6.4 Core Functions

The idea behind the IDNA function names are as follows: the idna_to_ascii_4i and idna_to_unicode_44i functions are the core IDNA primitives. The 4 indicate that the function takes UCS-4 strings (i.e., Unicode code points encoded in a 32-bit unsigned integer type) of the specified length. The i indicate that the data is written “inline” into the buffer. This means the caller is responsible for allocating (and de-allocating) the string, and providing the library with the allocated length of the string. The output length is written in the output length variable. The remaining functions all contain the z indicator, which means the strings are zero terminated. All output strings are allocated by the library, and must be de-allocated by the caller. The 4 indicator again means that the string is UCS-4, the 8 means the strings are UTF-8 and the l indicator means the strings are encoded in the encoding used by the current locale.

The functions provided are the following entry points:

idna_to_ascii_4i

Function: int idna_to_ascii_4i (const uint32_t * in, size_t inlen, char * out, int flags)

in: input array with unicode code points.

inlen: length of input array with unicode code points.

out: output zero terminated string that must have room for at least 63 characters plus the terminating zero.

flags: an Idna_flags value, e.g., IDNA_ALLOW_UNASSIGNED or IDNA_USE_STD3_ASCII_RULES.

The ToASCII operation takes a sequence of Unicode code points that make up one domain label and transforms it into a sequence of code points in the ASCII range (0..7F). If ToASCII succeeds, the original sequence and the resulting sequence are equivalent labels.

It is important to note that the ToASCII operation can fail. ToASCII fails if any step of it fails. If any step of the ToASCII operation fails on any label in a domain name, that domain name MUST NOT be used as an internationalized domain name. The method for deadling with this failure is application-specific.

The inputs to ToASCII are a sequence of code points, the AllowUnassigned flag, and the UseSTD3ASCIIRules flag. The output of ToASCII is either a sequence of ASCII code points or a failure condition.

ToASCII never alters a sequence of code points that are all in the ASCII range to begin with (although it could fail). Applying the ToASCII operation multiple times has exactly the same effect as applying it just once.

Return value: Returns 0 on success, or an Idna_rc error code.

idna_to_unicode_44i

Function: int idna_to_unicode_44i (const uint32_t * in, size_t inlen, uint32_t * out, size_t * outlen, int flags)

in: input array with unicode code points.

inlen: length of input array with unicode code points.

out: output array with unicode code points.

outlen: on input, maximum size of output array with unicode code points, on exit, actual size of output array with unicode code points.

flags: an Idna_flags value, e.g., IDNA_ALLOW_UNASSIGNED or IDNA_USE_STD3_ASCII_RULES.

The ToUnicode operation takes a sequence of Unicode code points that make up one domain label and returns a sequence of Unicode code points. If the input sequence is a label in ACE form, then the result is an equivalent internationalized label that is not in ACE form, otherwise the original sequence is returned unaltered.

ToUnicode never fails. If any step fails, then the original input sequence is returned immediately in that step.

The Punycode decoder can never output more code points than it inputs, but Nameprep can, and therefore ToUnicode can. Note that the number of octets needed to represent a sequence of code points depends on the particular character encoding used.

The inputs to ToUnicode are a sequence of code points, the AllowUnassigned flag, and the UseSTD3ASCIIRules flag. The output of ToUnicode is always a sequence of Unicode code points.

Return value: Returns Idna_rc error condition, but it must only be used for debugging purposes. The output buffer is always guaranteed to contain the correct data according to the specification (sans malloc induced errors). NB! This means that you normally ignore the return code from this function, as checking it means breaking the standard.

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