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manual: Various fixes to the mbstouwcs example, and mbrtowc update

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The example did not work because the null byte was not converted, and
mbrtowc was called with a zero-length input string.  This results in a
(size_t) -2 return value, so the function always returns NULL.

The size computation for the heap allocation of the result was
incorrect because it did not deal with integer overflow.

Error checking was missing, and the allocated memory was not freed on
error paths.  All error returns now set errno.  (Note that there is an
assumption that free does not clobber errno.)

The slightly unportable comparision against (size_t) -2 to catch both
(size_t) -1 and (size_t) -2 return values is gone as well.

A null wide character needs to be stored in the result explicitly, to
terminate it.

The description in the manual is updated to deal with these finer
points.  The (size_t) -2 behavior (consuming the input bytes) matches
what is specified in ISO C11.
This commit is contained in:
Florian Weimer 2018-04-05 12:52:19 +02:00
parent 0f33925269
commit cf138b0c83
3 changed files with 88 additions and 43 deletions
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11
ChangeLog
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@ -1,3 +1,14 @@
2018-04-05 Florian Weimer <fweimer@redhat.com>
* manual/examples/mbstouwcs.c (mbstouwcs): Fix loop termination,
integer overflow, memory leak on error, and indeterminate errno
value. Add a null wide character to terminate the result string.
* manual/charset.texi (Converting a Character): Mention embedded
null bytes in the mbrtowc input string. Explain what happens in
the -2 result case. Do not claim that mbrtowc is simple or
obvious to use. Adjust the description of the code example. Use
@code, not @var, for concrete variables.
2018-04-05 Florian Weimer <fweimer@redhat.com>
* manual/examples/mbstouwcs.c: New file.

