Bounty program for improvements to Tcl and certain Tcl packages.
Author:         Jan Nijtmans <>
Author:         Jan Nijtmans <>
State:          Draft
Type:           Project
Vote:           Pending
Created:        23-Aug-2011
Discussions-To: Tcl Core list
Keywords:       Tcl
Tcl-Version:    8.7


This TIP proposes to add full support for all characters in Unicode 10.0+, inclusive the characters >= U+010000.


In order to extend the range of the characters to more than 16 bits, the type Tcl_UniChar is not big enough any more to hold all possible characters. Changing the type of Tcl_UniChar to a 32-bit quantity is not an option, as it will result in a binary API incompatibility.

The solution proposed in this TIP is to keep Tcl_UniChar a 16-bit quantity, but to increase the value of TCL_UTF_MAX to 4 (from 3). Any conversions from UTF-8 to Tcl_UniChar will convert any valid 4-byte UTF-8 sequence to a sequence of two Surrogate characters. All conversions from UTF-16 to UTF-8 will make sure that any High Surrogate immediately followed by a Low Surrogate will result in a single 4-byte UTF-8 character.

This can be done in a binary compatible way: No source code of existing extensions need to be modified. As long as no characters >= U+010000 or Surrogates are used, all functions provided by the Tcl library will function as before. There are few functions which currently return a value of type Tcl_UniChar, those will be modified to return an int in stead.


As Unicode 10.0, and future Unicode versions, will supply more and more characters outside the 16-bit range, it would be useful if Tcl supports that as well.


This document proposes:

  • Change the functions Tcl_UniCharToUtf and UnicodeToUtfProc such that when they are fed with a valid High Surrogate followed by a Low Surrogate, the result will be a single 4-byte UTF-8 character.

  • Change the functions Tcl_UtfToUniChar and UtfToUnicodeProc such that when they are fed with a valid 4-byte UTF-8 character, the first call will return a High Surrogate character, and the next call will return a Low Surrogate character.

  • The following functions, which currently return a Tcl_UniChar, will be changed to return an int instead:

    * Tcl_UniCharAtIndex

    * Tcl_UniCharToLower

    * Tcl_UniCharToTitle

    * Tcl_UniCharToUpper

    * Tcl_GetUniChar

  • Extend tclUniData.c to include all Unicode 10.0 characters up to U+02FA20. A special case will be made for the functions Tcl_UniCharIsGraph and Tcl_UniCharIsPrint for the characters in the range U+0E0100 - U+0E01EF, otherwise it would almost double the Unicode table size.

  • Extend the Tcl_UtfToUniChar function such that the invalid bytes 0x80 up to 0x9F are interpreted as the cp1252 characters € up to Ÿ instead of the (not-existing) Unicode characters U+0080 - U+009F. Since cp1252 is the mostly used Windows codepage, which is upwards compatible with ISO 8859-1, this adheres more to what's intuitively expected behavior. Many current UTF-8 converters nowadays use this. See also here

  • If Tcl is compiled with -DTCL_UTF_MAX=6, use a different TCL_STUB_MAGIC value. Since extensions compiled with -DTCL_UTF_MAX=6 are binary incompatible with normally-compiled Tcl, this causes extensions compiled with this same options no longer being loadable in normal Tcl and reverse. Note that TCL_UTF_MAX=6 compiles are still not officially supported, a lot of additional fixes are needed to make it work right.

  • Change the Windows-only functions Tcl_WinTchar2Utf and Tcl_WinUtf2TChar to using only the Win32 API functions in its implementation. This means that the Tcl encoding system no longer needs to be initialized in order for those functions to work. (This cannot be done with TCL_UTF_MAX=3, because the Windows API might produce 4-byte UTF-8 sequences, which Tcl cannot handle then)


As long as no Surrogates or characters >= U+010000 are used, all functions behave exactly the same as before. The only way that Tcl_UniCharToUtf can produce a 4-byte output is when Surrogates or characters >= U+010000 are used.

Extension that want to be compatible with any Tcl version, can include tcl.h as follows:

#define TCL_UTF_MAX 4
#include <tcl.h>

or they can call the C compiler with the additional argument -DTCL_UTF_MAX=4, in order to be sure that UTF-8 representations of length 4 can be handled. This way, the extension can be used with any Tcl version, whether it supports Surrogates or not.

Apart from this, it is advisable to initialize the variable where the chPtr argument from Tcl_UtfToUniChar points to, as this location is used to remember whether the High Surrogate is already produced or not. Not doing so when the first character of a string is a character > U+010000 might result in a Low Surrogate character only. This danger, however unlikely, only exists for the first character in a string, and it only occurs when the (random) value is exactly equal to the expected High Surrogate.


In the current implementation "string length \U10000" will return 2 in stead of the expected 1. The reason for this is that many internal operations convert the representation into UTF-16 format, which occupies two surrogate characters. This should change in future Tcl implementations. Correcting this would involve many internal changes, and the risk of introducing crashes because of miscounting bytes (as have been reported in the past, during the tip-389 branch implementation development). Tcl is not the only language doing it this way: javascript does it as well.

This also means that functions line "string index", "string range" and "string reverse" might give unexpected results when characters > U+010000 are involved. Any index pointing to the middle of a 'double-length' Unicode character will be handled as if the index points to just after the character instead. So:

In Tcl 8.7 with TIP #389:

% string length "a\U100000b"
% scan %c \U100000
1048576  -> (this is the correct Unicode character)
% string length [string index "a\U100000b" 1]
2        -> (the Unicode character has length 2)
% string length [string index "a\U100000b" 2]
0        -> (So we cannot access the lower surrogate separately)

So, the "string length" of a Unicode character >U+FFFF is 2, and if you try to split it in two separate characters that won't work: It will then be split in a character with length 2 (the original one) and another character with length 0 (the empty string).

Also note that the regexp engine still cannot handle Unicode characters >U+FFFF, this engine will handle those as if they consist of 2 separate characters.

Those caveats are planned to be handled in "part 2" (TIP #497)

Reference Implementation

A reference implementation is available in the tip-389 branch.


This document has been placed in the public domain.