Check-in [ee6f77054c]

{"tip": {
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<th>Type</th>
<th>Tcl Version</th>
<th>Status</th>
<th>Title</th>
<th>Impl.</th>
</tr></thead><tbody>

<tr class='project projectdraft'>
<td valign='top'><a href='./tip/528.md'>528</a></td>
<td valign='top' colspan=2>Project</td>
<td valign='top'>Draft</td>
<td valign='top'># TIP 528: Deprecate Tk_Offset()</td>
<td valign='top'><a href='/tk/timeline?r=offsetof'>Link</a></td>
</tr>
<tr class='project projectdraft projectdraft85 project85'>
<td valign='top'><a href='./tip/527.md'>527</a></td>
<td valign='top'>Project</td>
<td valign='top'>8.5</td>
<td valign='top'>Draft</td>
<td valign='top'># TIP 527: New measurement facilities in TCL: New command timerate.</td>
<td valign='top'><a href='/tcl/timeline?r=sebres-8-6-timerate'>Link</a></td>

# TIP 528: Deprecate Tk_Offset()
	Author:         Jan Nijtmans <[email protected]>
	State:          Draft
	Type:           Project
	Vote:           Pending
	Created:        05-Dec-2018
	Post-History:   
	Tk-Version:    8.7
	Tk-Branch:     offsetof
-----

# Abstract

This TIP proposes to deprecate `Tk_Offset()` in favour of [`offsetof()`](https://en.wikipedia.org/wiki/Offsetof).

# Rationale

The `Tk_Offset()` macro comes from the past, when C's official [`offsetof()`](https://en.wikipedia.org/wiki/Offsetof)
macro didn't exist yet, or was not supported yet by all compilers.

The main difference between `Tk_Offset()` and [`offsetof()`](https://en.wikipedia.org/wiki/Offsetof)
is the type: `Tk_Offset()` gives an `int` while `offsetof()` gives a `size_t`. `size_t` is actually
more correct, because it cannot become negative and it allows for values >= 4Gb on 64-bit platforms.

# Proposed Change

This TIP proposes to deprecate the `Tk_Offset()` macro. Starting with
Tk 8.7, when compiling an extension with the flag `-DTK_NO_DEPRECATED`,
`Tk_Offset()` will no longer be provided by tk.h. All occurrences
of `Tk_Offset()` within Tk itself are replaced by `offsetof()`.

This TIP doesn't propose to remove `Tk_Offset()` altogether, since there
are too many extensions still using it.

Further on, it makes sense to change various struct member types from
`int` to `size_t`, to make room for this enhancements. For reasons
of upwards compatibility, providing a smooth upgrade path, when
compiling Tk against Tcl 8.x headers, the type of those struct
members will continue to be `int`.

While on it, this TIP proposes to change the `src` and `dst` members
of `Tk_ArgvInfo` from `char *` to `void *`. Although unrelated to
the other proposed changes, this allows various type casts to
be eliminated in the source code of Tk (and possibly in extensions).
Since those fields are used to store untyped pointers, this is
actually a bug-fix which should not need a TIP, but since it affects
the documentation it doesn't hurt to provide this change in a TIP anyway.

An implementation of this TIP can be found in the [\[offsetof\]](https://core.tcl.tk/tk/timeline?r=offsetof) branch.

# Copyright

This document has been placed in the public domain.