Manual Reference Pages - TCL_LIMITTYPESET (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 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
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
ClientData clientData (in)
Arbitrary pointer-sized word used to pass some context to the
Tcl_LimitHandlerDeleteProc *deleteProc (in)
Function to call whenever a handler is deleted. May be NULL if the
clientData requires no deletion.
Tcls 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.
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
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
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
typedef void Tcl_LimitHandlerProc(
The clientData argument to the handler will be whatever is
passed to the clientData argment 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
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 ||TCL_LIMITCHECK (3) ||8.5 |
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