These routines allows a C procedure to take a snapshot of the current interpreter result so that it can be restored after a call to Tcl_Eval or some other routine that modifies the interpreter result. These routines are passed a pointer to a structure that is used to store enough information to restore the interpreter result state. This structure can be allocated on the stack of the calling procedure. These routines do not save the state of any error information in the interpreter (e.g. the errorCode or errorInfo variables).
Tcl_SaveResult moves the string and object results of interp into the location specified by statePtr. Tcl_SaveResult clears the result for interp and leaves the result in its normal empty initialized state.
Tcl_RestoreResult moves the string and object results from statePtr back into interp. Any result or error that was already in the interpreter will be cleared. The statePtr is left in an uninitialized state and cannot be used until another call to Tcl_SaveResult.
Tcl_DiscardResult releases the saved interpreter state stored at statePtr. The state structure is left in an uninitialized state and cannot be used until another call to Tcl_SaveResult.
Once Tcl_SaveResult is called to save the interpreter result, either Tcl_RestoreResult or Tcl_DiscardResult must be called to properly clean up the memory associated with the saved state.