Provided by: tcl8.5-doc_8.5.19-4_all 
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.
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 parenthesized index.
int flags (in) OR-ed combination of the values TCL_TRACE_READS,
TCL_TRACE_WRITES, TCL_TRACE_UNSETS, TCL_TRACE_ARRAY,
TCL_GLOBAL_ONLY, TCL_NAMESPACE_ONLY,
TCL_TRACE_RESULT_DYNAMIC and TCL_TRACE_RESULT_OBJECT. 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.
const char *name1 (in) Name of scalar or array variable (without array index).
const 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
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 is not 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 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_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 current 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 specified 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 finished 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 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,
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_NAMESPACE_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 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 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 argument is not 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 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
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 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 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 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 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 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 is not 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 does not do any error checking to prevent trace procedures 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