Provided by: tcl8.6-doc_8.6.14+dfsg-1build1_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.
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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