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2 Library Functions

Below are the interfaces of the Libidn2 library documented.

2.1 Header file idn2.h

To use the functions documented in this chapter, you need to include the file idn2.h like this:

#include <idn2.h>

2.2 Core Functions

When you have the data encoded in UTF-8 form the direct interfaces to the library are as follows.

idn2_to_ascii_8z

Function: int idn2_to_ascii_8z (const char * input, char ** output, int flags)

input: zero terminated input UTF-8 string.

output: pointer to newly allocated output string.

flags: optional idn2_flags to modify behaviour.

Convert UTF-8 domain name to ASCII string using the IDNA2008 rules. The domain name may contain several labels, separated by dots. The output buffer must be deallocated by the caller.

The default behavior of this function (when flags are zero) is to apply the IDNA2008 rules without the TR46 amendments. As the TR46 non-transitional processing is nowadays ubiquitous, when unsure, it is recommended to call this function with the IDN2_NONTRANSITIONAL and the IDN2_NFC_INPUT flags for compatibility with other software.

Return value: Returns IDN2_OK on success, or error code.

Since: 2.0.0

idn2_to_unicode_8z8z

Function: int idn2_to_unicode_8z8z (const char * input, char ** output, int flags)

input: Input zero-terminated UTF-8 string.

output: Newly allocated UTF-8 output string.

flags: Currently unused.

Converts a possibly ACE encoded domain name in UTF-8 format into a UTF-8 string (punycode decoding). The output buffer will be zero-terminated and must be deallocated by the caller.

output may be NULL to test lookup of input without allocating memory.

Since: 2.0.0

idn2_lookup_u8

Function: int idn2_lookup_u8 (const uint8_t * src, uint8_t ** lookupname, int flags)

src: input zero-terminated UTF-8 string in Unicode NFC normalized form.

lookupname: newly allocated output variable with name to lookup in DNS.

flags: 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 _443._tcp.example.com, or IPs 1.2.3.4/24 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.

Returns: 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.

Since: 0.1

idn2_register_u8

Function: int idn2_register_u8 (const uint8_t * ulabel, const uint8_t * alabel, uint8_t ** insertname, int flags)

ulabel: input zero-terminated UTF-8 and Unicode NFC string, or NULL.

alabel: input zero-terminated ACE encoded string (xn–), or NULL.

insertname: newly allocated output variable with name to register in DNS.

flags: optional idn2_flags to modify behaviour.

Perform IDNA2008 register string conversion on domain label ulabel and alabel , as described in section 4 of RFC 5891. Note that the input ulabel must be encoded in UTF-8 and be in Unicode NFC form.

Pass IDN2_NFC_INPUT in flags to convert input ulabel to NFC form before further processing.

It is recommended to supply both ulabel and alabel for better error checking, but supplying just one of them will work. Passing in only alabel is better than only ulabel . See RFC 5891 section 4 for more information.

After version 0.11: insertname may be NULL to test conversion of src without allocating memory.

Returns: On successful conversion IDN2_OK is returned, when the given ulabel and alabel does not match each other IDN2_UALABEL_MISMATCH is returned, when either of the input labels are too long IDN2_TOO_BIG_LABEL is returned, when alabel does does not appear to be a proper A-label IDN2_INVALID_ALABEL is returned, or another error code is returned.

2.3 Locale Functions

As a convenience, the following functions are provided that will convert the input from the locale encoding format to UTF-8 and normalize the string using NFC, and then apply the core functions described earlier.

idn2_to_ascii_lz

Function: int idn2_to_ascii_lz (const char * input, char ** output, int flags)

input: zero terminated input UTF-8 string.

output: pointer to newly allocated output string.

flags: optional idn2_flags to modify behaviour.

Convert a domain name in locale’s encoding to ASCII string using the IDNA2008 rules. The domain name may contain several labels, separated by dots. The output buffer must be deallocated by the caller.

The default behavior of this function (when flags are zero) is to apply the IDNA2008 rules without the TR46 amendments. As the TR46 non-transitional processing is nowadays ubiquitous, when unsure, it is recommended to call this function with the IDN2_NONTRANSITIONAL and the IDN2_NFC_INPUT flags for compatibility with other software.

