As approved in TIP 27 [http://purl.org/tcl/tip/27.html], several
of the public C functions provided by the Tcl and Tk libraries
have had their declarations augmented with the addition of
CONST modifiers on pointers.  If you have C code, either an
application or a package, that makes calls into the Tcl or Tk
libraries, you may find that this code that compiled against
the public header files (''tcl.h'' and ''tk.h'') of releases
8.3.* of Tcl and Tk will no longer compile against the public
header files of releases 8.4*.  This page is meant to guide
you through making the required changes to your code to 
restore the ability to compile with the new headers.

There are two classes of people who will confront this
problem.  First are those who have written their own programs
or libraries that link the Tcl/Tk libraries.  Second are
those who have acquired such source code from somebody else,
and just need to get it to compile.  The latter group is not
likely interested in the finer points of programming, or
arguments for/against these source incompatibilties.  They
just want to get a working compile.  So I address that
community first:

-----
'''Step by step: getting your code to compile against Tcl/Tk 8.4'''

----
Most stuff can be fixed by putting CONST (or, if you're unlucky, CONST84) in your types.  Sometimes you might need to add a cast.  Anything that traditionally worked, will continue to work.  There's also a symbol you can #define to switch off the constness in the interface (whose name I don't remember)

The define is '''USE_NON_CONST'''.

----
Is the new constness adding value?  If so, turning it off seems like it would defeat that added
value.  If the new constness is not adding value, then it would seem like turning it off by default would be as good a solution as having it turned on.

And if the new constness is adding value, then will automatically adding CONST to types cause any other problems?

----
[DGP] The answers are subtler than the sketch above implies.
I will get to this.  Please be patient.

For example '''USE_NON_CONST''' does ''not'' disable all the
TIP 27 changes.  It is a more careful, more useful migration
tool than that.

----
See also [Changes in Tcl/Tk 8.4]
----
See also [http://sourceforge.net/mailarchive/message.php?msg_id=1235712]
and [http://sourceforge.net/mailarchive/message.php?msg_id=1236136]
(and subsequent messages in the thread)
for a fairly detailed report of the TIP 27 impact on two extensions.

Summary: In one case, adding CONST84 in a few places did 90% of the job, a few minor changes to the code fixed the rest.  In another case, -DUSE_NON_CONST was necessary, since the extension receives data from parts of the Core that were ''not'' CONSTified and passes it on to other parts that were.
----