#### 6.3.3 Converting Single Characters

The most fundamental of the conversion functions are those dealing with single characters. Please note that this does not always mean single bytes. But since there is very often a subset of the multibyte character set that consists of single byte sequences, there are functions to help with converting bytes. Frequently, ASCII is a subpart of the multibyte character set. In such a scenario, each ASCII character stands for itself, and all other characters have at least a first byte that is beyond the range 0 to 127.

— Function: wint_t btowc (int c)

> The `btowc` function (“byte to wide character”) converts a valid single byte character c in the initial shift state into the wide character equivalent using the conversion rules from the currently selected locale of the `LC_CTYPE` category.

> If `(unsigned char)` c is no valid single byte multibyte character or if c is `EOF`, the function returns `WEOF`.

> Please note the restriction of c being tested for validity only in the initial shift state. No `mbstate_t` object is used from which the state information is taken, and the function also does not use any static state.

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

Despite the limitation that the single byte value is always interpreted in the initial state, this function is actually useful most of the time. Most characters are either entirely single-byte character sets or they are extension to ASCII. But then it is possible to write code like this (not that this specific example is very useful):

         wchar_t *
         itow (unsigned long int val)
         {
           static wchar_t buf[30];
           wchar_t *wcp = &buf[29];
           *wcp = L'\0';
           while (val != 0)
             {
               *--wcp = btowc ('0' + val % 10);
               val /= 10;
             }
           if (wcp == &buf[29])
             *--wcp = L'0';
           return wcp;
         }

Why is it necessary to use such a complicated implementation and not simply cast `'0' + val % 10` to a wide character? The answer is that there is no guarantee that one can perform this kind of arithmetic on the character of the character set used for `wchar_t` representation. In other situations the bytes are not constant at compile time and so the compiler cannot do the work. In situations like this, using `btowc` is required.

There is also a function for the conversion in the other direction.

— Function: int wctob (wint_t c)

> The `wctob` function (“wide character to byte”) takes as the parameter a valid wide character. If the multibyte representation for this character in the initial state is exactly one byte long, the return value of this function is this character. Otherwise the return value is `EOF`.

> `wctob` was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h`.

There are more general functions to convert single character from multibyte representation to wide characters and vice versa. These functions pose no limit on the length of the multibyte representation and they also do not require it to be in the initial state.

— Function: size_t mbrtowc (wchar_t *restrict pwc, const char *restrict s, size_t n, mbstate_t *restrict ps)

> The `mbrtowc` function (“multibyte restartable to wide character”) converts the next multibyte character in the string pointed to by s into a wide character and stores it in the wide character string pointed to by pwc. The conversion is performed according to the locale currently selected for the `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 object pointed to by ps. If ps is a null pointer, a static, internal state variable used only by the `mbrtowc` function is used.

> If the next multibyte character corresponds to the NUL wide character, the return value of the function is 0 and the state object is afterwards in the initial state. If the next n or fewer bytes form a correct multibyte character, the return value is the number of bytes starting from s that form the multibyte character. The conversion state is updated according to the bytes consumed in the conversion. In both cases the wide character (either the `L'\0'` or the one found in the conversion) is stored in the string pointed to by pwc if pwc is not null.

> If the first n bytes of the multibyte string possibly form a valid multibyte character but there are more than n bytes needed to complete it, the return value of the function is `(size_t) -2` and no value is stored. Please note that this can happen even if n has a value greater than or equal to `MB_CUR_MAX` since the input might contain redundant shift sequences.

> If the first `n` bytes of the multibyte string cannot possibly form a valid multibyte character, no value is stored, the global variable `errno` is set to the value `EILSEQ`, and the function returns `(size_t) -1`. The conversion state is afterwards undefined.

> `mbrtowc` was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h`.

Use of `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):

         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.  */
                 return NULL;
               *wcp++ = towupper (tmp[0]);
               len -= nbytes;
               s += nbytes;
             }
           return result;
         }

The use of `mbrtowc` should be clear. A single wide character is stored in tmp`[0]`, and the number of consumed bytes is stored in the variable nbytes. If the conversion is successful, the uppercase variant of the wide character is stored in the result array and the pointer to the input string and the number of available bytes is adjusted.

The only non-obvious thing about `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 results 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.

— Function: size_t mbrlen (const char *restrict s, size_t n, mbstate_t *ps)

> The `mbrlen` function (“multibyte restartable length”) computes the number of at most n bytes starting at s, which form the next valid and complete multibyte character.

> If the next multibyte character corresponds to the NUL wide character, the return value is 0. If the next n bytes form a valid multibyte character, the number of bytes belonging to this multibyte character byte sequence is returned.

> If the first n bytes possibly form a valid multibyte character but the character is incomplete, the return value is `(size_t) -2`. Otherwise the multibyte character sequence is invalid and the return value is `(size_t) -1`.

> The multibyte sequence is interpreted in the state represented by the object pointed to by ps. If ps is a null pointer, a state object local to `mbrlen` is used.

> `mbrlen` was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h`.

