Programming Tutorials

Unicode and UTF-8 in C

By: Ramlak  

Starting with GNU glibc 2.2, the type wchar_t is officially intended to be used only for 32-bit ISO 10646 values, independent of the currently used locale. This is signalled to applications by the definition of the __STDC_ISO_10646__ macro as required by ISO C99. The ISO C multi-byte conversion functions (mbsrtowcs(), wcsrtombs(), etc.) are fully implemented in glibc 2.2 or higher and can be used to convert between wchar_t and any locale-dependent multibyte encoding, including UTF-8, ISO 8859-1, etc.

For example, you can write

  #include <stdio.h>
  #include <locale.h>

  int main()
  {
    if (!setlocale(LC_CTYPE, "")) {
      fprintf(stderr, "Can't set the specified locale! "
              "Check LANG, LC_CTYPE, LC_ALL.\n");
      return 1;
    }
    printf("%ls\n", L"Schöne Grüße");
    return 0;
  }

Call this program with the locale setting LANG=de_DE and the output will be in ISO 8859-1. Call it with LANG=de_DE.UTF-8 and the output will be in UTF-8. The %ls format specifier in printf calls wcsrtombs in order to convert the wide character argument string into the locale-dependent multi-byte encoding.

Many of C’s string functions are locale-independent and they just look at zero-terminated byte sequences:

  strcpy strncpy strcat strncat strcmp strncmp strdup strchr strrchr
  strcspn strspn strpbrk strstr strtok

Some of these (e.g. strcpy) can equally be used for single-byte (ISO 8859-1) and multi-byte (UTF-8) encoded character sets, as they need no notion of how many byte long a character is, while others (e.g., strchr) depend on one character being encoded in a single char value and are of less use for UTF-8 (strchr still works fine if you just search for an ASCII character in a UTF-8 string).

Other C functions are locale dependent and work in UTF-8 locales just as well:

  strcoll strxfrm

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