TIP #340: Const Qualification of Tcl_SetResult's Argument
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TIP:	340
Title:	Const Qualification of Tcl_SetResult's Argument
Version:	$Revision: 1.8 $
Authors:	Jan Nijtmans <nijtmans@users.sourceforge.net>
State:	Withdrawn
Type:	Project
Tcl-Version:	8.7
Vote:	Pending
Created:	Fri Nov 14 00:00:00 UTC 2008
Keywords:	Tcl_SetResult
Abstract
As a follow-up of [27], in Tcl 8.6 and Tk 8.6 much work has been done to clean up the remaining places where pointers were not const qualified. The question is, how can we prevent that in the future similar "mistakes" are made. The gcc compiler warning -Wwrite-strings helps in that, therefore this TIP proposes to add that to the CFLAGS_WARNING flag in Tcl and Tk when using gcc. But for this flag to be introduced, all warnings will have to be eliminated. In the HEAD, this is done already, except for one function: Tcl_SetResult. This function is explicitely mentioned in [27] not to be modified, because it cannot be handled without unsafe casting. This TIP proposes to deprecate Tcl_SetResult in full, and provide a new macro Tcl_SetStringResult in its place.
Rationale
The gcc manual mentions for the flag -Wwrite-strings:
When compiling C, give string constants the type const char[length] so that copying the address of one into a non-const char * pointer will get a warning .... These warnings will help you find at compile time code that can try to write into a string constant, but only if you have been very careful about using const in declarations and prototypes. Otherwise, it will just be a nuisance; this is why we did not make -Wall request these warnings.
Now that all Tcl and Tk API's are modified to be very careful about using const, this opens the way to add -Wwrite-strings to CFLAGS_WARNING when building with gcc. Other extensions can start to do the same, if they want to find out about this type of potential problem.
When considering the elimination of the warning when using 'Tcl_SetResult, I originally see two alternatives:
Change the implementation such that Tcl_SetResult(i, s, f) does the same as Tcl_SetObjResult(i, Tcl_NewStringObj(s, -1)), ignoring the last parameter.
This can be done without an unsafe type cast, but it has the disadvantage that after a (modified) Tcl_SetResult call the result is no longer available in interp->result. Older extensions might expect that, those will be silently broken. A the moment, serveral tests fail when doing this, because in various places of the Tcl core there are hack which still support older extensions which still use interp->result directly. It's a little short before Tcl 8.6 to do it now, but it should certainly be considered for the future.
The solution that originally was proposed in this TIP was to leave the Tcl_SetResult implementation as it is, only add a single type cast to prevent a gcc warning.
This violates the [27] conditions, but is in fact not more dangerous than the current situation. It is only dangerous, when the Tcl_SetResult call has another value than TCL_STATIC or TCL_VOLATILE as last argument
The final proposal is a new macro Tcl_SetStringResult that takes over the function of Tcl_SetResult. The function Tcl_SetResult will be deprecated in full.
There has been a discussion stating that changing the Tcl_SetResult signature is wrong, because Tcl_SetResult cannot be made const-correct.
Most Tcl_SetResult calls use TCL_STATIC or TCL_VOLATILE as last argument. In this case, the second argument is expected to be a const. The macro Tcl_SetStringResult takes care of that, since it is implemented in terms of Tcl_SetObjResult and Tcl_NewStringObj.
Very few Tcl_SetResult calls have some other value as last argument, most likely TCL_DYNAMIC. This TIP proposes to deprecate Tcl_SetResult for all values of freeProc. If the value is TCL_STATIC or TCL_VOLATILE, there is a new macro Tcl_SetStringResult which can be used in stead. For other values the call can be replaced with Tcl_SetStringResult as well, but then the caller is responsible to free the memory after the Tcl_SetStringResult call.
It turns out that Tcl had only 4 deprecated (as defined by this TIP) Tcl_SetResult calls, one of them was wrong [[Bug 2308236]], two of them were in tclTest.c meant to test the Tcl_SetResult function itself. Tk had only 5 such calls. All those calls have been modified now. Tcl and Tk now only calls Tcl_SetResult with either TCL_STATIC or TCL_VOLATILE
This proposal does not have the "forward compatibility" problem, that extensions using Tcl_SetStringResult compiled. it is even possible for extensions to use Tcl_SetStringResult with Tcl 8.5 and before:
 #ifndef
 #   define Tcl_SetStringResult(i,s) \par     Tcl_SetObjResult(i, Tcl_NewStringObj(s, -1))
 #endif
Reference Implementation
A new patch will be available in issue #2315890 [http://sourceforge.net/support/tracker.php/?aid=2315890].
Copyright
This document has been placed in the public domain.
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