The file tcl.h should be considered a public declaration of
the Tcl C functions, defines/macros, and structures
which a developer can ''safely'' depend on.
(Some [extension]s also use things from tclInt.h, but the 
price for that is that they require more maintenance in 
order to keep working. Stick with tcl.h as long as possible.)
Most of the actual function declarations are found in the 
tclDecls.h file; this has to do with the [Stubs] mechanism.

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Tcl C API functions include (but the following is a very incomplete list):

   * Tcl_Main
   * [Tcl_Init]
   * TclObjTest_Init
   * TclThread_Init
   * Tcl_SetVar
   * [Tcl_CreateObjCommand]

See also [Tcl API] and the references there.
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[Category Tcl Library]