HPUX TraceVar[3]
TraceVar in anderen Kapiteln des hpux Handbuch:
Tcl_TraceVar(3) Tcl Tcl_TraceVar(3)
Tcl Library Procedures Tcl Library Procedures
___________________________________________________________________________
NAME
Tcl_TraceVar, Tcl_TraceVar2, Tcl_UntraceVar, Tcl_UntraceVar2,
Tcl_VarTraceInfo, Tcl_VarTraceInfo2 - monitor accesses to a variable
SYNOPSIS
#include <tcl.h>
int
Tcl_TraceVar(interp, varName, flags, proc, clientData)
int
Tcl_TraceVar2(interp, name1, name2, flags, proc, clientData)
Tcl_UntraceVar(interp, varName, flags, proc, clientData)
Tcl_UntraceVar2(interp, name1, name2, flags, proc, clientData)
ClientData
Tcl_VarTraceInfo(interp, varName, flags, proc, prevClientData)
ClientData
Tcl_VarTraceInfo2(interp, name1, name2, flags, proc, prevClientData)
ARGUMENTS
Tcl_Interp *interp (in) Interpreter containing
variable.
char *varName (in) Name of variable. May
refer to a scalar
variable, to an array
variable with no index,
or to an array variable
with a parenthesized
index.
int flags (in) OR-ed combination of the
values TCL_TRACE_READS,
TCL_TRACE_WRITES, and
TCL_TRACE_UNSETS, and
TCL_GLOBAL_ONLY. Not
all flags are used by
all procedures. See
below for more
information.
Tcl_VarTraceProc *proc (in) Procedure to invoke
whenever one of the
- 1 - Formatted: August 11, 1996
Tcl_TraceVar(3) Tcl Tcl_TraceVar(3)
Tcl Library Procedures Tcl Library Procedures
traced operations
occurs.
ClientData clientData (in) Arbitrary one-word value
to pass to proc.
char *name1 (in) Name of scalar or array
variable (without array
index).
char *name2 (in) For a trace on an
element of an array,
gives the index of the
element. For traces on
scalar variables or on
whole arrays, is NULL.
ClientData prevClientData (in) If non-NULL, gives last
value returned by
Tcl_VarTraceInfo or
Tcl_VarTraceInfo2, so
this call will return
information about next
trace. If NULL, this
call will return
information about first
trace.
___________________________________________________________________________
DESCRIPTION
Tcl_TraceVar allows a C procedure to monitor and control access to a
Tcl variable, so that the C procedure is invoked whenever the variable
is read or written or unset. If the trace is created successfully
then Tcl_TraceVar returns TCL_OK. If an error occurred (e.g. varName
specifies an element of an array, but the actual variable isn't an
array) then TCL_ERROR is returned and an error message is left in
interp->result.
The flags argument to Tcl_TraceVar indicates when the trace procedure
is to be invoked and provides information for setting up the trace.
It consists of an OR-ed combination of any of the following values:
TCL_GLOBAL_ONLY
Normally, the variable will be looked up at the current level of
procedure call; if this bit is set then the variable will be
looked up at global level, ignoring any active procedures.
TCL_TRACE_READS
- 2 - Formatted: August 11, 1996
Tcl_TraceVar(3) Tcl Tcl_TraceVar(3)
Tcl Library Procedures Tcl Library Procedures
Invoke proc whenever an attempt is made to read the variable.
TCL_TRACE_WRITES
Invoke proc whenever an attempt is made to modify the variable.
TCL_TRACE_UNSETS
Invoke proc whenever the variable is unset. A variable may be
unset either explicitly by an unset command, or implicitly when a
procedure returns (its local variables are automatically unset)
or when the interpreter is deleted (all variables are
automatically unset).
