tcl_tracevar(3)

NAME

Tcl_TraceVar, Tcl_TraceVar2, Tcl_UntraceVar, Tcl_Untrace
Var2, 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.

CONST 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.

int flags (in) OR-ed combi

nation of
the values
TCL_TRACE_READS,
TCL_TRACE_WRITES,
TCL_TRACE_UN

SETS,
TCL_TRACE_AR

RAY,
TCL_GLOB

AL_ONLY,
TCL_NAMES
PACE_ONLY,
TCL_TRACE_RE

SULT_DYNAMIC
and
TCL_TRACE_RE

SULT_OBJECT.
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.

CONST char *name1 (in) Name of

scalar or
array vari
able (with
out array
index).

CONST 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
on whole
arrays, is
NULL.

ClientData prevClientData (in) If non-NULL,

gives last
value
returned by
Tcl_VarTra ceInfo or Tcl_VarTra ceInfo2, 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 the interpreter's 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_NAMESPACE_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 in the cur
rent namespace, 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.

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).

TCL_TRACE_ARRAY

Invoke proc whenever the array command is invoked.
This gives the trace procedure a chance to update
the array before array names or array get is
called. Note that this is called before an array
set, but that will trigger write traces.

TCL_TRACE_RESULT_DYNAMIC

The result of invoking the proc is a dynamically
allocated string that will be released by the Tcl
library via a call to ckfree. Must not be speci
fied at the same time as TCL_TRACE_RESULT_OBJECT.

TCL_TRACE_RESULT_OBJECT

The result of invoking the proc is a Tcl_Obj* (cast
to a char*) with a reference count of at least one.
The ownership of that reference will be transferred
to the Tcl core for release (when the core has fin
ished with it) via a call to Tcl_DecrRefCount. Must not be specified at the same time as
TCL_TRACE_RESULT_DYNAMIC.

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 applicationspecific 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, TCL_TRACE_ARRAY, 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_NAMES
PACE_ONLY will be set whenever the variable being accessed
is a namespace 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 struc
tures (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 nor
mally 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 traces set on a given variable. The return value from
Tcl_VarTraceInfo is the clientData associated with a par ticular trace. The trace must be on the variable speci
fied by the interp, varName, and flags arguments (only the TCL_GLOBAL_ONLY and TCL_NAMESPACE_ONLY bits 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 argu ment 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 consists of two parts.
Name1 gives the name of a scalar variable or array, and
name2 gives the name of an element within an array. When
name2 is NULL, name1 may contain both an array and an ele ment name: if the name contains an open parenthesis and
ends with a close parenthesis, then the value between the
parentheses is treated as an element name (which can have
any string value) and the characters before the first open
parenthesis are treated as the name of an array variable.
If name2 is NULL and name1 does not refer to an array ele ment 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, write, and array 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
accesses to the variable by the trace 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 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 array tracing has been specified, the trace procedure
will be invoked at the beginning of the array command
implementation, before any of the operations like get,
set, or names have been invoked. The trace procedure can
modify the array elements with Tcl_SetVar and Tcl_SetVar2.

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 mostrecently-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, 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, unless (exactly one of) the TCL_TRACE_RESULT_DYNAMIC and
TCL_TRACE_RESULT_OBJECT flags is set, which specify that
the result is either a dynamic string (to be released with
ckfree) or a Tcl_Obj* (cast to char* and to be released with Tcl_DecrRefCount) containing the 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 proce
dure 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
what they do if the TCL_INTERP_DESTROYED bit is set. It
is not safe for the procedures to invoke any Tcl proce
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.

Array traces are not yet integrated with the Tcl "info
exists" command, nor is there Tcl-level access to array
traces.

KEYWORDS

clientData, trace, variable

Tcl 7.4 Tcl_TraceVar(3)