Unix/Linux Go Back    


SuSE 11.3 - man page for trace (suse section n)

Linux & Unix Commands - Search Man Pages
Man Page or Keyword Search:   man
Select Man Page Set:       apropos Keyword Search (sections above)


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

_________________________________________________________________________________________________

NAME
       trace - Monitor variable accesses, command usages and command executions

SYNOPSIS
       trace option ?arg arg ...?
_________________________________________________________________

DESCRIPTION
       This  command  causes Tcl commands to be executed whenever certain operations are invoked.
       The legal options (which may be abbreviated) are:

       trace add type name ops ?args?
	      Where type is command, execution, or variable.

	      trace add command name ops commandPrefix
		     Arrange for commandPrefix to be executed (with additional	arguments)  when-
		     ever command name is modified in one of the ways given by the list ops. Name
		     will be resolved using the usual namespace resolution  rules  used  by  com-
		     mands. If the command does not exist, an error will be thrown.

		     Ops indicates which operations are of interest, and is a list of one or more
		     of the following items:

		     rename Invoke commandPrefix whenever the traced command  is  renamed.   Note
			    that  renaming  to	the empty string is considered deletion, and will
			    not be traced with "rename".

		     delete Invoke commandPrefix when the traced command is deleted. Commands can
			    be	deleted explicitly by using the rename command to rename the com-
			    mand to an empty string. Commands are also deleted	when  the  inter-
			    preter is deleted, but traces will not be invoked because there is no
			    interpreter in which to execute them.

		     When the trace triggers, depending on the operations being traced, a  number
		     of  arguments are appended to commandPrefix so that the actual command is as
		     follows:
			    commandPrefix oldName newName op
		     OldName and newName give the traced command's current (old)  name,  and  the
		     name  to  which  it is being renamed (the empty string if this is a "delete"
		     operation).  Op indicates what operation is being performed on the  command,
		     and is one of rename or delete as defined above.  The trace operation cannot
		     be used to stop a command from being deleted.  Tcl will  always  remove  the
		     command once the trace is complete.  Recursive renaming or deleting will not
		     cause further traces of the same type to be evaluated,  so  a  delete  trace
		     which itself deletes the command, or a rename trace which itself renames the
		     command will not cause further trace evaluations to occur.  Both oldName and
		     newName are fully qualified with any namespace(s) in which they appear.

	      trace add execution name ops commandPrefix
		     Arrange  for  commandPrefix to be executed (with additional arguments) when-
		     ever command name is executed, with traces occurring at the points indicated
		     by the list ops.  Name will be resolved using the usual namespace resolution
		     rules used by commands.  If the command does not exist,  an  error  will  be
		     thrown.

		     Ops indicates which operations are of interest, and is a list of one or more
		     of the following items:

		     enter  Invoke commandPrefix whenever the  command	name  is  executed,  just
			    before the actual execution takes place.

		     leave  Invoke  commandPrefix  whenever  the  command  name is executed, just
			    after the actual execution takes place.

		     enterstep
			    Invoke commandPrefix for every Tcl command which is executed from the
			    start  of  the  execution  of the procedure name until that procedure
			    finishes. CommandPrefix is invoked just before the	actual	execution
			    of	the  Tcl  command  being reported takes place.	For example if we
			    have "proc foo {} { puts "hello" }", then an enterstep trace would be
			    invoked just before "puts "hello"" is executed.  Setting an enterstep
			    trace on a command name that does not refer to a procedure	will  not
			    result in an error and is simply ignored.

		     leavestep
			    Invoke commandPrefix for every Tcl command which is executed from the
			    start of the execution of the procedure  name  until  that	procedure
			    finishes. CommandPrefix is invoked just after the actual execution of
			    the Tcl command being reported  takes  place.   Setting  a	leavestep
			    trace  on  a command name that does not refer to a procedure will not
			    result in an error and is simply ignored.

		     When the trace triggers, depending on the operations being traced, a  number
		     of  arguments are appended to commandPrefix so that the actual command is as
		     follows:

		     For enter and enterstep operations:
			    commandPrefix command-string op
		     Command-string gives the complete current command being executed (the traced
		     command  for  a enter operation, an arbitrary command for a enterstep opera-
		     tion), including all arguments in their fully expanded form.   Op	indicates
		     what  operation  is  being performed on the command execution, and is one of
		     enter or enterstep as defined above.  The trace operation	can  be  used  to
		     stop  the	command  from executing, by deleting the command in question.  Of
		     course when the command is subsequently executed, an "invalid command" error
		     will occur.

		     For leave and leavestep operations:
			    command command-string code result op
		     Command-string gives the complete current command being executed (the traced
		     command for a enter operation, an arbitrary command for a	enterstep  opera-
		     tion), including all arguments in their fully expanded form.  Code gives the
		     result code of that execution, and result the result string.   Op	indicates
		     what  operation  is  being performed on the command execution, and is one of
		     leave or leavestep as defined above.  Note that the creation of many  enter-
		     step  or leavestep traces can lead to unintuitive results, since the invoked
		     commands from one trace can themselves lead to further  command  invocations
		     for other traces.