Whenever one of the specified operations occurs on the variable, proc
will be invoked. It should have arguments and result that match the
type Tcl_VarTraceProc:
typedef char *Tcl_VarTraceProc(
ClientData clientData,
Tcl_Interp *interp,
char *name1,
char *name2,
int flags);
The clientData and interp parameters will have the same values as
those passed to Tcl_TraceVar when the trace was created. ClientData
typically points to an application-specific data structure that
describes what to do when proc is invoked. Name1 and name2 give the
name of the traced variable in the normal two-part form (see the
description of Tcl_TraceVar2 below for details). Flags is an OR-ed
combination of bits providing several pieces of information. One of
the bits TCL_TRACE_READS, TCL_TRACE_WRITES, or TCL_TRACE_UNSETS will
be set in flags to indicate which operation is being performed on the
variable. The bit TCL_GLOBAL_ONLY will be set whenever the variable
being accessed is a global one not accessible from the current level
of procedure call: the trace procedure will need to pass this flag
back to variable-related procedures like Tcl_GetVar if it attempts to
access the variable. The bit TCL_TRACE_DESTROYED will be set in flags
if the trace is about to be destroyed; this information may be useful
to proc so that it can clean up its own internal data structures (see
the section TCL_TRACE_DESTROYED below for more details). Lastly, the
bit TCL_INTERP_DESTROYED will be set if the entire interpreter is
being destroyed. When this bit is set, proc must be especially
careful in the things it does (see the section TCL_INTERP_DESTROYED
below). The trace procedure's return value should normally be NULL;
see ERROR RETURNS below for information on other possibilities.
Tcl_UntraceVar may be used to remove a trace. If the variable
specified by interp, varName, and flags has a trace set with flags,
proc, and clientData, then the corresponding trace is removed. If no
such trace exists, then the call to Tcl_UntraceVar has no effect. The
same bits are valid for flags as for calls to Tcl_TraceVars.
- 3 - Formatted: August 11, 1996
Tcl_TraceVar(3) Tcl Tcl_TraceVar(3)
Tcl Library Procedures Tcl Library Procedures
Tcl_VarTraceInfo may be used to retrieve information about traces set
on a given variable. The return value from Tcl_VarTraceInfo is the
clientData associated with a particular trace. The trace must be on
the variable specified by the interp, varName, and flags arguments
(only the TCL_GLOBAL_ONLY bit from flags is used; other bits are
ignored) and its trace procedure must the same as the proc argument.
If the prevClientData argument is NULL then the return value
corresponds to the first (most recently created) matching trace, or
NULL if there are no matching traces. If the prevClientData argument
isn't NULL, then it should be the return value from a previous call to
Tcl_VarTraceInfo. In this case, the new return value will correspond
to the next matching trace after the one whose clientData matches
prevClientData, or NULL if no trace matches prevClientData or if there
are no more matching traces after it. This mechanism makes it
possible to step through all of the traces for a given variable that
have the same proc.
TWO-PART NAMES
The procedures Tcl_TraceVar2, Tcl_UntraceVar2, and Tcl_VarTraceInfo2
are identical to Tcl_TraceVar, Tcl_UntraceVar, and Tcl_VarTraceInfo,
respectively, except that the name of the variable has already been
separated by the caller into two parts. Name1 gives the name of a
scalar variable or array, and name2 gives the name of an element
within an array. If name2 is NULL it means that either the variable
is a scalar or the trace is to be set on the entire array rather than
an individual element (see WHOLE-ARRAY TRACES below for more
information).
ACCESSING VARIABLES DURING TRACES
During read and write traces, the trace procedure can read, write, or
unset the traced variable using Tcl_GetVar2, Tcl_SetVar2, and other
procedures. While proc is executing, traces are temporarily disabled
for the variable, so that calls to Tcl_GetVar2 and Tcl_SetVar2 will
not cause proc or other trace procedures to be invoked again.
Disabling only occurs for the variable whose trace procedure is
active; accesses to other variables will still be traced. However, |
if a variable is unset during a read or write trace then unset traces |
will be invoked. |
During unset traces the variable has already been completely expunged.
It is possible for the trace procedure to read or write the variable,
but this will be a new version of the variable. Traces are not
disabled during unset traces as they are for read and write traces,
but existing traces have been removed from the variable before any
trace procedures are invoked. If new traces are set by unset trace
procedures, these traces will be invoked on accesses to the variable
by the trace procedures.
- 4 - Formatted: August 11, 1996
Tcl_TraceVar(3) Tcl Tcl_TraceVar(3)
Tcl Library Procedures Tcl Library Procedures
CALLBACK TIMING
When read tracing has been specified for a variable, the trace
procedure will be invoked whenever the variable's value is read. This
includes set Tcl commands, $-notation in Tcl commands, and invocations
of the Tcl_GetVar and Tcl_GetVar2 procedures. Proc is invoked just
before the variable's value is returned. It may modify the value of
the variable to affect what is returned by the traced access. If it |
unsets the variable then the access will return an error just as if |
the variable never existed. |
When write tracing has been specified for a variable, the trace
procedure will be invoked whenever the variable's value is modified.
