### 5.6 Collation Functions

In some locales, the conventions for lexicographic ordering differ from the strict numeric ordering of character codes. For example, in Spanish most glyphs with diacritical marks such as accents are not considered distinct letters for the purposes of collation. On the other hand, the two-character sequence ‘`ll`’ is treated as a single letter that is collated immediately after ‘`l`’.

You can use the functions `strcoll` and `strxfrm` (declared in the headers file `string.h`) and `wcscoll` and `wcsxfrm` (declared in the headers file `wchar`) to compare strings using a collation ordering appropriate for the current locale. The locale used by these functions in particular can be specified by setting the locale for the `LC_COLLATE` category; see Locales. In the standard C locale, the collation sequence for `strcoll` is the same as that for `strcmp`. Similarly, `wcscoll` and `wcscmp` are the same in this situation.

Effectively, the way these functions work is by applying a mapping to transform the characters in a string to a byte sequence that represents the string's position in the collating sequence of the current locale. Comparing two such byte sequences in a simple fashion is equivalent to comparing the strings with the locale's collating sequence.

The functions `strcoll` and `wcscoll` perform this translation implicitly, in order to do one comparison. By contrast, `strxfrm` and `wcsxfrm` perform the mapping explicitly. If you are making multiple comparisons using the same string or set of strings, it is likely to be more efficient to use `strxfrm` or `wcsxfrm` to transform all the strings just once, and subsequently compare the transformed strings with `strcmp` or `wcscmp`.

— Function: int strcoll (const char *s1, const char *s2)

> Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

> The `strcoll` function is similar to `strcmp` but uses the collating sequence of the current locale for collation (the `LC_COLLATE` locale).

— Function: int wcscoll (const wchar_t *ws1, const wchar_t *ws2)

> Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

> The `wcscoll` function is similar to `wcscmp` but uses the collating sequence of the current locale for collation (the `LC_COLLATE` locale).

Here is an example of sorting an array of strings, using `strcoll` to compare them. The actual sort algorithm is not written here; it comes from `qsort` (see Array Sort Function). The job of the code shown here is to say how to compare the strings while sorting them. (Later on in this section, we will show a way to do this more efficiently using `strxfrm`.)

         /* This is the comparison function used with qsort. */

         int
         compare_elements (const void *v1, const void *v2)
         {
           char * const *p1 = v1;
           char * const *p2 = v2;

           return strcoll (*p1, *p2);
         }

         /* This is the entry point---the function to sort
            strings using the locale's collating sequence. */

         void
         sort_strings (char **array, int nstrings)
         {
           /* Sort temp_array by comparing the strings. */
           qsort (array, nstrings,
                  sizeof (char *), compare_elements);
         }

— Function: size_t strxfrm (char *restrict to, const char *restrict from, size_t size)

> Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

> The function `strxfrm` transforms the string from using the collation transformation determined by the locale currently selected for collation, and stores the transformed string in the array to. Up to size characters (including a terminating null character) are stored.

> The behavior is undefined if the strings to and from overlap; see Copying and Concatenation.

> The return value is the length of the entire transformed string. This value is not affected by the value of size, but if it is greater or equal than size, it means that the transformed string did not entirely fit in the array to. In this case, only as much of the string as actually fits was stored. To get the whole transformed string, call `strxfrm` again with a bigger output array.

> The transformed string may be longer than the original string, and it may also be shorter.

> If size is zero, no characters are stored in to. In this case, `strxfrm` simply returns the number of characters that would be the length of the transformed string. This is useful for determining what size the allocated array should be. It does not matter what to is if size is zero; to may even be a null pointer.

— Function: size_t wcsxfrm (wchar_t *restrict wto, const wchar_t *wfrom, size_t size)

> Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

> The function `wcsxfrm` transforms wide character string wfrom using the collation transformation determined by the locale currently selected for collation, and stores the transformed string in the array wto. Up to size wide characters (including a terminating null character) are stored.