The attentive reader now will note that `mbrlen` can be implemented as

         mbrtowc (NULL, s, n, ps != NULL ? ps : &internal)

This is true and in fact is mentioned in the official specification. How can this function be used to determine the length of the wide character string created from a multibyte character string? It is not directly usable, but we can define a function `mbslen` using it:

         size_t
         mbslen (const char *s)
         {
           mbstate_t state;
           size_t result = 0;
           size_t nbytes;
           memset (&state, '\0', sizeof (state));
           while ((nbytes = mbrlen (s, MB_LEN_MAX, &state)) > 0)
             {
               if (nbytes >= (size_t) -2)
                 /* Something is wrong.  */
                 return (size_t) -1;
               s += nbytes;
               ++result;
             }
           return result;
         }

This function simply calls `mbrlen` for each multibyte character in the string and counts the number of function calls. Please note that we here use `MB_LEN_MAX` as the size argument in the `mbrlen` call. This is acceptable since a) this value is larger then the length of the longest multibyte character sequence and b) we know that the string s ends with a NUL byte, which cannot be part of any other multibyte character sequence but the one representing the NUL wide character. Therefore, the `mbrlen` function will never read invalid memory.

Now that this function is available (just to make this clear, this function is not part of the GNU C Library) we can compute the number of wide character required to store the converted multibyte character string s using

         wcs_bytes = (mbslen (s) + 1) * sizeof (wchar_t);

Please note that the `mbslen` function is quite inefficient. The implementation of `mbstouwcs` with `mbslen` would have to perform the conversion of the multibyte character input string twice, and this conversion might be quite expensive. So it is necessary to think about the consequences of using the easier but imprecise method before doing the work twice.

— Function: size_t wcrtomb (char *restrict s, wchar_t wc, mbstate_t *restrict ps)

> The `wcrtomb` function (“wide character restartable to multibyte”) converts a single wide character into a multibyte string corresponding to that wide character.

> If s is a null pointer, the function resets the state stored in the objects pointed to by ps (or the internal `mbstate_t` object) to the initial state. This can also be achieved by a call like this:

>               wcrtombs (temp_buf, L'\0', ps)

> since, if s is a null pointer, `wcrtomb` performs as if it writes into an internal buffer, which is guaranteed to be large enough.

> If wc is the NUL wide character, `wcrtomb` emits, if necessary, a shift sequence to get the state ps into the initial state followed by a single NUL byte, which is stored in the string s.

> Otherwise a byte sequence (possibly including shift sequences) is written into the string s. This only happens if wc is a valid wide character (i.e., it has a multibyte representation in the character set selected by locale of the `LC_CTYPE` category). If wc is no valid wide character, nothing is stored in the strings s, `errno` is set to `EILSEQ`, the conversion state in ps is undefined and the return value is `(size_t) -1`.

> If no error occurred the function returns the number of bytes stored in the string s. This includes all bytes representing shift sequences.

> One word about the interface of the function: there is no parameter specifying the length of the array s. Instead the function assumes that there are at least `MB_CUR_MAX` bytes available since this is the maximum length of any byte sequence representing a single character. So the caller has to make sure that there is enough space available, otherwise buffer overruns can occur.

> `wcrtomb` was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h`.

Using `wcrtomb` is as easy as using `mbrtowc`. The following example appends a wide character string to a multibyte character string. Again, the code is not really useful (or correct), it is simply here to demonstrate the use and some problems.

         char *
         mbscatwcs (char *s, size_t len, const wchar_t *ws)
         {
           mbstate_t state;
           /* Find the end of the existing string.  */
           char *wp = strchr (s, '\0');
           len -= wp - s;
           memset (&state, '\0', sizeof (state));
           do
             {
               size_t nbytes;
               if (len < MB_CUR_LEN)
                 {
                   /* We cannot guarantee that the next
                      character fits into the buffer, so
                      return an error.  */
                   errno = E2BIG;
                   return NULL;
                 }
               nbytes = wcrtomb (wp, *ws, &state);
               if (nbytes == (size_t) -1)
                 /* Error in the conversion.  */
                 return NULL;
               len -= nbytes;
               wp += nbytes;
             }
           while (*ws++ != L'\0');
           return s;
         }

First the function has to find the end of the string currently in the array s. The `strchr` call does this very efficiently since a requirement for multibyte character representations is that the NUL byte is never used except to represent itself (and in this context, the end of the string).

After initializing the state object the loop is entered where the first task is to make sure there is enough room in the array s. We abort if there are not at least `MB_CUR_LEN` bytes available. This is not always optimal but we have no other choice. We might have less than `MB_CUR_LEN` bytes available but the next multibyte character might also be only one byte long. At the time the `wcrtomb` call returns it is too late to decide whether the buffer was large enough. If this solution is unsuitable, there is a very slow but more accurate solution.

           ...
           if (len < MB_CUR_LEN)
             {
               mbstate_t temp_state;
               memcpy (&temp_state, &state, sizeof (state));
               if (wcrtomb (NULL, *ws, &temp_state) > len)
                 {
                   /* We cannot guarantee that the next
                      character fits into the buffer, so
                      return an error.  */
                   errno = E2BIG;
                   return NULL;
                 }
             }
           ...

Here we perform the conversion that might overflow the buffer so that we are afterwards in the position to make an exact decision about the buffer size. Please note the `NULL` argument for the destination buffer in the new `wcrtomb` call; since we are not interested in the converted text at this point, this is a nice way to express this. The most unusual thing about this piece of code certainly is the duplication of the conversion state object, but if a change of the state is necessary to emit the next multibyte character, we want to have the same shift state change performed in the real conversion. Therefore, we have to preserve the initial shift state information.

There are certainly many more and even better solutions to this problem. This example is only provided for educational purposes.