Tcl_TraceVar(3) Tcl Library Procedures Tcl_TraceVar(3)______________________________________________________________________________NAME
Tcl_TraceVar, Tcl_TraceVar2, Tcl_UntraceVar, Tcl_UntraceVar2, Tcl_Var‐
TraceInfo, 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 vari‐
able, to an array vari‐
able with no index, or to
an array variable with a
parenthesized index. If
the name references an
element of an array, then
it must be in writable
memory: Tcl will make
temporary modifications
to it while looking up
the name.
int flags (in) OR-ed combination of the
values TCL_TRACE_READS,
TCL_TRACE_WRITES, and
TCL_TRACE_UNSETS,
TCL_PARSE_PART1, 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
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 informa‐
tion 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 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
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 typi‐
cally 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 pro‐
cedure 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 sec‐
tion 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 speci‐
fied 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_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 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 corre‐
sponds 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 pre‐
vClientData, 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. 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). As a special case, if the flag
TCL_PARSE_PART1 is specified, name1 may contain both an array and an
element 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 the flag TCL_PARSE_PART1 is given, name2 should be NULL
since the array and element names are taken from name1.
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 dis‐
abled 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 proce‐
dures, 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 proce‐
dure 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 proce‐
dure 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 vari‐
able 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. 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 par‐
tially-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 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.
BUGS
Tcl doesn't do any error checking to prevent trace procedures from mis‐
using the interpreter during traces with TCL_INTERP_DESTROYED set.
KEYWORDS
clientData, trace, variable
Tcl 7.4 Tcl_TraceVar(3)
