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Tcl_TraceVar(3) 		      Tcl Library Procedures			  Tcl_TraceVar(3)

_________________________________________________________________________________________________

NAME
       Tcl_TraceVar,  Tcl_TraceVar2,  Tcl_UntraceVar, Tcl_UntraceVar2, Tcl_VarTraceInfo, Tcl_Var-
       TraceInfo2 - 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 paren-
						     thesized 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 explic-
	      itly by an unset command, or implicitly when a procedure returns (its  local  vari-
	      ables  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_Decr-
	      RefCount.  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 vari-
       able 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 cur-
       rent 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,  var-
       Name,  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 pre-
       vClientData 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_Var-
       TraceInfo.   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  pre-
       vClientData  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 exe-
       cuting, 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 proce-
       dures 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 mod-
       ify 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 modi-
       fied, 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 begin-
       ning 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_Set-
       Var 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 speci-
       fied  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-cre-
       ated  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  individ-
       ual-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 comple-
       tion.   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 inter-
       preter'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 proce-
       dure 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  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 does not do any error checking to prevent trace procedures from  misusing  the  inter-
       preter 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)
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