| Tcl_LimitCheck(3tcl) | Tcl Library Procedures | Tcl_LimitCheck(3tcl) |
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> int Tcl_LimitCheck(interp) int Tcl_LimitReady(interp) int Tcl_LimitExceeded(interp) int Tcl_LimitTypeExceeded(interp, type) int Tcl_LimitTypeEnabled(interp, type) void Tcl_LimitTypeSet(interp, type) void Tcl_LimitTypeReset(interp, type) int Tcl_LimitGetCommands(interp) void Tcl_LimitSetCommands(interp, commandLimit) void Tcl_LimitGetTime(interp, timeLimitPtr) void Tcl_LimitSetTime(interp, timeLimitPtr) int Tcl_LimitGetGranularity(interp, type) void Tcl_LimitSetGranularity(interp, type, granularity) void Tcl_LimitAddHandler(interp, type, handlerProc, clientData, deleteProc) void Tcl_LimitRemoveHandler(interp, type, handlerProc, clientData)
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 the main interpreter) to handle the error.
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.
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 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
| 8.5 | Tcl |