# TIP 664: Enable compiler warnings for missing int → Tcl_Size conversions
	Author:		Jan Nijtmans <jan.nijtmans@gmail.com>
	State:		Final
	Type:		Project
	Vote:		Done
	Created:	02-05-2023
	Tcl-Version:	9.0
	Tcl-Branch:	tip-664
	Vote-Summary:	Accepted 7/0/0
	Votes-For:	AF, AK, DKF, JN, KW, MC, SL
	Votes-Against:	none
	Votes-Present:	none
-----

# Abstract

The TIPs [481](https://core.tcl-lang.org/tips/doc/trunk/tip/481.md)
and [616](https://core.tcl-lang.org/tips/doc/trunk/tip/616.md)
introduced macro constructs which hide the C API changes from Tcl 8.6
to Tcl 9 and disable compiler warnings. This tip proposes to enable
those compiler warnings, as a help for people who want to make their
extension fully capable of handling strings > 2^31.

If users want to disable this warning they can do that by using
their normal compiler options, e.g. (for clang/gcc):
<pre>
    configure CFLAGS=-Wno-incompatible-pointer-types
</pre>
Users disabling/ignoring this warning are not harmed in any way,
although it is not recommended. This should encourage, rather
than force, extension writers to update their code.

All _battery-included_ extensions (`Itcl`, `tdbc*`, `thread`) are
already converted to use `Tcl_Size` everywhere necessary.

# Rationale

When TIPs [481](https://core.tcl-lang.org/tips/doc/trunk/tip/481.md)
and [616](https://core.tcl-lang.org/tips/doc/trunk/tip/616.md) were
implemented/proposed, the `Tcl_Size` type didn't exist yet. The
main goal for Tcl 9.0 was - at that time - to keep everything
source-compatible as much as possible.
That means that extensions using `Tcl_GetStringFromObj(obj, &len)`
- where `len` points to an `int` variable - should keep working as-is.

Currently, extension-writers are starting to make their extensions
Tcl-9-ready, which means that they are able to use string/list lengths
> 2^31. That means the `int len` variable has to be replaced
by a `Tcl_Size len` variable. But Tcl doesn't provide any
help finding those instances. This TIP makes it much easier
for extension-writers to find those places in their code,
without any functional disadvantage: If the compiler warning
is ignored, still the extension will work as expected as
long as no strings/lists exceed the 2^31 limit.

# Implementation

Implementation is in [Tcl branch "tip-664"](https://core.tcl-lang.org/tcl/timeline?r=tip-664).

It only removes a lot of type-casts from `tclDecls.h`, which were
originally put there just to prevent this compiler-warning. There
is no change in implementation of any functions, so the only
effect of this proposal is to enable compiler-warnings for this
specific case, nothing else.

# Compatibility

If an extension which is originally written for Tcl 8.6 is built
with Tcl 9 and the compiler warnings are ignored by the developer,
and no strings or lists are used larger than 2^31 in size,
everything will function as before. Any use of strings or lists
larger then that in the extension will result in an error-message,
wherever possible. Only  `Tcl_GetStringFromObj()` and `Tcl_GetUnicodeFromObj()`
are not capable of returning an error-message, they will panic (as they do now).

# Copyright

This document has been placed in the public domain.
