Tkinter: incompatible pointer types warnings when built with Tcl 9

[chrstphrchvz]

Tcl/Tk 9 changes C APIs to use a different type for sizes to allow transferring ≥32-bit sized things. The new type is Tcl_Size, which as of TIP 660 is defined as either ptrdiff_t in Tcl 9, or int in Tcl 8.7 for binary compatibility.

My impression is that Tcl/Tk wrappers such as Tkinter, which are primarily used for Tk GUI, have little if any need for this. But as of TIP 664, usage of APIs which previously expected int * must be updated for Tcl 9. There was previously effort in Tcl to continue allowing int * usage for compatibility, and maybe that would have been good enough for Tkinter, but others in the Tcl/Tk inner circle (who already wish to abandon Tcl/Tk < 9 entirely) rejected that approach. -Wincompatible-pointer-types warnings now seen under Tcl 9:

./Modules/_tkinter.c:504:21: warning: incompatible pointer types passing 'int *' to parameter of type 'Tcl_Size *' (aka 'long *') [-Wincompatible-pointer-types]
  504 |     const char *s = Tcl_GetStringFromObj(value, &len);
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
tcl90/include/tclDecls.h:4235:36: note: expanded from macro 'Tcl_GetStringFromObj'
 4235 |                 (Tcl_GetStringFromObj)((objPtr), (sizePtr)))
      |                                                  ^~~~~~~~~
tcl90/include/tclDecls.h:1754:15: note: passing argument to parameter 'lengthPtr' here
 1754 |                                 Tcl_Size *lengthPtr);
      |                                           ^
./Modules/_tkinter.c:1138:29: warning: incompatible pointer types passing 'int *' to parameter of type 'Tcl_Size *' (aka 'long *') [-Wincompatible-pointer-types]
 1138 |         char *data = (char*)Tcl_GetByteArrayFromObj(value, &size);
      |                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
tcl90/include/tclDecls.h:4229:41: note: expanded from macro 'Tcl_GetByteArrayFromObj'
 4229 |                 (Tcl_GetBytesFromObj)(NULL, (objPtr), (sizePtr)))
      |                                                       ^~~~~~~~~
tcl90/include/tclDecls.h:1751:32: note: passing argument to parameter 'numBytesPtr' here
 1751 |                                 Tcl_Obj *objPtr, Tcl_Size *numBytesPtr);
      |                                                            ^
./Modules/_tkinter.c:1168:18: warning: incompatible pointer types passing 'int *' to parameter of type 'Tcl_Size *' (aka 'long *') [-Wincompatible-pointer-types]
 1168 |         status = Tcl_ListObjLength(interp, value, &size);
      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
tcl90/include/tclDecls.h:4244:44: note: expanded from macro 'Tcl_ListObjLength'
 4244 |                 (Tcl_ListObjLength)((interp), (listPtr), (lengthPtr)))
      |                                                          ^~~~~~~~~~~
tcl90/include/tclDecls.h:1790:33: note: passing argument to parameter 'lengthPtr' here
 1790 |                                 Tcl_Obj *listPtr, Tcl_Size *lengthPtr);
      |                                                             ^
./Modules/_tkinter.c:2104:13: warning: incompatible pointer types passing 'int *' to parameter of type 'Tcl_Size *' (aka 'long *') [-Wincompatible-pointer-types]
 2104 |         if (Tcl_ListObjGetElements(Tkapp_Interp(self),
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 2105 |                                    ((PyTclObject*)arg)->value,
      |                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 2106 |                                    &objc, &objv) == TCL_ERROR) {
      |                                    ~~~~~~~~~~~~~
tcl90/include/tclDecls.h:4241:49: note: expanded from macro 'Tcl_ListObjGetElements'
 4241 |                 (Tcl_ListObjGetElements)((interp), (listPtr), (objcPtr), (objvPtr)))
      |                                                               ^~~~~~~~~
tcl90/include/tclDecls.h:1786:33: note: passing argument to parameter 'objcPtr' here
 1786 |                                 Tcl_Obj *listPtr, Tcl_Size *objcPtr,
      |                                                             ^
./Modules/_tkinter.c:2136:9: warning: incompatible pointer types passing 'int *' to parameter of type 'Tcl_Size *' (aka 'long *') [-Wincompatible-pointer-types]
 2136 |     if (Tcl_SplitList(Tkapp_Interp(self), list,
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 2137 |                       &argc, &argv) == TCL_ERROR)  {
      |                       ~~~~~~~~~~~~~
tcl90/include/tclDecls.h:4250:40: note: expanded from macro 'Tcl_SplitList'
 4250 |                 (Tcl_SplitList)((interp), (listStr), (argcPtr), (argvPtr)))
      |                                                      ^~~~~~~~~
tcl90/include/tclDecls.h:1796:36: note: passing argument to parameter 'argcPtr' here
 1796 |                                 const char *listStr, Tcl_Size *argcPtr,
      |                                                                ^
5 warnings generated.

Code intending to remain compatible with Tcl 8.6 is suggested to use the following after including tcl.h:

#ifndef TCL_SIZE_MAX
typedef int Tcl_Size;
# define Tcl_GetSizeIntFromObj Tcl_GetIntFromObj
# define Tcl_NewSizeIntObj Tcl_NewIntObj
# define TCL_SIZE_MAX      INT_MAX
# define TCL_SIZE_MODIFIER ""
#endif

I intend to open a PR updating _tkinter.c to pass Tcl_Size * where needed.

There are several other instances where Tkinter assumes Tcl expectsint or something not larger than INT_MAX, but migrating those to Tcl_Size and TCL_SIZE_MAX seems optional.

Linked PRs

• gh-112672: use Tcl_Size to avoid incompatible pointer type warnings #112681
• [3.13] gh-112672: Fix builtin Tkinter with Tcl 9.0 (GH-112681) #120208
• [3.12] gh-112672: Fix builtin Tkinter with Tcl 9.0 (GH-112681) #120209

[chrstphrchvz]

Code intending to remain compatible with Tcl 8.6 is suggested to use the following after including tcl.h:

#ifndef TCL_SIZE_MAX
typedef int Tcl_Size;
# define Tcl_GetSizeIntFromObj Tcl_GetIntFromObj
# define Tcl_NewSizeIntObj Tcl_NewIntObj
# define TCL_SIZE_MAX      INT_MAX
# define TCL_SIZE_MODIFIER ""
#endif

There is ongoing debate upstream about what is actually needed here, most recently in https://sourceforge.net/p/tcl/mailman/tcl-core/thread/74564788-7d69-4c42-9205-c5f3e6a1409d%40elmicron.de/
Some want Tcl_Size to be a macro and not a typedef. Some do not want to force downstream projects/users to need “boilerplate” for 8.6 compatibility. Others do not care about Tcl/Tk < 9.0 and will gladly pass the buck downstream.

[serhiy-storchaka]

In future we may change more code to support longer data in Python->Tcl conversion. But this requires testing, because not always the library supports larger than 2GiB data even if the size type is 64-bit (for example SQLite has 32-bit limitation even in its 64-bit API),