Tcl_Cancel manual page - Tcl Library Procedures

Tcl_Interp *interp (in): Interpreter in which to cancel the script.
Tcl_Obj *resultObjPtr (in): Error message to use in the cancellation, or NULL to use a default message. If not NULL, this object will have its reference count decremented before Tcl_CancelEval returns.
int flags (in): OR'ed combination of flag bits that specify additional options. For Tcl_CancelEval, only TCL_CANCEL_UNWIND is currently supported. For Tcl_Canceled, only TCL_LEAVE_ERR_MSG and TCL_CANCEL_UNWIND are currently supported.
void *clientData (in): Currently reserved for future use. It should be set to NULL.

Tcl_CancelEval cancels or unwinds the script in progress soon after the next invocation of asynchronous handlers, causing TCL_ERROR to be the return code for that script. This function is thread-safe and may be called from any thread in the process.

Tcl_Canceled checks if the script in progress has been canceled and returns TCL_ERROR if it has. Otherwise, TCL_OK is returned. Extensions can use this function to check to see if they should abort a long running command. This function is thread sensitive and may only be called from the thread the interpreter was created in.

FLAG BITS

Any OR'ed combination of the following values may be used for the flags argument to procedures such as Tcl_CancelEval:

TCL_CANCEL_UNWIND: This flag is used by Tcl_CancelEval and Tcl_Canceled. For Tcl_CancelEval, if this flag is set, the script in progress is canceled and the evaluation stack for the interpreter is unwound. For Tcl_Canceled, if this flag is set, the script in progress is considered to be canceled only if the evaluation stack for the interpreter is being unwound.
TCL_LEAVE_ERR_MSG: This flag is only used by Tcl_Canceled; it is ignored by other procedures. If an error is returned and this bit is set in flags, then an error message will be left in the interpreter's result, where it can be retrieved with Tcl_GetObjResult or Tcl_GetStringResult. If this flag bit is not set then no error message is left and the interpreter's result will not be modified.

REFERENCE COUNT MANAGEMENT

Tcl_CancelEval always decrements the reference count of its resultObjPtr argument (if that is non-NULL). It is expected to be usually called with an object with zero reference count. If the object is shared with some other location (including the Tcl evaluation stack) it should have its reference count incremented before calling this function.

KEYWORDS

cancel, unwind