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_________________________________________________________________
NAME
Tcl_TraceVar, Tcl_TraceVar2, Tcl_UntraceVar,
Tcl_UntraceVar2, Tcl_VarTraceInfo, Tcl_VarTraceInfo2 - moni-
tor 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 vari-
able. May
refer to a
scalar vari-
able, to an
array variable
with no index,
or to an array
variable with
a
parenthesized
index. If the
name refer-
ences an ele-
ment of an
array, then it
must be in
writable
memory: Tcl
will make tem-
porary
modifications
to it while
looking up the
name.
int flags (in) OR-ed combina-
tion of the
values
TCL_TRACE_READS,
TCL_TRACE_WRITES,
and
TCL_TRACE_UNSETS,
TCL_TRACE_ARRAY,
and
TCL_GLOBAL_ONLY.
Not all flags
are used by
all pro-
cedures. 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 (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 vari-
ables 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 infor-
mation 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 set-
ting 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 inter-
preter 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.
Whenever one of the specified operations occurs on the vari-
able, 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 combi-
nation 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_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 normally be NULL; see
ERROR RETURNS below for information on other possibilities.
Tcl_UntraceVar may be used to remove a trace. If the vari-
able 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_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 partic-
ular 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 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 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 name2 is NULL |
and name1 does not refer to an array element 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 vari-
able, 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 pro-
cedures, 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 pro-
cedures). 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 array tracing has been specified, the trace procedure
will be invoked at the beginning of the array command imple-
mentation, 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 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,
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 con-
taining 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 pro-
cedures 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 dam-
age 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 inter-
preter, 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 pro-
cedures 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
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