idn2\_lookup\_ul(3) libidn2 idn2\_lookup\_ul(3)

# [NAME](#NAME)

idn2\_lookup\_ul - API function

# [SYNOPSIS](#SYNOPSIS)

**#include <idn2.h>**

**int idn2\_lookup\_ul(const char * src, char \** lookupname, int flags);**

# [ARGUMENTS](#ARGUMENTS)

[const char * src](#const)

input zero-terminated locale encoded string.

[char \** lookupname](#char)

newly allocated output variable with name to lookup in DNS.

[int flags](#int)

optional **idn2\_flags** to modify behaviour.

# [DESCRIPTION](#DESCRIPTION)

Perform IDNA2008 lookup string conversion on domain name *src* , as described in section 5 of RFC 5891. Note that the input is assumed to be encoded in the locale's default coding system, and will be transcoded to UTF-8 and NFC normalized by this function.

Pass **IDN2\_ALABEL\_ROUNDTRIP** in *flags* to convert any input A-labels to U-labels and perform additional testing. This is default since version 2.2. To switch this behavior off, pass IDN2\_NO\_ALABEL\_ROUNDTRIP

Pass **IDN2\_TRANSITIONAL** to enable Unicode TR46 transitional processing, and **IDN2\_NONTRANSITIONAL** to enable Unicode TR46 non-transitional processing.

Multiple flags may be specified by binary or:ing them together, for example **IDN2\_ALABEL\_ROUNDTRIP** | **IDN2\_NONTRANSITIONAL**.

The **IDN2\_NFC\_INPUT** in *flags* is always enabled in this function.

After version 0.11: *lookupname* may be NULL to test lookup of *src* without allocating memory.

# [RETURNS](#RETURNS)

On successful conversion **IDN2\_OK** is returned, if conversion from locale to UTF-8 fails then **IDN2\_ICONV\_FAIL** is returned, if the output domain or any label would have been too long **IDN2\_TOO\_BIG\_DOMAIN** or **IDN2\_TOO\_BIG\_LABEL** is returned, or another error code is returned.

# [SINCE](#SINCE)

0.1

# [REPORTING BUGS](#REPORTING_BUGS)

Report bugs to <help-libidn@gnu.org>.  
General guidelines for reporting bugs: https://www.gnu.org/gethelp/  
Libidn2 home page: https://www.gnu.org/software/libidn2/

# [COPYRIGHT](#COPYRIGHT)

Copyright © 2002-2025 Simon Josefsson.  
Copying and distribution of this file, with or without modification, are permitted in any medium without royalty provided the copyright notice and this notice are preserved.

# [SEE ALSO](#SEE_ALSO)

The full documentation for **libidn2** is maintained as a Texinfo manual. If the **info** and **libidn2** programs are properly installed at your site, the command

**info libidn2**

should give you access to the complete manual. As an alternative you may obtain the manual from:

**https://www.gnu.org/software/libidn/libidn2/manual/**

2.3.8 libidn2

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Document: idn2_lookup_ul(3) from libidn2 2.3.8-r9 (Chainguard OS)
Source: https://gitlab.com/libidn/libidn2