Returns: IDN2_OK on success, or error code. Same as described in idn2_lookup_ul() documentation.

Since: 2.0.0

idn2_to_unicode_8zlz

Function: int idn2_to_unicode_8zlz (const char * input, char ** output, int flags)

input: Input zero-terminated UTF-8 string.

output: Newly allocated output string in current locale’s character set.

flags: Currently unused.

Converts a possibly ACE encoded domain name in UTF-8 format into a string encoded in the current locale’s character set (punycode decoding). The output buffer will be zero-terminated and must be deallocated by the caller.

output may be NULL to test lookup of input without allocating memory.

Since: 2.0.0

idn2_to_unicode_lzlz

Function: int idn2_to_unicode_lzlz (const char * input, char ** output, int flags)

input: Input zero-terminated string encoded in the current locale’s character set.

output: Newly allocated output string in current locale’s character set.

flags: Currently unused.

Converts a possibly ACE encoded domain name in the locale’s character set into a string encoded in the current locale’s character set (punycode decoding). The output buffer will be zero-terminated and must be deallocated by the caller.

output may be NULL to test lookup of input without allocating memory.

Since: 2.0.0

idn2_lookup_ul

Function: int idn2_lookup_ul (const char * src, char ** lookupname, int flags)

src: input zero-terminated locale encoded string.

lookupname: newly allocated output variable with name to lookup in DNS.

flags: 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 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

idn2_register_ul

Function: int idn2_register_ul (const char * ulabel, const char * alabel, char ** insertname, int flags)

ulabel: input zero-terminated locale encoded string, or NULL.

alabel: input zero-terminated ACE encoded string (xn–), or NULL.

insertname: newly allocated output variable with name to register in DNS.

flags: optional idn2_flags to modify behaviour.

Perform IDNA2008 register string conversion on domain label ulabel and alabel , as described in section 4 of RFC 5891. Note that the input ulabel 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.

It is recommended to supply both ulabel and alabel for better error checking, but supplying just one of them will work. Passing in only alabel is better than only ulabel . See RFC 5891 section 4 for more information.

After version 0.11: insertname may be NULL to test conversion of src without allocating memory.

Returns: On successful conversion IDN2_OK is returned, when the given ulabel and alabel does not match each other IDN2_UALABEL_MISMATCH is returned, when either of the input labels are too long IDN2_TOO_BIG_LABEL is returned, when alabel does does not appear to be a proper A-label IDN2_INVALID_ALABEL is returned, when ulabel locale to UTF-8 conversion failed IDN2_ICONV_FAIL is returned, or another error code is returned.

2.4 Control Flags

The flags parameter can take on the following values, or a bit-wise inclusive or of any subset of the parameters:

Global flag: idn2_flags IDN2_NFC_INPUT

Apply NFC normalization on input.

Global flag: idn2_flags IDN2_ALABEL_ROUNDTRIP

Apply additional round-trip conversion of A-label inputs.

Global flag: idn2_flags IDN2_TRANSITIONAL

Perform Unicode TR46 transitional processing.

Global flag: idn2_flags IDN2_NONTRANSITIONAL

Perform Unicode TR46 non-transitional processing (default).

Global flag: idn2_flags IDN2_NO_TR46

Disable any TR#46 transitional or non-transitional processing.

Global flag: idn2_flags IDN2_USE_STD3_ASCII_RULES

Use STD3 ASCII rules. This is a TR#46 flag and is a no-op when IDN2_NO_TR46 is specified.

2.5 Error Handling

idn2_strerror

Function: const char * idn2_strerror (int rc)

rc: return code from another libidn2 function.

Convert internal libidn2 error code to a humanly readable string. The returned pointer must not be de-allocated by the caller.

Return value: A humanly readable string describing error.

idn2_strerror_name

Function: const char * idn2_strerror_name (int rc)

rc: return code from another libidn2 function.

Convert internal libidn2 error code to a string corresponding to internal header file symbols. For example, idn2_strerror_name(IDN2_MALLOC) will return the string "IDN2_MALLOC".

The caller must not attempt to de-allocate the returned string.

Return value: A string corresponding to error code symbol.

