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idn2_lookup_u8(3) libidn2 idn2_lookup_u8(3)


idn2_lookup_u8 - API function


#include <idn2.h>

int idn2_lookup_u8(const uint8_t * src, uint8_t ** lookupname, int flags);


input zero-terminated UTF-8 string in Unicode NFC normalized form.
newly allocated output variable with name to lookup in DNS.
optional idn2_flags to modify behaviour.


Perform IDNA2008 lookup string conversion on domain name src , as described in section 5 of RFC 5891. Note that the input string must be encoded in UTF-8 and be in Unicode NFC form.

Pass IDN2_NFC_INPUT in flags to convert input to NFC form before further processing. IDN2_TRANSITIONAL and IDN2_NONTRANSITIONAL do already imply IDN2_NFC_INPUT.

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.

After version 2.0.3: IDN2_USE_STD3_ASCII_RULES disabled by default. Previously we were eliminating non-STD3 characters from domain strings such as, or IPs provided to libidn2 functions. That was an unexpected regression for applications switching from libidn and thus it is no longer applied by default. Use IDN2_USE_STD3_ASCII_RULES to enable that behavior again.

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


On successful conversion IDN2_OK 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.




Report bugs to <>.
General guidelines for reporting bugs:
Libidn2 home page:


Copyright © 2002-2024 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.


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:
2.3.7 libidn2