Tcl_LimitAddHandler man page

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


#include <tcl.h>




Tcl_LimitTypeExceeded(interp, type)

Tcl_LimitTypeEnabled(interp, type)

Tcl_LimitTypeSet(interp, type)

Tcl_LimitTypeReset(interp, type)


Tcl_LimitSetCommands(interp, commandLimit)

Tcl_LimitGetTime(interp, timeLimitPtr)

Tcl_LimitSetTime(interp, timeLimitPtr)

Tcl_LimitGetGranularity(interp, type)

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 checked.
int type (in)
The type of limit that the operation refers to. This must be either TCL_LIMIT_COMMANDS or TCL_LIMIT_TIME.
int commandLimit (in)
The maximum number of commands (as reported by info cmdcount) that may be executed in the interpreter.
Tcl_Time *timeLimitPtr (in/out)
A pointer to a structure that will either have the new time limit read from (Tcl_LimitSetTime) 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 callbacks 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 function 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.

Limit Configuration

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.

Limit Callbacks

To add a handler callback to be invoked when a limit is exceeded, call Tcl_LimitAddHandler. The handlerProc argument describes the function that will actually be called; it should have the following prototype:

typedef void Tcl_LimitHandlerProc(
        ClientData clientData,
        Tcl_Interp *interp);

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(
        ClientData clientData);

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

Referenced By

Tcl_LimitCheck(3), Tcl_LimitExceeded(3), Tcl_LimitGetCommands(3), Tcl_LimitGetGranularity(3), Tcl_LimitGetTime(3), Tcl_LimitReady(3), Tcl_LimitRemoveHandler(3), Tcl_LimitSetCommands(3), Tcl_LimitSetGranularity(3), Tcl_LimitSetTime(3), Tcl_LimitTypeEnabled(3), Tcl_LimitTypeExceeded(3), Tcl_LimitTypeReset(3) and Tcl_LimitTypeSet(3) are aliases of Tcl_LimitAddHandler(3).

Tcl 8.5 Tcl Library Procedures