As approved in TIP 27 [L1 ], 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 ...while being as lazy as possible...
Step 1: Upgrade to Tcl/Tk 8.4b2.
The whole task is much easier with the very latest Tcl/Tk.
Step 2: Get updated code.
OK, so you have code that doesn't compile with 8.4 headers. Check back where you got it and see if an updated release is available that will compile. Better to get the fixes from the author/vendor than have to improvise some yourself.
Step 3: Define USE_NON_CONST
OK, so there are no updates available. The incompatibilities between 8.3 and 8.4 headers are completely removed if the symbol USE_NON_CONST is defined when the header files are included. If you have code that compiled with 8.3 headers, defining that symbol will make the same code compile with the 8.4 headers.
The simplest way to do this is to add -DUSE_NON_CONST to the CFLAGS definition during the compile, but that assumes a conventional Makefile. The surest way to do this is to find all the instances of
#include <tcl.h>
in the source code and insert
#define USE_NON_CONST
before them.
Step 4: There is no Step 4
That's it! It should work. If not, turn to the provider of the broken package for more help.
CONST migration for package authors
See also Changes in Tcl/Tk 8.4
See also [L2 ] and [L3 ] (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.