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6.3.4 Converting Multibyte and Wide Character Strings

The functions described in the previous section only convert a single character at a time. Most operations to be performed in real-world programs include strings and therefore the ISO C standard also defines conversions on entire strings. However, the defined set of functions is quite limited; therefore, the GNU C library contains a few extensions that can help in some important situations.

— Function: size_t mbsrtowcs (wchar_t *restrict dst, const char **restrict src, size_t len, mbstate_t *restrict ps)

The mbsrtowcs function (“multibyte string restartable to wide character string”) converts an NUL-terminated multibyte character string at *src into an equivalent wide character string, including the NUL wide character at the end. The conversion is started using the state information from the object pointed to by ps or from an internal object of mbsrtowcs if ps is a null pointer. Before returning, the state object is updated to match the state after the last converted character. The state is the initial state if the terminating NUL byte is reached and converted.

If dst is not a null pointer, the result is stored in the array pointed to by dst; otherwise, the conversion result is not available since it is stored in an internal buffer.

If len wide characters are stored in the array dst before reaching the end of the input string, the conversion stops and len is returned. If dst is a null pointer, len is never checked.

Another reason for a premature return from the function call is if the input string contains an invalid multibyte sequence. In this case the global variable errno is set to EILSEQ and the function returns (size_t) -1.

In all other cases the function returns the number of wide characters converted during this call. If dst is not null, mbsrtowcs stores in the pointer pointed to by src either a null pointer (if the NUL byte in the input string was reached) or the address of the byte following the last converted multibyte character.

mbsrtowcs was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

The definition of the mbsrtowcs function has one important limitation. The requirement that dst has to be a NUL-terminated string provides problems if one wants to convert buffers with text. A buffer is normally no collection of NUL-terminated strings but instead a continuous collection of lines, separated by newline characters. Now assume that a function to convert one line from a buffer is needed. Since the line is not NUL-terminated, the source pointer cannot directly point into the unmodified text buffer. This means, either one inserts the NUL byte at the appropriate place for the time of the mbsrtowcs function call (which is not doable for a read-only buffer or in a multi-threaded application) or one copies the line in an extra buffer where it can be terminated by a NUL byte. Note that it is not in general possible to limit the number of characters to convert by setting the parameter len to any specific value. Since it is not known how many bytes each multibyte character sequence is in length, one can only guess.

There is still a problem with the method of NUL-terminating a line right after the newline character, which could lead to very strange results. As said in the description of the mbsrtowcs function above the conversion state is guaranteed to be in the initial shift state after processing the NUL byte at the end of the input string. But this NUL byte is not really part of the text (i.e., the conversion state after the newline in the original text could be something different than the initial shift state and therefore the first character of the next line is encoded using this state). But the state in question is never accessible to the user since the conversion stops after the NUL byte (which resets the state). Most stateful character sets in use today require that the shift state after a newline be the initial state–but this is not a strict guarantee. Therefore, simply NUL-terminating a piece of a running text is not always an adequate solution and, therefore, should never be used in generally used code.

The generic conversion interface (see Generic Charset Conversion) does not have this limitation (it simply works on buffers, not strings), and the GNU C library contains a set of functions that take additional parameters specifying the maximal number of bytes that are consumed from the input string. This way the problem of mbsrtowcs's example above could be solved by determining the line length and passing this length to the function.

— Function: size_t wcsrtombs (char *restrict dst, const wchar_t **restrict src, size_t len, mbstate_t *restrict ps)

The wcsrtombs function (“wide character string restartable to multibyte string”) converts the NUL-terminated wide character string at *src into an equivalent multibyte character string and stores the result in the array pointed to by dst. The NUL wide character is also converted. The conversion starts in the state described in the object pointed to by ps or by a state object locally to wcsrtombs in case ps is a null pointer. If dst is a null pointer, the conversion is performed as usual but the result is not available. If all characters of the input string were successfully converted and if dst is not a null pointer, the pointer pointed to by src gets assigned a null pointer.

If one of the wide characters in the input string has no valid multibyte character equivalent, the conversion stops early, sets the global variable errno to EILSEQ, and returns (size_t) -1.

Another reason for a premature stop is if dst is not a null pointer and the next converted character would require more than len bytes in total to the array dst. In this case (and if dest is not a null pointer) the pointer pointed to by src is assigned a value pointing to the wide character right after the last one successfully converted.

Except in the case of an encoding error the return value of the wcsrtombs function is the number of bytes in all the multibyte character sequences stored in dst. Before returning the state in the object pointed to by ps (or the internal object in case ps is a null pointer) is updated to reflect the state after the last conversion. The state is the initial shift state in case the terminating NUL wide character was converted.

The wcsrtombs function was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

The restriction mentioned above for the mbsrtowcs function applies here also. There is no possibility of directly controlling the number of input characters. One has to place the NUL wide character at the correct place or control the consumed input indirectly via the available output array size (the len parameter).

— Function: size_t mbsnrtowcs (wchar_t *restrict dst, const char **restrict src, size_t nmc, size_t len, mbstate_t *restrict ps)

The mbsnrtowcs function is very similar to the mbsrtowcs function. All the parameters are the same except for nmc, which is new. The return value is the same as for mbsrtowcs.

This new parameter specifies how many bytes at most can be used from the multibyte character string. In other words, the multibyte character string *src need not be NUL-terminated. But if a NUL byte is found within the nmc first bytes of the string, the conversion stops here.

This function is a GNU extension. It is meant to work around the problems mentioned above. Now it is possible to convert a buffer with multibyte character text piece for piece without having to care about inserting NUL bytes and the effect of NUL bytes on the conversion state.

A function to convert a multibyte string into a wide character string and display it could be written like this (this is not a really useful example):

     showmbs (const char *src, FILE *fp)
       mbstate_t state;
       int cnt = 0;
       memset (&state, '\0', sizeof (state));
       while (1)
           wchar_t linebuf[100];
           const char *endp = strchr (src, '\n');
           size_t n;
           /* Exit if there is no more line.  */
           if (endp == NULL)
           n = mbsnrtowcs (linebuf, &src, endp - src, 99, &state);
           linebuf[n] = L'\0';
           fprintf (fp, "line %d: \"%S\"\n", linebuf);

There is no problem with the state after a call to mbsnrtowcs. Since we don't insert characters in the strings that were not in there right from the beginning and we use state only for the conversion of the given buffer, there is no problem with altering the state.

— Function: size_t wcsnrtombs (char *restrict dst, const wchar_t **restrict src, size_t nwc, size_t len, mbstate_t *restrict ps)

The wcsnrtombs function implements the conversion from wide character strings to multibyte character strings. It is similar to wcsrtombs but, just like mbsnrtowcs, it takes an extra parameter, which specifies the length of the input string.

No more than nwc wide characters from the input string *src are converted. If the input string contains a NUL wide character in the first nwc characters, the conversion stops at this place.

The wcsnrtombs function is a GNU extension and just like mbsnrtowcs helps in situations where no NUL-terminated input strings are available.