---
name: "idn2_lookup_ul"
section: "3"
platform: "linux"
package: "libidn2"
version: "2.3.8-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/idn2_lookup_ul"
---

# idn2\_lookup\_ul(3)

API function

## NAME

idn2\_lookup\_ul - API function

## SYNOPSIS

**#include \<idn2.h\>**

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

## ARGUMENTS

- const char \* src

  input zero-terminated locale encoded string.

- char \*\* lookupname

  newly allocated output variable with name to lookup in DNS.

- int flags

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

## 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

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

0\.1

## 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 © 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

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/**