*nix Documentation Project
·  Home
 +   man pages
·  Linux HOWTOs
·  FreeBSD Tips
·  *niX Forums

  man pages->IRIX man pages -> Tcl/trace (3)              


trace(3Tcl)							   trace(3Tcl)

NAME    [Toc]    [Back]

     trace - Monitor variable accesses

SYNOPSIS    [Toc]    [Back]

     trace option ?arg arg ...?

DESCRIPTION    [Toc]    [Back]

     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.

	  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.  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

									Page 1

trace(3Tcl)							   trace(3Tcl)

	  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 command.

	  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 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.

									Page 2

trace(3Tcl)							   trace(3Tcl)

	  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.

KEYWORDS    [Toc]    [Back]

     read, variable, write, trace, unset

									PPPPaaaaggggeeee 3333
[ Back ]
 Similar pages
Name OS Title
tracevar IRIX monitor accesses to a variable
linkvar IRIX link Tcl variable to C variable
nfswatch Tru64 Monitor an NFS server
prom IRIX PROM monitor
moninfo IRIX Monitor characteristics database
rpc.statd HP-UX network status monitor
sysmonpp IRIX System Monitor Preprocessor
statd HP-UX network status monitor
imon IRIX inode monitor device
setmonitor IRIX sets the monitor type
Copyright © 2004-2005 DeniX Solutions SRL
newsletter delivery service