trace(n)						       Tcl Built-In Commands							  trace(n)

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NAME

       trace - Monitor variable accesses

SYNOPSIS

       trace option ?arg arg ...?
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DESCRIPTION

       This command causes Tcl commands to be executed whenever certain operations are invoked.  At present, only variable tracing is implemented.
       The legal option's (which may be abbreviated) are:

       trace variable name ops command
	      Arrange for command to be executed whenever variable name is accessed in one of the ways given by ops.  Name may refer to  a  normal
	      variable,  an  element  of  an  array,  or to an array as a whole (i.e. name may be just the name of an array, with no parenthesized
	      index).  If name refers to a whole array, then command is invoked whenever any element of the array is manipulated.  If the variable
	      does  not exist, it will be created but will not be given a value, so it will be visible to namespace which queries, but not to info
	      exists queries.

	      Ops indicates which operations are of interest, and consists of one or more of the following letters:

	      r      Invoke command whenever the variable is read.

	      w      Invoke command whenever the variable is written.

	      u      Invoke command whenever the variable is unset.  Variables can be unset explicitly with the unset command, or implicitly  when
		     procedures  return  (all  of  their  local variables are unset).  Variables are also unset when interpreters are deleted, but
		     traces will not be invoked because there is no interpreter in which to execute them.

	      When the trace triggers, three arguments are appended to command so that the actual command is as follows:
		     command name1 name2 op
	      Name1 and name2 give the name(s) for the variable being accessed:  if the variable is a scalar then name1 gives the variable's  name
	      and  name2  is an empty string; if the variable is an array element then name1 gives the name of the array and name2 gives the index
	      into the array; if an entire array is being deleted and the trace was registered on the overall array, rather than a single element,
	      then  name1 gives the array name and name2 is an empty string.  Name1 and name2 are not necessarily the same as the name used in the
	      trace variable command:  the upvar command allows a procedure to reference a variable under a different  name.   Op  indicates  what
	      operation is being performed on the variable, and is one of r, w, or u as defined above.

	      Command  executes  in the same context as the code that invoked the traced operation:  if the variable was accessed as part of a Tcl
	      procedure, then command will have access to the same local variables as code in the procedure.  This context may be  different  than
	      the context in which the trace was created.  If command invokes a procedure (which it normally does) then the procedure will have to
	      use upvar or uplevel if it wishes to access the traced variable.	Note also that name1 may not necessarily be the same as  the  name
	      used  to	set the trace on the variable;	differences can occur if the access is made through a variable defined with the upvar com-
	      mand.

	      For read and write traces, command can modify the variable to affect the result of the traced operation.	If  command  modifies  the
	      value  of  a  variable during a read or write trace, then the new value will be returned as the result of the traced operation.  The
	      return value from  command is ignored except that if it returns an error of any sort then the traced operation also returns an error
	      with  the  same  error message returned by the trace command (this mechanism can be used to implement read-only variables, for exam-
	      ple).  For write traces, command is invoked after the variable's value has been changed; it can write a new value into the  variable
	      to override the original value specified in the write operation.	To implement read-only variables, command will have to restore the
	      old value of the variable.

	      While command is executing during a read or write trace, traces on the variable are temporarily disabled.  This means that reads and
	      writes invoked by command will occur directly, without invoking command (or any other traces) again.  However, if command unsets the
	      variable then unset traces will be invoked.

	      When an unset trace is invoked, the variable has already been deleted:  it will appear to be undefined with no traces.  If an  unset
	      occurs  because  of  a  procedure return, then the trace will be invoked in the variable context of the procedure being returned to:
	      the stack frame of the returning procedure will no longer exist.	Traces are not disabled during unset traces, so if an unset  trace
	      command creates a new trace and accesses the variable, the trace will be invoked.  Any errors in unset traces are ignored.

	      If there are multiple traces on a variable they are invoked in order of creation, most-recent first.  If one trace returns an error,
	      then no further traces are invoked for the variable.  If an array element has a trace set, and there is also  a  trace  set  on  the
	      array as a whole, the trace on the overall array is invoked before the one on the element.

	      Once  created,  the  trace remains in effect either until the trace is removed with the trace vdelete command described below, until
	      the variable is unset, or until the interpreter is deleted.  Unsetting an element of array will remove any traces on  that  element,
	      but will not remove traces on the overall array.

	      This command returns an empty string.

       trace vdelete name ops command
	      If  there  is  a	trace set on variable name with the operations and command given by ops and command, then the trace is removed, so
	      that command will never again be invoked.  Returns an empty string.

       trace vinfo name
	      Returns a list containing one element for each trace currently set on variable name.  Each element of the list is itself a list con-
	      taining  two  elements,  which  are the ops and command associated with the trace.  If name doesn't exist or doesn't have any traces
	      set, then the result of the command will be an empty string.

KEYWORDS

       read, variable, write, trace, unset

Tcl																	  trace(n)