69
manual/charset.texi
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@ -643,8 +643,8 @@ and they also do not require it to be in the initial state.
@cindex stateful
The @code{mbrtowc} function (``multibyte restartable to wide
character'') converts the next multibyte character in the string pointed
to by @var{s} into a wide character and stores it in the wide character
string pointed to by @var{pwc}. The conversion is performed according
to by @var{s} into a wide character and stores it in the location
pointed to by @var{pwc}. The conversion is performed according
to the locale currently selected for the @code{LC_CTYPE} category. If
the conversion for the character set used in the locale requires a state,
the multibyte string is interpreted in the state represented by the
@ -652,7 +652,7 @@ object pointed to by @var{ps}. If @var{ps} is a null pointer, a static,
internal state variable used only by the @code{mbrtowc} function is
used.
If the next multibyte character corresponds to the NUL wide character,
If the next multibyte character corresponds to the null wide character,
the return value of the function is @math{0} and the state object is
afterwards in the initial state. If the next @var{n} or fewer bytes
form a correct multibyte character, the return value is the number of
@ -665,50 +665,59 @@ by @var{pwc} if @var{pwc} is not null.
If the first @var{n} bytes of the multibyte string possibly form a valid
multibyte character but there are more than @var{n} bytes needed to
complete it, the return value of the function is @code{(size_t) -2} and
no value is stored. Please note that this can happen even if @var{n}
has a value greater than or equal to @code{MB_CUR_MAX} since the input
might contain redundant shift sequences.
no value is stored in @code{*@var{pwc}}. The conversion state is
updated and all @var{n} input bytes are consumed and should not be
submitted again. Please note that this can happen even if @var{n} has a
value greater than or equal to @code{MB_CUR_MAX} since the input might
contain redundant shift sequences.
If the first @code{n} bytes of the multibyte string cannot possibly form
a valid multibyte character, no value is stored, the global variable
@code{errno} is set to the value @code{EILSEQ}, and the function returns
@code{(size_t) -1}. The conversion state is afterwards undefined.
As specified, the @code{mbrtowc} function could deal with multibyte
sequences which contain embedded null bytes (which happens in Unicode
encodings such as UTF-16), but @theglibc{} does not support such
multibyte encodings. When encountering a null input byte, the function
will either return zero, or return @code{(size_t) -1)} and report a
@code{EILSEQ} error. The @code{iconv} function can be used for
converting between arbitrary encodings. @xref{Generic Conversion
Interface}.
@pindex wchar.h
@code{mbrtowc} was introduced in @w{Amendment 1} to @w{ISO C90} and
is declared in @file{wchar.h}.
@end deftypefun
Use of @code{mbrtowc} is straightforward. A function that copies a
multibyte string into a wide character string while at the same time
converting all lowercase characters into uppercase could look like this
(this is not the final version, just an example; it has no error
checking, and sometimes leaks memory):
A function that copies a multibyte string into a wide character string
while at the same time converting all lowercase characters into
uppercase could look like this:
@smallexample
@include mbstouwcs.c.texi
@end smallexample
The use of @code{mbrtowc} should be clear. A single wide character is
stored in @code{@var{tmp}[0]}, and the number of consumed bytes is stored
in the variable @var{nbytes}. If the conversion is successful, the
uppercase variant of the wide character is stored in the @var{result}
array and the pointer to the input string and the number of available
bytes is adjusted.
In the inner loop, a single wide character is stored in @code{wc}, and
the number of consumed bytes is stored in the variable @code{nbytes}.
If the conversion is successful, the uppercase variant of the wide
character is stored in the code{result} array and the pointer to the
input string and the number of available bytes is adjusted. If the
@code{mbrtowc} function returns zero, the null input byte has not been
converted, so it must be stored explicitly in the result.
The only non-obvious thing about @code{mbrtowc} might be the way memory
is allocated for the result. The above code uses the fact that there
can never be more wide characters in the converted result than there are
bytes in the multibyte input string. This method yields a pessimistic
guess about the size of the result, and if many wide character strings
have to be constructed this way or if the strings are long, the extra
memory required to be allocated because the input string contains
multibyte characters might be significant. The allocated memory block can
be resized to the correct size before returning it, but a better solution
might be to allocate just the right amount of space for the result right
away. Unfortunately there is no function to compute the length of the wide
character string directly from the multibyte string. There is, however, a
function that does part of the work.
The above code uses the fact that there can never be more wide
characters in the converted result than there are bytes in the multibyte
input string. This method yields a pessimistic guess about the size of
the result, and if many wide character strings have to be constructed
this way or if the strings are long, the extra memory required to be
allocated because the input string contains multibyte characters might
be significant. The allocated memory block can be resized to the
correct size before returning it, but a better solution might be to
allocate just the right amount of space for the result right away.
Unfortunately there is no function to compute the length of the wide
character string directly from the multibyte string. There is, however,
a function that does part of the work.
@deftypefun size_t mbrlen (const char *restrict @var{s}, size_t @var{n}, mbstate_t *@var{ps})
@standards{ISO, wchar.h}

51
manual/examples/mbstouwcs.c
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@ -1,3 +1,4 @@
#include <stdbool.h>
#include <stdlib.h>
#include <string.h>
#include <wchar.h>
@ -7,22 +8,46 @@
wchar_t *
mbstouwcs (const char *s)
{
size_t len = strlen (s);
wchar_t *result = malloc ((len + 1) * sizeof (wchar_t));
wchar_t *wcp = result;
wchar_t tmp[1];
mbstate_t state;
size_t nbytes;
memset (&state, '\0', sizeof (state));
while ((nbytes = mbrtowc (tmp, s, len, &state)) > 0)
{
if (nbytes >= (size_t) -2)
/* Invalid input string. */
/* Include the null terminator in the conversion. */
size_t len = strlen (s) + 1;
wchar_t *result = reallocarray (NULL, len, sizeof (wchar_t));
if (result == NULL)
return NULL;
*wcp++ = towupper (tmp[0]);
wchar_t *wcp = result;
mbstate_t state;
memset (&state, '\0', sizeof (state));
while (true)
{
wchar_t wc;
size_t nbytes = mbrtowc (&wc, s, len, &state);
if (nbytes == 0)
{
/* Terminate the result string. */
*wcp = L'\0';
break;
}
else if (nbytes == (size_t) -2)
{
/* Truncated input string. */
errno = EILSEQ;
free (result);
return NULL;
}
else if (nbytes == (size_t) -1)
{
/* Some other error (including EILSEQ). */
free (result);
return NULL;
}
else
{
/* A character was converted. */
*wcp++ = towupper (wc);
len -= nbytes;
s += nbytes;
}
}
return result;
}