Dynamically links a code library (typically written in [C], [C++], or some comparable language) with the program containing the Tcl interpreter. Most extensions use this mechanism.

    :   '''load''' ''fileName'' ?''packageName''? ?''interp''?

http://www.purl.org/tcl/home/man/tcl8.5/TclCmd/load.htm 

If ''fileName'' does not contain any directory separators, the library will be searched for in OS-specific ways. If ''packageName'' is not provided, it is derived from the ''fileName'' by trimming certain prefixes and suffixes and applying capitalization rules (so `/usr/local/lib/libfoobar.so` would become `Foobar`); it is used to locate the name of the initialization function inside the library. If ''interp'' is not supplied, the library is initialized for the current interpreter.

<<discussion>>

   * What does one need to create an extension to be loaded?

   * What does one need to load Tk?


----
[Clif Flynt] has made available a useful C extension to load shared 
libraries that don't know anything about tcl's _Init function.

The extension allows one  to invoke dlopen
to load libraries that do not have Tcl entry points (for use by other
things that do have legitimate foo_Init entry points).

The dlopen extension is available at:

http://noucorp.com/cgi-bin/noucorp/generic.tcl?dir=/var/www/html/tcl/utilities

It's a minimal, single C file extension, but it has been known to work on both
Windows and Posix (just retested on Linux)

----
**Basics of DLL Creation for Tcl extension**

You have a design for some software that will be implemented in a dll written in C (or C++ code with extern "C" {} around it). You must write the active code in a function, and add some initialisation code to register the dll's functions with tcl (Tcl_CreateCommand or Tcl_CreateObjCommand is faster).

The initialisation required in the code that compiles to make tclext.dll (the `Tclext_Init` name is composed from the dll name by Tcl):
======c
  // this is your useful procedure. It can be a C++ or C linked procedure.
 int tclextfunction (ClientData clientData, Tcl_Interp *interp, int argc, char *argv[]) {
     // dosomething - clientdata is a pointer to something the programmer may or may not define
     return TCL_OK;
 }
 #ifdef __cplusplus
 extern "C" {
 #endif
 int PROBE_EXPORT Tclext_Init(Tcl_Interp *interp)
 {
 //        tcl stubs and tk stubs are needed for dynamic loading, you must have this set as a compiler option
 #ifdef USE_TCL_STUBS
    if (Tcl_InitStubs(interp, TCL_VERSION, 1) == NULL) {
                        Tcl_SetResult(interp, "Tcl_InitStubs failed",TCL_STATIC);
                return TCL_ERROR;
    }
 #endif
 #ifdef USE_TK_STUBS
    if (Tk_InitStubs(interp, TCL_VERSION, 1) == NULL) {
                        Tcl_SetResult(interp, "Tk_InitStubs failed",TCL_STATIC);
                return TCL_ERROR;
    }
 #endif
    Tcl_CreateCommand( "tclextfunction", tclextfunction ); // register your functions with Tcl
 }
 #ifdef __cplusplus
 }
 #endif
======
Then in Tcl script use 
======
  load tclext.dll
  tclextfunction  a b c
======
to pass the strings a,b,c to tclextfunction in the array argv - argc will be 3 of course. The ClientData is a pointer to anything (perhaps a global data structure or a class defined by a ClientData statement often dynamically allocated in the `Tclext_Init` function).

The reader should be able to find all the required .h files from the net without any more help from me.

----
So what do you do when load, or, even more mysteriously, [package],
fails?  There are many, many ways for this to happen, and typical
built-in diagnostics from supplied loaders are inscrutable.

One handy tool for such situations is a dependency utility.  Under
Windows, DependencyWalker
[http://www.dependencywalker.com/]
walks dependencies. When you let it load a specific dll, it 
assists you in determining what other DLLs it needs.



<<discussion>>
----
**See also**
   * [ffidl]
   * [twapi]
   * [tcom] / [COMet]
   * [Tcl_InitStubs]
   * [info loaded]
   * [info sharedlibextension]
----
!!!!!!
[Tcl syntax help]
- [Arts and crafts of Tcl-Tk programming]
%| [Category Command] of [Tcl] |%
!!!!!!