Provided by: tcl8.6-doc_8.6.13+dfsg-2_all 
NAME
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
SYNOPSIS
#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)
ARGUMENTS
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.
_________________________________________________________________________________________________
DESCRIPTION
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
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).
KEYWORDS
interpreter, resource, limit, commands, time, callback