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5 Punycode Functions

Punycode is a simple and efficient transfer encoding syntax designed for use with Internationalized Domain Names in Applications. It uniquely and reversibly transforms a Unicode string into an ASCII string. ASCII characters in the Unicode string are represented literally, and non-ASCII characters are represented by ASCII characters that are allowed in host name labels (letters, digits, and hyphens). A general algorithm called Bootstring allows a string of basic code points to uniquely represent any string of code points drawn from a larger set. Punycode is an instance of Bootstring that uses particular parameter values, appropriate for IDNA.

5.1 Header file punycode.h

To use the functions explained in this chapter, you need to include the file punycode.h using: 

     #include <punycode.h>

5.2 Unicode Code Point Data Type

The punycode function uses a special type to denote Unicode code points. It is guaranteed to always be a 32 bit unsigned integer.

— Punycode Unicode code point: uint32_t punycode_uint

A unsigned integer that hold Unicode code points.

5.3 Core Functions

Note that the current implementation will fail if the input_length exceed 4294967295 (the size of punycode_uint). This restriction may be removed in the future. Meanwhile applications are encouraged to not depend on this problem, and use sizeof to initialize input_length and output_length.

The functions provided are the following two entry points:

punycode_encode

— Function: int punycode_encode (size_t input_length, const punycode_uint [] input, const unsigned char [] case_flags, size_t * output_length, char [] output)

input_length: The number of code points in the input array and the number of flags in the case_flags array.

input: An array of code points. They are presumed to be Unicode code points, but that is not strictly REQUIRED. The array contains code points, not code units. UTF-16 uses code units D800 through DFFF to refer to code points 10000..10FFFF. The code points D800..DFFF do not occur in any valid Unicode string. The code points that can occur in Unicode strings (0..D7FF and E000..10FFFF) are also called Unicode scalar values.

case_flags: A NULL pointer or an array of boolean values parallel to the input array. Nonzero (true, flagged) suggests that the corresponding Unicode character be forced to uppercase after being decoded (if possible), and zero (false, unflagged) suggests that it be forced to lowercase (if possible). ASCII code points (0..7F) are encoded literally, except that ASCII letters are forced to uppercase or lowercase according to the corresponding case flags. If case_flags is a NULL pointer then ASCII letters are left as they are, and other code points are treated as unflagged.

output_length: The caller passes in the maximum number of ASCII code points that it can receive. On successful return it will contain the number of ASCII code points actually output.

output: An array of ASCII code points. It is *not* null-terminated; it will contain zeros if and only if the input contains zeros. (Of course the caller can leave room for a terminator and add one if needed.)

Converts a sequence of code points (presumed to be Unicode code points) to Punycode.

Return value: The return value can be any of the Punycode_status values defined above except PUNYCODE_BAD_INPUT. If not PUNYCODE_SUCCESS, then output_size and output might contain garbage.

punycode_decode

— Function: int punycode_decode (size_t input_length, const char [] input, size_t * output_length, punycode_uint [] output, unsigned char [] case_flags)

input_length: The number of ASCII code points in the input array.

input: An array of ASCII code points (0..7F).

output_length: The caller passes in the maximum number of code points that it can receive into the output array (which is also the maximum number of flags that it can receive into the case_flags array, if case_flags is not a NULL pointer). On successful return it will contain the number of code points actually output (which is also the number of flags actually output, if case_flags is not a null pointer). The decoder will never need to output more code points than the number of ASCII code points in the input, because of the way the encoding is defined. The number of code points output cannot exceed the maximum possible value of a punycode_uint, even if the supplied output_length is greater than that.

output: An array of code points like the input argument of punycode_encode() (see above).

case_flags: A NULL pointer (if the flags are not needed by the caller) or an array of boolean values parallel to the output array. Nonzero (true, flagged) suggests that the corresponding Unicode character be forced to uppercase by the caller (if possible), and zero (false, unflagged) suggests that it be forced to lowercase (if possible). ASCII code points (0..7F) are output already in the proper case, but their flags will be set appropriately so that applying the flags would be harmless.

Converts Punycode to a sequence of code points (presumed to be Unicode code points).

Return value: The return value can be any of the Punycode_status values defined above. If not PUNYCODE_SUCCESS, then output_length, output, and case_flags might contain garbage.

5.4 Error Handling

punycode_strerror

— Function: const char * punycode_strerror (Punycode_status rc)

rc: an Punycode_status return code.

Convert a return code integer to a text string. This string can be used to output a diagnostic message to the user.

PUNYCODE_SUCCESS: Successful operation. This value is guaranteed to always be zero, the remaining ones are only guaranteed to hold non-zero values, for logical comparison purposes.

PUNYCODE_BAD_INPUT: Input is invalid.

PUNYCODE_BIG_OUTPUT: Output would exceed the space provided.

PUNYCODE_OVERFLOW: Input needs wider integers to process.

Return value: Returns a pointer to a statically allocated string containing a description of the error with the return code rc.