- Tcl_TraceVar, Tcl_TraceVar2, Tcl_UntraceVar, Tcl_UntraceVar2, Tcl_VarTraceInfo, Tcl_VarTraceInfo2 - monitor accesses to a variable
- TWO-PART NAMES
- ACCESSING VARIABLES DURING TRACES
- CALLBACK TIMING
- WHOLE-ARRAY TRACES
- MULTIPLE TRACES
- ERROR RETURNS
- UNDEFINED VARIABLES
- TCL_TRACE_DESTROYED FLAG
Tcl_TraceVar, Tcl_TraceVar2, Tcl_UntraceVar, Tcl_UntraceVar2, Tcl_VarTraceInfo, Tcl_VarTraceInfo2 - monitor accesses to a variable
Tcl_TraceVar(interp, varName, flags, proc, clientData)
Tcl_TraceVar2(interp, name1, name2, flags, proc, clientData)
Tcl_UntraceVar(interp, varName, flags, proc, clientData)
Tcl_UntraceVar2(interp, name1, name2, flags, proc, clientData)
Tcl_VarTraceInfo(interp, varName, flags, proc, prevClientData)
Tcl_VarTraceInfo2(interp, name1, name2, flags, proc, prevClientData)
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.
- 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.
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_TRACE_ARRAY, 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 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,
- 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
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:
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.
Invoke proc whenever an attempt is made to read the variable.
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).
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 variable,
proc will be invoked.
It should have arguments and result that match the type
typedef char *Tcl_VarTraceProc(
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
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,
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_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 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.
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
During read, write, and array traces, the
trace procedure can read, write, or unset the traced
variable using Tcl_GetVar2, Tcl_SetVar2, and
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
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
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
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
It may modify the value of the variable to override the change
and to determine the value actually returned by the traced
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
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
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
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
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
If proc returns a non-NULL value it signifies that an
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
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
The return value from proc is only used during read and
During unset traces, the return value is ignored and all relevant
trace procedures will always be invoked.
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.
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.
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.
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.
Tcl doesn't do any error checking to prevent trace procedures
from misusing the interpreter during traces with TCL_INTERP_DESTROYED
Array traces are not yet integrated with the Tcl "info exists" command,
nor is there Tcl-level access to array traces.
clientData, trace, variable
Copyright © 1989-1993 The Regents of the University of California.
Copyright © 1994-1996 Sun Microsystems, Inc.
Copyright © 1995-1997 Roger E. Critchlow Jr.
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