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3 Converting from libidn

This library is backwards (API) compatible with the libidn library (

Although it is recommended for new software to use the native libidn2 functions (i.e., the ones prefixed with idn2), old software isn’t always feasible to modify.

3.1 Converting with minimal modifications

As such, libidn2, provides compatibility macros which switch all libidn functions, to libidn2 functions in a backwards compatible way. To take advantage of these compatibility functions, it is sufficient to replace the idna.h header in legacy code, with idn2.h. That would transform the software from using libidn, i.e., IDNA2003, to using libidn2 with IDNA2008 non-transitional encoding.

3.2 Converting to native APIs

However, it is recommended to switch applications to the IDN2 native APIs. The following table provides a mapping of libidn code snippets to libidn2, for switching to IDNA2008.

rc = idna_to_ascii_8z (buf, &p, IDNA_USE_STD3_ASCII_RULES);
if (rc != IDNA_SUCCESS)
rc = idn2_to_ascii_8z (buf, &p, IDN2_USE_STD3_ASCII_RULES);
if (rc != IDN2_OK)
rc = idna_to_ascii_8z (buf, &p, 0 /* any other flags */);
if (rc != IDNA_SUCCESS)
/* we recommend to use the default flags (0), so that
 * the default behavior of libidn2 applies. */
rc = idn2_to_ascii_8z (buf, &p, 0);
if (rc != IDN2_OK)
rc = idna_to_unicode_8z8z (buf, &p, 0 /* any flags */);
if (rc != IDNA_SUCCESS)
rc = idn2_to_unicode_8z8z (buf, &p, 0);
if (rc != IDN2_OK)

Note that, although the table only lists the UTF-8 functions, the mapping is identical for every other one on the family of toUnicode and toAscii. As the IDNA2003 details differ signicantly to IDNA2008, not all flags used in the libidn functions map to any specific flags; it is typically safe to use the suggested libidn2 flags. Exceptionally the libidn flag IDNA_USE_STD3_ASCII_RULES is mapped to IDN2_USE_STD3_ASCII_RULES.

3.3 Converting with backwards compatibility

In several cases where IDNA2008 mappings do not exist whereas IDNA2003 mappings do, software like browsers take a backwards compatible approach. That is convert the domain to IDNA2008 form, and if that fails try the IDNA2003 conversion. The following example demonstrates that approach.

rc = idn2_to_ascii_8z (buf, &p, IDN2_NONTRANSITIONAL); /* IDNA2008 */
if (rc == IDN2_DISALLOWED)
  rc = idn2_to_ascii_8z (buf, &p, IDN2_TRANSITIONAL); /* IDNA2003 - compatible */

3.4 Using libidn and libidn2 code

In the special case of software that needs to support both libraries (e.g., both IDNA2003 and IDNA2008), you must define IDN2_SKIP_LIBIDN_COMPAT prior to including idn2.h in order to disable compatibility code which overlaps with libidn functionality. That would allow software to use both libraries’ functions.

3.5 Stringprep and libidn2

The original libidn library includes functionality for the stringprep processing in stringprep.h. That functionality was an integral part of an IDNA2003 implementation, but it does not apply to IDNA2008. Furthermore, stringprep processing has been replaced by the PRECIS framework (RFC8264).

For the reasons above, libidn2 does not implement stringprep or any other string processing protocols unrelated to IDNA2008. Applications requiring the stringprep processing should continue using the original libidn, and new applications should consider using the PRECIS framework.

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