Version 43 of Stubs

Updated 2016-08-15 04:45:01 by Rachel

Stubs are used to avoid having a program that embeds Tcl, or a shared library that extends Tcl be linked to the Tcl shared library.

Recommendations

For me stubs is simply a brilliant feature of tcl.
Georgios Petasis

Description

The stubs mechanism provides a cross-platform dynamic linking mechanism using tables of function pointers. Tcl, Tk, and other extensions and libraries each provide a table of pointers to their interface functions. The client code then obtains a pointer to this table and uses it to look up anc call functions.

When an extension is linked against the Tcl shared library to produce an extension shared library, the runtime linker is then responsible for linking in the Tcl shared library when the exension shared library is loaded. A particular name and version may be recorded in the shared library at when the shared library is created, which can then be problematic when distributing the extension shared library to other parties who bring their own Tcl shared library.

Instead, the extension library defines USE_TCL_STUBS and includes tcl.h, which redefines all Tcl_* function calls as lookups into the Tcl stubs table. The extension library is then linked to the Tcl stubs static library, which gives the extension library a public function called Tcl_InitStubs. The extension also makes a public function, usually called Extensionname_Init available. When Tcl loads the extension shared library, it calls this initialisation function, passing it a handle to the interpreter, This initialisation function then calls Tcl_InitStubs, which gets the stubs table from the interpreter and makes it available to the extension.

This approach achieves the following things:

  1. Adds support for backlinking (resolving symbols in the loaded library to the the loading library) to all platforms, thus making it possible for static executables to dynamically load extensions on any platform.
  2. Minimises the involvement of the operating system's runtime linker, which provides a more consistent procedure for loading an extension, or any other library which exports a stub interface.
  3. Avoids symbol pollution, which broadens the range of Extensions and libraries built with different compilers will work together even if it is not possible to link them normally. The only requirement for them to be able to work together is that function calls are compatible. PYK 2015-12-20: Can someone provide details on what this actually means? What is it that would make libraries incompatible otherwise? Symbol collision?
  4. As extensions do not have a hard coded reference to the library it is possible to use them with any library which is compatible. e.g. an extension which was originally built for Tcl 8.0 should work with Tcl 8.1, and an extension built for Tcl 8.1 could work with Tcl 8.0.

A shared object that doesn't use stubs to call interface functions of other libraries such as Tcl, but instead is directly linked to those libraries when it is created, it is bound to those libraries by a particular name and version, and it will be necessary to make sure parties to whom the shared object is distributed provide compatible libraries. It will probably also be necessary recompile such a shared object when switching to newer version of the shared libraries it depends on.

A Tcl extension that uses stubs to call into the Tcl or Tk C API can be loaded by any version of Tcl that supports the API required by the extension. Many extensions use stubs in this way, and any extension that is included in starkit must use stubs.

If it provides a C API, an extension (or other library) may also provide a stubs table of its own. This allows other code to dynamically bind to it. For example, TclOO provides a stubs table that other Tcl extensions can use. TclOO, Tk, and memchan are all examples of libraries that both use stubs tables and provide stubs tables.

Compare and Contrast

To see the differences take a look at the following which describes what happens when Tcl loads two different extensions.

Loading an extension which is linked directly to Tcl:

    1. Tcl asks the operating system to load the extension.
    2. The operating system loads the extension and then tries to resolve any undefined symbols. This process is very operating system dependent but involves one or more of the following steps.
      1. Resolve any symbols which are defined in the current process context.
      2. Find and load any libraries that the extension is dependent on. This involves searching paths defined through a variety of operating system dependent methods.
        1. Resolve any symbols which are defined in the new libraries.
      3. Tcl then calls the extension's initialisation entry point.

Loading an extension which uses Tcl's stub interface:

    1. Tcl asks the operating system to load the extension.
    2. The operating system does so, resolving any non Tcl symbols.
    3. Tcl calls the extension's initialisation entry point.
    4. The extension obtains the pointer(s) to Tcl's stub table(s) and uses that to call Tcl.

See Also

Writing extensions for stubs and pre-stubs Tcl
Extension Stubs Tables
How to provide a stubs table.
How to Use the Tcl Stubs Library
InitStubs TkInitStubs
the important man pages.
Tk ticket 1716117 , improve genstubs/checkstubs
Tcl ticket 1819422 , tclStubsPtr out of libtcl
Tk ticket 1920030 , Ttk stubs table

Who's responsible for the Stubs implementation?

Perhaps Jean-Claude Wippler suggested it to Paul Duffin in 1999, and Paul and Jan Nijtmans, with whom Jean-Claude had also been discussing ideas, implemented it in 2000. Others involved in the first generation were ??? (backlinking details) ...

DKF: I certainly remember suggesting something like the mechanism we ended up with back before Paul produced his initial version, though his initial version was a lot more elegant...

Questions

How can I tell if a binary extension was built with Stubs?

Under Linux and similer Unixes, check with ldd. No Tcl or Tk library references should appear. For Windows, do the same with "dumpbin /dependents".

Where is it most important to use Stubs?

Well, one place is that development against a Tclkit/Starkit/Starpack environment, extensions that are not built using Stubs result in more difficulty. (Or is it actually that they cannot be used at all?)

What are my options if the extension I want to use isn't Stubs-compatible?

Is all this stubs stuff necessary? Why does the build system have to be so difficult. Believe us: this is far less difficult than the alternative. Yes, libtool and autoconf are maddening, but there are things outside the Tcl world that demonstrate we don't have it so bad. DKF briefly defends stubs in a follow-up [L1 ] to a lengthy thread that itself touches several aspects of generation.


Helmut Giese notes that, even when compiler providers agree on object formats, they might still construct libraries in incompatible ways. In particular, under Windows, those who choose to work with gcc-based compilation might need to rebuild tclstub84.lib before Stubs-enabled extensions load correctly.


Georgios Petasis and Michael Schlenker observe that Stubs-less languages need "batteries included'', because reliance on version-specific extensions would otherwise be prohibitively onerous: "the C interface of these languages seems so primitive that you have to recompile everything each time a new version is out."

Phils take on stubs Stubs - Another explanation


KBK: "Even in version lockstep, stubs can be handy if only to keep all of your libraries following the same linkage conventions."


hat0 Please note that, in building a cross-platform stub-enabled extension, that Windows may require this modification to your C code's declaration:

- int Extension_Init(Tcl_Interp *interp)
+ int DLLEXPORT Extension_Init(Tcl_Interp *interp)

Without this, linking may fail in curious ways, with errors such like:

Cannot export ??_C@_03KBFG@Tcl?$AA@: symbol not found
Cannot export ??_C@_03PFGM@?$CJ?3?5?$AA@: symbol not found
Cannot export ??_C@_0BC@JJNH@?0?5actual?5version?5?$AA@: symbol not found