wchar_t is defined in wchar.h
Currently, if the developers want to use only wchar_t
, they can not do
this without getting type conversion warnings from the compiler. If
wchar_t
will be made the same type as wint_t
, it will be good for both parties.
The developers who want to have both wint_t
and wchar_t
in their
programs (for example if they want their code to be compiled not only under
glibc) can do this without getting compiler warnings. The developers who
want to use only wchar_t
(to avoid unnecessary hassle with using wint_t
and
explicit typecasting) can also do this without getting compiler warnings.
And it will not bring any incompatibility or portability problems, except that if code using only wchar_t
will be compiled on the machine which uses original wchar.h
, the compiler will print those pesky warnings (if -Wconversion
is enabled), but the compiled program will work absolutely the same way.
The C standard (9899:201x 7.29) mentions:
wchar_t and wint_t can be the same integer type.
Also, in glibc wide characters are always
ISO10646/Unicode/UCS-4, so they always use 4 bytes. Thus, nothing
prevents making wchar_t
the same type as wint_t
in glibc.
But it seems that developers of glibc do not want to make wint_t
and
wchar_t
the same type for some reason. As such, I want to change the local copy of
wchar.h
.
ISO10646/Unicode/UCS-4 uses 2^31
values for the extended character set
(MSB being unused):
0xxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx
Notice, that a 4-byte type can hold 2^31
extra values (MSB being "1"):
1xxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx
Any of those extra values can be used to represent WEOF
, thus one
4-byte type can be used to hold all the character set and WEOF
.
Notice, that no recompilation of glibc is necessary to use the modified wchar.h
, because
wint_t
can be signed or unsigned (since both -1
and 0xffffffff
have MSB
"1", in any representation, and since MSB is not used in
ISO10646/Unicode/UCS-4).
Definition of wchar_t
is done somewhere in the following excerpt from wchar.h
.
How to change it to make wchar_t
the same type as wint_t
?
#ifndef _WCHAR_H
#if !defined __need_mbstate_t && !defined __need_wint_t
# define _WCHAR_H 1
# include <features.h>
#endif
#ifdef _WCHAR_H
/* Get FILE definition. */
# define __need___FILE
# if defined __USE_UNIX98 || defined __USE_XOPEN2K
# define __need_FILE
# endif
# include <stdio.h>
/* Get va_list definition. */
# define __need___va_list
# include <stdarg.h>
# include <bits/wchar.h>
/* Get size_t, wchar_t, wint_t and NULL from <stddef.h>. */
# define __need_size_t
# define __need_wchar_t
# define __need_NULL
#endif
#if defined _WCHAR_H || defined __need_wint_t || !defined __WINT_TYPE__
# undef __need_wint_t
# define __need_wint_t
# include <stddef.h>
/* We try to get wint_t from <stddef.h>, but not all GCC versions define it
there. So define it ourselves if it remains undefined. */
# ifndef _WINT_T
/* Integral type unchanged by default argument promotions that can
hold any value corresponding to members of the extended character
set, as well as at least one value that does not correspond to any
member of the extended character set. */
# define _WINT_T
typedef unsigned int wint_t;
# else
/* Work around problems with the <stddef.h> file which doesn't put
wint_t in the std namespace. */
# if defined __cplusplus && defined _GLIBCPP_USE_NAMESPACES \
&& defined __WINT_TYPE__
__BEGIN_NAMESPACE_STD
typedef __WINT_TYPE__ wint_t;
__END_NAMESPACE_STD
# endif
# endif
/* Tell the caller that we provide correct C++ prototypes. */
# if defined __cplusplus && __GNUC_PREREQ (4, 4)
# define __CORRECT_ISO_CPP_WCHAR_H_PROTO
# endif
#endif
#if (defined _WCHAR_H || defined __need_mbstate_t) && !defined ____mbstate_t_defined
# define ____mbstate_t_defined 1
/* Conversion state information. */
typedef struct
{
int __count;
union
{
# ifdef __WINT_TYPE__
__WINT_TYPE__ __wch;
# else
wint_t __wch;
# endif
char __wchb[4];
} __value; /* Value so far. */
} __mbstate_t;
#endif
#undef __need_mbstate_t
/* The rest of the file is only used if used if __need_mbstate_t is not
defined. */
#ifdef _WCHAR_H
# ifndef __mbstate_t_defined
__BEGIN_NAMESPACE_C99
/* Public type. */
typedef __mbstate_t mbstate_t;
__END_NAMESPACE_C99
# define __mbstate_t_defined 1
# endif
#ifdef __USE_GNU
__USING_NAMESPACE_C99(mbstate_t)
#endif
#ifndef WCHAR_MIN
/* These constants might also be defined in <inttypes.h>. */
# define WCHAR_MIN __WCHAR_MIN
# define WCHAR_MAX __WCHAR_MAX
#endif
#ifndef WEOF
# define WEOF (0xffffffffu)
#endif
/* For XPG4 compliance we have to define the stuff from <wctype.h> here
as well. */
#if defined __USE_XOPEN && !defined __USE_UNIX98
# include <wctype.h>
#endif
__BEGIN_DECLS
__BEGIN_NAMESPACE_STD
/* This incomplete type is defined in <time.h> but needed here because
of `wcsftime'. */
struct tm;
__END_NAMESPACE_STD
/* XXX We have to clean this up at some point. Since tm is in the std
namespace but wcsftime is in __c99 the type wouldn't be found
without inserting it in the global namespace. */
__USING_NAMESPACE_STD(tm)
Note that wint_t
was introduced because wchar_t
might be a type subject to 'default promotion' rules when passed to printf()
et al. This matters, for example, when calling printf()
:
wchar_t wc = …;
printf("%lc", wc);
The value of wc
might be converted to wint_t
. If you're writing a function like printf()
which needs to use the va_arg()
macro from <stdarg.h>
, then you should use the type wint_t
to get the value.
The standard notes that wint_t
might be the same type as wchar_t
, but if wchar_t
is a (16-bit) short
(or unsigned short
), wint_t
might be (32-bit) int
. To a first approximation, wint_t
only matters when wchar_t
is a 16-bit type. The full rules are, of course, more complex. For example, int
could be a 16-bit type — but this is rarely a problem.
7.29 Extended multibyte and wide character utilities
<wchar.h>
7.29.1 Introduction
¶1 The header
<wchar.h>
defines four macros, and declares four data types, one tag, and many functions.326)2 The types declared are
wchar_t
andsize_t
(both described in 7.19);mbstate_t
which is a complete object type other than an array type that can hold the conversion state information necessary to convert between sequences of multibyte characters and wide characters;
wint_t
which is an integer type unchanged by default argument promotions that can hold any value corresponding to members of the extended character set, as well as at least one value that does not correspond to any member of the extended character set (see
WEOF
below);327) …326) See ‘‘future library directions’’ (7.31.16).
327)wchar_t
andwint_t
can be the same integer type.§7.19 Common definitions
<stddef.h>
¶2 … and
wchar_t
which is an integer type whose range of values can represent distinct codes for all members of the largest extended character set specified among the supported locales; the null character shall have the code value zero. Each member of the basic character set shall have a code value equal to its value when used as the lone character in an integer character constant if an implementation does not define
__STDC_MB_MIGHT_NEQ_WC__
.
See Why the argument type of putchar()
, fputc()
, and putc()
is not char
for one place where the 'default promotion' rules from the C standard are quoted. There are probably other questions where the information is available too.