Stubs: A dynamic linking mechanism for Tcl, Tk and extensions. Georgios Petasis enthuses, "For me stubs is simply a brilliant feature of tcl."
What does it do ?
How does it do it ?
Basically the stub mechanism provides a cross-platform dynamic linking mechanism using tables of function pointers.
To see the differences take a look at the following which describes what happens when Tcl loads two different extensions.
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. (Backlinking). 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. a) Resolve any symbols which are defined in the new libraries. 3) Tcl then calls the extension's initialisation entry point.
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.
Levels of Stubs Support
There are two/three levels of stubs support.
Level 0 - The extension doesn't support stubs at all. On many platforms (but not all) this means it is bound to an exact version of Tcl, but it always means that you'll probably have to recompile to upgrade. - Lars H: Upgrade what? The extension or Tcl? (The latter is probably more annoying.)
Level 1 - The extension is stubs-enabled, using the stubs mechanism to dynamically bind to the Tcl (and Tk and other) library so that it can be loaded by any version of interpreter that supports the required version of API. Many many extensions provide this level of support (e.g. TclUDP) and this is the minimum level of support required for an extension to be supported inside a starkit.
Level 2 - The extension is a stubs-source and exports its own stubs table. This allows other code to dynamically bind to it. This is only useful where an extension provides its own meaningful C API. While this is theoretically separate from whether it is stubs-enabled, it is normally the case that all stubs-sources are also enabled. Tk and memchan are examples of stubs-sources.
Links
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...
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??
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 explaination
KBK: "Even in version lockstep, stubs can be handy if only to keep all of your libraries following the same linkage conventions."