
standard. Instead of just allowing the two standard mappings, a | |
locale can contain others. Again, the @code{localedef} program | |
already supports generating such locale data files. | |
@comment wctype.h | |
@comment ISO | |
@deftp {Data Type} wctrans_t | |
This data type is defined as a scalar type which can hold a value | |
representing the locale-dependent character mapping. There is no way to | |
construct such a value apart from using the return value of the | |
@code{wctrans} function. | |
@pindex wctype.h | |
@noindent | |
This type is defined in @file{wctype.h}. | |
@end deftp | |
@comment wctype.h | |
@comment ISO | |
@deftypefun wctrans_t wctrans (const char *@var{property}) | |
The @code{wctrans} function has to be used to find out whether a named | |
mapping is defined in the current locale selected for the | |
@code{LC_CTYPE} category. If the returned value is non-zero, you can use | |
it afterwards in calls to @code{towctrans}. If the return value is | |
zero no such mapping is known in the current locale. | |
Beside locale-specific mappings there are two mappings which are | |
guaranteed to be available in every locale: | |
@multitable @columnfractions .5 .5 | |
@item | |
@code{"tolower"} @tab @code{"toupper"} | |
@end multitable | |
@pindex wctype.h | |
@noindent | |
These functions are declared in @file{wctype.h}. | |
@end deftypefun | |
@comment wctype.h | |
@comment ISO | |
@deftypefun wint_t towctrans (wint_t @var{wc}, wctrans_t @var{desc}) | |
@code{towctrans} maps the input character @var{wc} | |
according to the rules of the mapping for which @var{desc} is a | |
descriptor, and returns the value it finds. @var{desc} must be | |
obtained by a successful call to @code{wctrans}. | |
@pindex wctype.h | |
@noindent | |
This function is declared in @file{wctype.h}. | |
@end deftypefun | |
For the generally available mappings, the @w{ISO C} standard defines | |
convenient shortcuts so that it is not necessary to call @code{wctrans} | |
for them. | |
@comment wctype.h | |
@comment ISO | |
@deftypefun wint_t towlower (wint_t @var{wc}) | |
If @var{wc} is an upper-case letter, @code{towlower} returns the corresponding | |
lower-case letter. If @var{wc} is not an upper-case letter, | |
@var{wc} is returned unchanged. | |
@noindent | |
@code{towlower} can be implemented using | |
@smallexample | |
towctrans (wc, wctrans ("tolower")) | |
@end smallexample | |
@pindex wctype.h | |
@noindent | |
This function is declared in @file{wctype.h}. | |
@end deftypefun | |
@comment wctype.h | |
@comment ISO | |
@deftypefun wint_t towupper (wint_t @var{wc}) | |
If @var{wc} is a lower-case letter, @code{towupper} returns the corresponding | |
upper-case letter. Otherwise @var{wc} is returned unchanged. | |
@noindent | |
@code{towupper} can be implemented using | |
@smallexample | |
towctrans (wc, wctrans ("toupper")) | |
@end smallexample | |
@pindex wctype.h | |
@noindent | |
This function is declared in @file{wctype.h}. | |
@end deftypefun | |
The same warnings given in the last section for the use of the wide | |
character classification functions apply here. It is not possible to | |
simply cast a @code{char} type value to a @code{wint_t} and use it as an | |
argument to @code{towctrans} calls. |