This includes set commands, commands that modify variables as side
effects (such as catch and scan), and calls to the Tcl_SetVar and
Tcl_SetVar2 procedures). Proc will be invoked after the variable's
value has been modified, but before the new value of the variable has
been returned. It may modify the value of the variable to override
the change and to determine the value actually returned by the traced
access. If it deletes the variable then the traced access will return |
an empty string. |
When unset tracing has been specified, the trace procedure will be
invoked whenever the variable is destroyed. The traces will be called
after the variable has been completely unset.
WHOLE-ARRAY TRACES
If a call to Tcl_TraceVar or Tcl_TraceVar2 specifies the name of an
array variable without an index into the array, then the trace will be
set on the array as a whole. This means that proc will be invoked
whenever any element of the array is accessed in the ways specified by
flags. When an array is unset, a whole-array trace will be invoked
just once, with name1 equal to the name of the array and name2 NULL;
it will not be invoked once for each element.
MULTIPLE TRACES
It is possible for multiple traces to exist on the same variable.
When this happens, all of the trace procedures will be invoked on each
access, in order from most-recently-created to least-recently-created.
When there exist whole-array traces for an array as well as traces on
individual elements, the whole-array traces are invoked before the
individual-element traces. If a read or write trace unsets the |
variable then all of the unset traces will be invoked but the |
remainder of the read and write traces will be skipped. |
ERROR RETURNS
Under normal conditions trace procedures should return NULL,
- 5 - Formatted: August 11, 1996
Tcl_TraceVar(3) Tcl Tcl_TraceVar(3)
Tcl Library Procedures Tcl Library Procedures
indicating successful completion. If proc returns a non-NULL value it
signifies that an error occurred. The return value must be a pointer
to a static character string containing an error message. If a trace
procedure returns an error, no further traces are invoked for the
access and the traced access aborts with the given message. Trace
procedures can use this facility to make variables read-only, for
example (but note that the value of the variable will already have
been modified before the trace procedure is called, so the trace
procedure will have to restore the correct value).
The return value from proc is only used during read and write tracing.
During unset traces, the return value is ignored and all relevant
trace procedures will always be invoked.
RESTRICTIONS
A trace procedure can be called at any time, even when there is a
partically-formed result in the interpreter's result area. If the
trace procedure does anything that could damage this result (such as
calling Tcl_Eval) then it must save the original values of the
interpreter's result and freeProc fields and restore them before it
returns.
UNDEFINED VARIABLES
It is legal to set a trace on an undefined variable. The variable
will still appear to be undefined until the first time its value is
set. If an undefined variable is traced and then unset, the unset
will fail with an error (``no such variable''), but the trace
procedure will still be invoked.
TCL_TRACE_DELETED FLAG
In an unset callback to proc, the TCL_TRACE_DELETED bit is set in
flags if the trace is being removed as part of the deletion. Traces
on a variable are always removed whenever the variable is deleted;
the only time TCL_TRACE_DELETED isn't set is for a whole-array trace
invoked when only a single element of an array is unset.
TCL_INTERP_DESTROYED
When an interpreter is destroyed, unset traces are called for all of
its variables. The TCL_INTERP_DESTROYED bit will be set in the flags
argument passed to the trace procedures. Trace procedures must be
extremely careful in what they do if the TCL_INTERP_DESTROYED bit is
set. It is not safe for the procedures to invoke any Tcl procedures
on the interpreter, since its state is partially deleted. All that
trace procedures should do under these circumstances is to clean up
and free their own internal data structures.
- 6 - Formatted: August 11, 1996
Tcl_TraceVar(3) Tcl Tcl_TraceVar(3)
Tcl Library Procedures Tcl Library Procedures
BUGS
Tcl doesn't do any error checking to prevent trace procedures from
misusing the interpreter during traces with TCL_INTERP_DESTROYED set.
KEYWORDS
clientData, trace, variable
- 7 - Formatted: August 11, 1996