Check-in [c348a9d6eb]

<th>#</th>
<th>Type</th>
<th>Tcl Version</th>
<th>Status</th>
<th>Title</th>
</tr></thead><tbody>

<tr class='project projectdraft projectdraft87 project87'>
<td valign='top'><a href='./tip/512.md'>512</a></td>
<td valign='top'>Project</td>
<td valign='top'>8.7</td>
<td valign='top'>Draft</td>
<td valign='top'># TIP 512: No stub for Tcl_SetExitProc()</td>
</tr>
<tr class='project projectdraft projectdraft87 project87'>
<td valign='top'><a href='./tip/511.md'>511</a></td>
<td valign='top'>Project</td>
<td valign='top'>8.7</td>
<td valign='top'>Draft</td>
<td valign='top'># TIP 511: Implement Tcl_AsyncMarkFromSignal()</td>
</tr>

# TIP 512: No stub for Tcl_SetExitProc()
	Author:         Jan Nijtmans <[email protected]>
	State:          Draft
	Type:           Project
	Vote:           
	Created:        25-June-2018
	Post-History:   
	Keywords:       Tcl
	Tcl-Version:	8.7
-----

# Abstract

This TIP proposes to remove the stub entries for `Tcl_SetExitProc()`,
`Tcl_SetPanicProc()`, `Tcl_FindExecutable()` and (for Tk) `Tk_MainEx()`.
The normal exported symbols for those functions will be kept intact.

# Context

This TIP is inspired by Tcl commit ["Add custom exit procedure for
tcltests executable"](http://core.tcl.tk/tcl/info/6f650b4271a1ef2e)
(thanks to Poor Yorick!). This commit demonstrates exactly what's the problem
when using `Tcl_SetExitProc()` in a stub-enabled extension.

# Rationale

When running test-cases in Windows (using mingw-64 or msvc, but using the
win/Makefile system not the nmake system), the tcltest executable
crashes after shutdown with some tests. The reason for this is that
the additional test-code "tcltest" is compiled as stub-enabled extension
which is dynamically loaded into tclsh. So, there is no "tcltest"
executable in this environment, all there is is tclsh86.exe and
tcltest86.dll.

If a stub-enabled extension registers an Exit-handler, the following
happens at exit time. During the clean-up, Tcl unloads all extensions.
When the clean-up is done, the last step is calling the registered
Exit-handler. But ... since the exit handler function was located
inside the dll, it's address space is gone already. This results
in the mentioned crash.

The same problem exists in `Tcl_SetPanicProc()`: If a stub-enabled
extension registers a Panic proc which is located in its own
address space, the extension might be gone when the panicproc is
finally called.

`Tcl_FindExecutable()` and `Tk_MainEx()` don't register functions,
but those functions - too - are meant to be used at application
level, not at extension level. Therefore those don't belong in
the stub table either.

# Specification

The above mentioned functions are marked with _nostub_, and the
tools/genStubs.tcl tooling is adapted to recognize this special
keyword. It means that the function entry in the stub table is
marked as deprecated, but the normal function entry is leaf as-is.

When Tcl is compiled with -DTCL\_NO\_DEPRECATED, the stub entries for
the above functions will no longer be available. The stub table
entries for those function will be filled with 0 instead.

Starting with Tcl 9.0, those stub entries will be removed completely.
In stead, those functions will be defined in tcl.h, as normal
exported symbols, just like `Tcl_MainEx()` (for example).

# Implementation

Currently, the proposed implementation is available in [tip-512 branch]
(https://core.tcl.tk/tk/timeline?r=tip-512).

# Copyright

This document has been placed in the public domain.