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RedHat 9 (Linux i386) - man page for trace (redhat section n)

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


       trace - Monitor variable accesses

       trace option ?arg arg ...?

       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  abbre-
       viated) 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 paren-
	      thesized 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  inter-
		     preters  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

	      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;  differ-
	      ences  can  occur  if  the access is made through a variable defined with the upvar

	      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 opera-
	      tion.  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 example).  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 containing 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.

       read, variable, write, trace, unset

Tcl											 trace(n)

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