manual/ctype.texi

@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 ISO
@deftypefun int isblank (int @var{c})
Returns true if @var{c} is a blank character; that is, a space or a tab.
This function was originally a GNU extension, but was added in @w{ISO C99}.
@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 ISO
@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 was originally a GNU extension, but was added in @w{ISO C99}.
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.