2.6 Return Codes

The functions normally return 0 on success or a negative error code.

Return code: idn2_rc IDN2_OK

Successful return.

Return code: idn2_rc IDN2_MALLOC

Memory allocation error.

Return code: idn2_rc IDN2_NO_CODESET

Could not determine locale string encoding format.

Return code: idn2_rc IDN2_ICONV_FAIL

Could not transcode locale string to UTF-8.

Return code: idn2_rc IDN2_ENCODING_ERROR

Unicode data encoding error.

Return code: idn2_rc IDN2_NFC

Error normalizing string.

Return code: idn2_rc IDN2_PUNYCODE_BAD_INPUT

Punycode invalid input.

Return code: idn2_rc IDN2_PUNYCODE_BIG_OUTPUT

Punycode output buffer too small.

Return code: idn2_rc IDN2_PUNYCODE_OVERFLOW

Punycode conversion would overflow.

Return code: idn2_rc IDN2_TOO_BIG_DOMAIN

Domain name longer than 255 characters.

Return code: idn2_rc IDN2_TOO_BIG_LABEL

Domain label longer than 63 characters.

Return code: idn2_rc IDN2_INVALID_ALABEL

Input A-label is not valid.

Return code: idn2_rc IDN2_UALABEL_MISMATCH

Input A-label and U-label does not match.

Return code: idn2_rc IDN2_INVALID_FLAGS

Invalid combination of flags.

Return code: idn2_rc IDN2_NOT_NFC

String is not NFC.

Return code: idn2_rc IDN2_2HYPHEN

String has forbidden two hyphens.

Return code: idn2_rc IDN2_HYPHEN_STARTEND

String has forbidden starting/ending hyphen.

Return code: idn2_rc IDN2_LEADING_COMBINING

String has forbidden leading combining character.

Return code: idn2_rc IDN2_DISALLOWED

String has disallowed character.

Return code: idn2_rc IDN2_CONTEXTJ

String has forbidden context-j character.

Return code: idn2_rc IDN2_CONTEXTJ_NO_RULE

String has context-j character with no rull.

Return code: idn2_rc IDN2_CONTEXTO

String has forbidden context-o character.

Return code: idn2_rc IDN2_CONTEXTO_NO_RULE

String has context-o character with no rull.

Return code: idn2_rc IDN2_UNASSIGNED

String has forbidden unassigned character.

Return code: idn2_rc IDN2_BIDI

String has forbidden bi-directional properties.

Return code: idn2_rc IDN2_DOT_IN_LABEL

Label has forbidden dot (TR46).

Return code: idn2_rc IDN2_INVALID_TRANSITIONAL

Label has character forbidden in transitional mode (TR46).

Return code: idn2_rc IDN2_INVALID_NONTRANSITIONAL

Label has character forbidden in non-transitional mode (TR46).

2.7 Memory Handling

idn2_free

Function: void idn2_free (void * ptr)

ptr: pointer to deallocate

Call free(3) on the given pointer.

This function is typically only useful on systems where the library malloc heap is different from the library caller malloc heap, which happens on Windows when the library is a separate DLL.

2.8 Version Check

It is often desirable to check that the version of Libidn2 used is indeed one which fits all requirements. Even with binary compatibility new features may have been introduced but due to problem with the dynamic linker an old version is actually used. So you may want to check that the version is okay right after program startup.

idn2_check_version

Function: const char * idn2_check_version (const char * req_version)

req_version: version string to compare with, or NULL.

Check IDN2 library version. This function can also be used to read out the version of the library code used. See IDN2_VERSION for a suitable req_version string, it corresponds to the idn2.h header file version. Normally these two version numbers match, but if you are using an application built against an older libidn2 with a newer libidn2 shared library they will be different.

Return value: Check that the version of the library is at minimum the one given as a string in req_version and return the actual version string of the library; return NULL if the condition is not met. If NULL is passed to this function no check is done and only the version string is returned.

The normal way to use the function is to put something similar to the following first in your main:

  if (!idn2_check_version (IDN2_VERSION))
    {
      printf ("idn2_check_version() failed:\n"
              "Header file incompatible with shared library.\n");
      exit(EXIT_FAILURE);
    }

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