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
parenthe-
sized index.
If the name
references
an element
of an array,
then it must
be in
writable
memory: Tcl
modifica-
tions to it
while look-
ing up the
name.
int flags (in) OR-ed combi-
nation 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 when-
ever one of
the traced
operations
occurs.
ClientData clientData (in) Arbitrary
one-word
value to
pass to
proc.
char *name1 (in) Name of
scalar or
array vari-
able (with-
out array
index).
char *name2 (in) For a trace
on an ele-
ment of an
array, gives
the index of
the element.
For traces
on scalar
variables or
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 speci-
fies 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 combi-
nation 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
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.
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 pro-
cedure 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 inter-
preter 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 proce-
dure'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 cor-
responding 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_TraceVar.
Tcl_VarTraceInfo may be used to retrieve information about
ticular trace. The trace must be on the variable speci-
fied 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 cre-
ated) 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 vari-
ables 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 com-
pletely expunged. It is possible for the trace procedure
to read or write the variable, but this will be a new ver-
sion 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
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 proce-
dures). Proc will be invoked after the variable's value
has been modified, but before the new value of the vari-
able 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 com-
pletely 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.
Under normal conditions trace procedures should return
NULL, 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 mes-
sage. 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 partially-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_DESTROYED FLAG
In an unset callback to proc, the TCL_TRACE_DESTROYED 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_DESTROYED 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 pro-
cedures. Trace procedures must be extremely careful in
dures 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.
BUGS
Tcl doesn't do any error checking to prevent trace proce-
dures from misusing the interpreter during traces with
TCL_INTERP_DESTROYED set.
KEYWORDS
clientData, trace, variable