Tcl_LimitCheck(3) Tcl Library Procedures Tcl_LimitCheck(3)
Tcl_LimitAddHandler, Tcl_LimitCheck, Tcl_LimitExceeded, Tcl_LimitGetCommands, Tcl_LimitGetGranularity, Tcl_LimitGetTime, Tcl_LimitReady,
Tcl_LimitRemoveHandler, Tcl_LimitSetCommands, Tcl_LimitSetGranularity, Tcl_LimitSetTime, Tcl_LimitTypeEnabled, Tcl_LimitTypeExceeded,
Tcl_LimitTypeReset, Tcl_LimitTypeSet - manage and check resource limits on interpreters
Tcl_LimitSetGranularity(interp, type, granularity)
Tcl_LimitAddHandler(interp, type, handlerProc, clientData, deleteProc)
Tcl_LimitRemoveHandler(interp, type, handlerProc, clientData)
Tcl_Interp *interp (in) Interpreter that the limit being managed applies to or that will have its limits
int type (in) The type of limit that the operation refers to. This must be either TCL_LIMIT_COM-
MANDS or TCL_LIMIT_TIME.
int commandLimit (in) The maximum number of commands (as reported by info cmdcount) that may be executed in
Tcl_Time *timeLimitPtr (in/out) A pointer to a structure that will either have the new time limit read from (Tcl_Lim-
itSetTime) or the current time limit written to (Tcl_LimitGetTime).
int granularity (in) Divisor that indicates how often a particular limit should really be checked. Must
be at least 1.
Tcl_LimitHandlerProc *handlerProc (in) Function to call when a particular limit is exceeded. If the handlerProc removes or
raises the limit during its processing, the limited interpreter will be permitted to
continue to process after the handler returns. Many handlers may be attached to the
same interpreter limit; their order of execution is not defined, and they must be
identified by handlerProc and clientData when they are deleted.
ClientData clientData (in) Arbitrary pointer-sized word used to pass some context to the handlerProc function.
Tcl_LimitHandlerDeleteProc *deleteProc (in) Function to call whenever a handler is deleted. May be NULL if the clientData
requires no deletion.
Tcl's interpreter resource limit subsystem allows for close control over how much computation time a script may use, and is useful for
cases where a program is divided into multiple pieces where some parts are more trusted than others (e.g. web application servers).
Every interpreter may have a limit on the wall-time for execution, and a limit on the number of commands that the interpreter may execute.
Since checking of these limits is potentially expensive (especially the time limit), each limit also has a checking granularity, which is a
divisor for an internal count of the number of points in the core where a check may be performed (which is immediately before executing a
command and at an unspecified frequency between running commands, which can happen in empty-bodied while loops).
The final component of the limit engine is a callback scheme which allows for notifications of when a limit has been exceeded. These call-
backs can just provide logging, or may allocate more resources to the interpreter to permit it to continue processing longer.
When a limit is exceeded (and the callbacks have run; the order of execution of the callbacks is unspecified) execution in the limited
interpreter is stopped by raising an error and setting a flag that prevents the catch command in that interpreter from trapping that error.
It is up to the context that started execution in that interpreter (typically a master interpreter) to handle the error.
LIMIT CHECKING API
To check the resource limits for an interpreter, call Tcl_LimitCheck, which returns TCL_OK if the limit was not exceeded (after processing
callbacks) and TCL_ERROR if the limit was exceeded (in which case an error message is also placed in the interpreter result). That func-
tion should only be called when Tcl_LimitReady returns non-zero so that granularity policy is enforced. This API is designed to be similar
in usage to Tcl_AsyncReady and Tcl_AsyncInvoke.
When writing code that may behave like catch in respect of errors, you should only trap an error if Tcl_LimitExceeded returns zero. If it
returns non-zero, the interpreter is in a limit-exceeded state and errors should be allowed to propagate to the calling context. You can
also check whether a particular type of limit has been exceeded using Tcl_LimitTypeExceeded.
To check whether a limit has been set (but not whether it has actually been exceeded) on an interpreter, call Tcl_LimitTypeEnabled with the
type of limit you want to check. To enable a particular limit call Tcl_LimitTypeSet, and to disable a limit call Tcl_LimitTypeReset.
The level of a command limit may be set using Tcl_LimitSetCommands, and retrieved using Tcl_LimitGetCommands. Similarly for a time limit
with Tcl_LimitSetTime and Tcl_LimitGetTime respectively, but with that API the time limit is copied from and to the Tcl_Time structure that
the timeLimitPtr argument points to.
The checking granularity for a particular limit may be set using Tcl_LimitSetGranularity and retrieved using Tcl_LimitGetGranularity. Note
that granularities must always be positive.
To add a handler callback to be invoked when a limit is exceeded, call Tcl_LimitAddHandler. The handlerProc argument describes the func-
tion that will actually be called; it should have the following prototype:
typedef void Tcl_LimitHandlerProc(
The clientData argument to the handler will be whatever is passed to the clientData argument to Tcl_LimitAddHandler, and the interp is the
interpreter that had its limit exceeded.
The deleteProc argument to Tcl_LimitAddHandler is a function to call to delete the clientData value. It may be TCL_STATIC or NULL if no
deletion action is necessary, or TCL_DYNAMIC if all that is necessary is to free the structure with Tcl_Free. Otherwise, it should refer
to a function with the following prototype:
typedef void Tcl_LimitHandlerDeleteProc(
A limit handler may be deleted using Tcl_LimitRemoveHandler; the handler removed will be the first one found (out of the handlers added
with Tcl_LimitAddHandler) with exactly matching type, handlerProc and clientData arguments. This function always invokes the deleteProc on
the clientData (unless the deleteProc was NULL or TCL_STATIC).
interpreter, resource, limit, commands, time, callback
Tcl 8.5 Tcl_LimitCheck(3)