In a header file of your C/C++ application or extension that includes
the Tcl library, add these lines: 

  #include <tcl.h>
  ...
  #if ((TCL_MAJOR_VERSION < 8) || ((TCL_MAJOR_VERSION == 8) \
      && (TCL_MINOR_VERSION == 0)))
  #undef TCL_STORAGE_CLASS
  #define TCL_STORAGE_CLASS
  typedef struct Tcl_SavedResult {
      char *result;
      Tcl_FreeProc *freeProc;
  } Tcl_SavedResult;
  EXTERN void Tcl_DiscardResult _ANSI_ARGS_((Tcl_SavedResult *));
  EXTERN void Tcl_RestoreResult _ANSI_ARGS_((Tcl_Interp *, Tcl_SavedResult *));
  EXTERN void Tcl_SaveResult _ANSI_ARGS_((Tcl_Interp *, Tcl_SavedResult *));
  #undef TCL_STORAGE_CLASS
  #define TCL_STORAGE_CLASS DLLIMPORT
  #endif

All that TCL_STORAGE_CLASS monkey business is to make sure
all compilers understand that your code, not the Tcl library, will be
providing the Tcl_*Result() functions. 

Somewhere in your program/library code, add these lines: 

  #if ((TCL_MAJOR_VERSION < 8) || ((TCL_MAJOR_VERSION == 8) \
      && (TCL_MINOR_VERSION == 0)))
  void
  Tcl_SaveResult(Tcl_Interp *interp, Tcl_SavedResult *savedPtr)
  {
    char *result = Tcl_GetStringResult(interp);

    if (interp->freeProc == TCL_STATIC) {
        /*
           String might go away with another Tcl_Eval() call,
           so make a copy.
         */
        int length = strlen(result);
        savedPtr->result = Tcl_Alloc(length + 1);
        strcpy(savedPtr->result, result);
        savedPtr->freeProc = TCL_DYNAMIC;
    } else {
        savedPtr->result = result;
        savedPtr->freeProc = interp->freeProc;
        /*
           We've taken responsibility for savedPtr->result, so make
           sure the interp doesn't free it for us.
         */
        interp->freeProc = TCL_STATIC;
    }
    Tcl_ResetResult(interp);
  }

  void
  Tcl_RestoreResult(Tcl_Interp *interp, Tcl_SavedResult *savedPtr)
  {
    Tcl_SetResult(interp, savedPtr->result, savedPtr->freeProc);
  }

  void
  Tcl_DiscardResult(Tcl_SavedResult *savedPtr)
  {
    if (savedPtr->freeProc == TCL_DYNAMIC) {
        Tcl_Free(savedPtr->result);
    } else {
        (*savedPtr->freeProc)(savedPtr->result);
    }
  }
  #endif

There! Now you can call Tcl_SaveResult(), Tcl_RestoreResult(),
and Tcl_DiscardResult() even if you happen
to link against a pre-Tcl 8.1 library.

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