Version 0 of Stubs

Updated 1999-03-10 16:35:14

Stubs: A dynamic linking mechanism for Tcl, Tk and extensions.

What does it do ?

1. Adds support for backlinking to all platforms, thus making it possible for static executables to dynamically load extensions on any platform.
2. Eliminates all the operating system specific problems associated with dynamically linking an extension to Tcl/Tk and any other library which exports a stub interface. This may sound like a wild claim but it is true simply because the operating system does not have to do anything.
3. 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.
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.

How does it do it ?

Basically the stub mechanism provides a cross platform dynamic linking mechanism using tables of function pointers.

• A library which wishes to provide a stub interface populates a function table with the addresses of its interface functions.
• A pointer to that table is then made available to any extension which needs to access that library's interface.
• Once an extension has obtained the pointer it simply uses it to get the address of the function it wants to call and calls it.
• At no time in this process does the operating system become involved.

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. (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.
         3) 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.