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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
to or that will
have its limits
int type (in) The type of limit
to. This must be
int commandLimit (in) The maximum
reported by info
may be executed
Tcl_Time *timeLimitPtr (in/out) A pointer to a
will either have
the new time
limit read from
or the current
int granularity (in) Divisor that
should really be
checked. Must be
at least 1.
Tcl_LimitHandlerProc *handlerProc (in) Function to call
when a particular
exceeded. If the
removes or raises
the limit during
be permitted to
process after the
Many handlers may
be attached to
execution is not
defined, and they
they are deleted.
ClientData clientData (in) Arbitrary
word used to pass
some context to
Tcl_LimitHandlerDeleteProc *deleteProc (in) Function to call
deleted. May be
NULL if the
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.
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
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(
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)