> The behavior is undefined if the strings wto and wfrom overlap; see Copying and Concatenation.

> The return value is the length of the entire transformed wide character string. This value is not affected by the value of size, but if it is greater or equal than size, it means that the transformed wide character string did not entirely fit in the array wto. In this case, only as much of the wide character string as actually fits was stored. To get the whole transformed wide character string, call `wcsxfrm` again with a bigger output array.

> The transformed wide character string may be longer than the original wide character string, and it may also be shorter.

> If size is zero, no characters are stored in to. In this case, `wcsxfrm` simply returns the number of wide characters that would be the length of the transformed wide character string. This is useful for determining what size the allocated array should be (remember to multiply with `sizeof (wchar_t)`). It does not matter what wto is if size is zero; wto may even be a null pointer.

Here is an example of how you can use `strxfrm` when you plan to do many comparisons. It does the same thing as the previous example, but much faster, because it has to transform each string only once, no matter how many times it is compared with other strings. Even the time needed to allocate and free storage is much less than the time we save, when there are many strings.

         struct sorter { char *input; char *transformed; };

         /* This is the comparison function used with qsort
            to sort an array of struct sorter. */

         int
         compare_elements (const void *v1, const void *v2)
         {
           const struct sorter *p1 = v1;
           const struct sorter *p2 = v2;

           return strcmp (p1->transformed, p2->transformed);
         }

         /* This is the entry point---the function to sort
            strings using the locale's collating sequence. */

         void
         sort_strings_fast (char **array, int nstrings)
         {
           struct sorter temp_array[nstrings];
           int i;

           /* Set up temp_array.  Each element contains
              one input string and its transformed string. */
           for (i = 0; i < nstrings; i++)
             {
               size_t length = strlen (array[i]) * 2;
               char *transformed;
               size_t transformed_length;

               temp_array[i].input = array[i];

               /* First try a buffer perhaps big enough.  */
               transformed = (char *) xmalloc (length);

               /* Transform array[i].  */
               transformed_length = strxfrm (transformed, array[i], length);

               /* If the buffer was not large enough, resize it
                  and try again.  */
               if (transformed_length >= length)
                 {
                   /* Allocate the needed space. +1 for terminating
                      NUL character.  */
                   transformed = (char *) xrealloc (transformed,
                                                    transformed_length + 1);

                   /* The return value is not interesting because we know
                      how long the transformed string is.  */
                   (void) strxfrm (transformed, array[i],
                                   transformed_length + 1);
                 }

               temp_array[i].transformed = transformed;
             }

           /* Sort temp_array by comparing transformed strings. */
           qsort (temp_array, sizeof (struct sorter),
                  nstrings, compare_elements);

           /* Put the elements back in the permanent array
              in their sorted order. */
           for (i = 0; i < nstrings; i++)
             array[i] = temp_array[i].input;

           /* Free the strings we allocated. */
           for (i = 0; i < nstrings; i++)
             free (temp_array[i].transformed);
         }

The interesting part of this code for the wide character version would look like this:

         void
         sort_strings_fast (wchar_t **array, int nstrings)
         {
           ...
               /* Transform array[i].  */
               transformed_length = wcsxfrm (transformed, array[i], length);

               /* If the buffer was not large enough, resize it
                  and try again.  */
               if (transformed_length >= length)
                 {
                   /* Allocate the needed space. +1 for terminating
                      NUL character.  */
                   transformed = (wchar_t *) xrealloc (transformed,
                                                       (transformed_length + 1)
                                                       * sizeof (wchar_t));

                   /* The return value is not interesting because we know
                      how long the transformed string is.  */
                   (void) wcsxfrm (transformed, array[i],
                                   transformed_length + 1);
                 }
           ...

Note the additional multiplication with `sizeof (wchar_t)` in the `realloc` call.

Compatibility Note: The string collation functions are a new feature of ISO C90. Older C dialects have no equivalent feature. The wide character versions were introduced in Amendment 1 to ISO C90.