		     CommandPrefix  executes  in  the  same  context as the code that invoked the
		     traced operation: thus the commandPrefix, if invoked from a procedure,  will
		     have  access to the same local variables as code in the procedure. This con-
		     text may be different than the context in which the trace	was  created.  If
		     commandPrefix  invokes  a procedure (which it normally does) then the proce-
		     dure will have to use upvar or uplevel commands if it wishes to  access  the
		     local variables of the code which invoked the trace operation.

		     While  commandPrefix  is executing during an execution trace, traces on name
		     are temporarily disabled. This allows the commandPrefix to execute  name  in
		     its  body without invoking any other traces again.  If an error occurs while
		     executing the commandPrefix, then the command name as a  whole  will  return
		     that same error.

		     When  multiple  traces  are set on name, then for enter and enterstep opera-
		     tions, the traced commands are invoked in	the  reverse  order  of  how  the
		     traces  were originally created; and for leave and leavestep operations, the
		     traced commands are invoked in the original order of creation.

		     The behavior of execution traces is currently undefined for a  command  name
		     imported into another namespace.

	      trace add variable name ops commandPrefix
		     Arrange  for commandPrefix to be executed whenever variable name is accessed
		     in one of the ways given by the list ops.	Name may refer to a normal  vari-
		     able,  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  commandPrefix 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 is a list of one or more
		     of the following items:

		     array  Invoke  commandPrefix  whenever  the variable is accessed or modified
			    via the array command, provided that name is not a scalar variable at
			    the  time  that  the  array  command is invoked.  If name is a scalar
			    variable, the access via the  array  command  will	not  trigger  the
			    trace.

		     read   Invoke commandPrefix whenever the variable is read.

		     write  Invoke commandPrefix whenever the variable is written.

		     unset  Invoke  commandPrefix  whenever the variable is unset.  Variables can
			    be unset explicitly with the unset command, or implicitly when proce-
			    dures 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 commandPrefix so
		     that the actual command is as follows:
			    commandPrefix 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 refer-
		     ence a variable under a different name.   Op  indicates  what  operation  is
		     being  performed  on  the	variable,  and is one of read, write, or unset as
		     defined above.

		     CommandPrefix 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 commandPrefix 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 commandPrefix 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, commandPrefix can modify the variable to affect
		     the result of the traced operation.  If commandPrefix 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  commandPrefix
		     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, commandPrefix is invoked	after  the  vari-
		     able'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, commandPrefix will have to restore the old value of the
		     variable.

		     While commandPrefix is executing during a read or write trace, traces on the
		     variable are temporarily disabled.  This means that reads and writes invoked
		     by commandPrefix will occur directly, without invoking commandPrefix (or any
		     other  traces)  again.   However,	if commandPrefix 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 cre-
		     ation,  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 remove variable 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 remove type name opList commandPrefix
	      Where type is either command, execution or variable.

	      trace remove command name opList commandPrefix
		     If there is a trace set on command name  with  the  operations  and  command
		     given  by	opList and commandPrefix, then the trace is removed, so that com-
		     mandPrefix will never again be invoked.  Returns an empty string.	 If  name
		     does not exist, the command will throw an error.

	      trace remove execution name opList commandPrefix
		     If  there	is  a  trace  set on command name with the operations and command
		     given by opList and commandPrefix, then the trace is removed, so  that  com-
		     mandPrefix  will never again be invoked.  Returns an empty string.   If name
		     does not exist, the command will throw an error.

	      trace remove variable name opList commandPrefix
		     If there is a trace set on variable name with  the  operations  and  command
		     given  by	opList and commandPrefix, then the trace is removed, so that com-
		     mandPrefix will never again be invoked.  Returns an empty string.

       trace info type name
	      Where type is either command, execution or variable.

	      trace info command name
		     Returns a list containing one element for each trace currently set  on  com-
		     mand  name.  Each	element  of the list is itself a list containing two ele-
		     ments, which are the opList and commandPrefix associated with the trace.  If
		     name does not have any traces set, then the result of the command will be an
		     empty string.  If name does not exist, the command will throw an error.

	      trace info execution name
		     Returns a list containing one element for each trace currently set  on  com-
		     mand  name.  Each	element  of the list is itself a list containing two ele-
		     ments, which are the opList and commandPrefix associated with the trace.  If
		     name does not have any traces set, then the result of the command will be an
		     empty string.  If name does not exist, the command will throw an error.

	      trace info variable name
		     Returns a list containing one element for each trace currently set on  vari-
		     able  name.   Each  element of the list is itself a list containing two ele-
		     ments, which are the opList and commandPrefix associated with the trace.  If
		     name  does not exist or does not have any traces set, then the result of the
		     command will be an empty string.

       For backwards compatibility, three other subcommands are available:

	      trace variable name ops command
		     This is equivalent to trace add variable name ops command.

	      trace vdelete name ops command
		     This is equivalent to trace remove variable name ops command

	      trace vinfo name
		     This is equivalent to trace info variable name

       These subcommands are deprecated and will likely be removed in a future	version  of  Tcl.
       They  use an older syntax in which array, read, write, unset are replaced by a, r, w and u
       respectively, and the ops argument is not a list, but simply a string concatenation of the
       operations, such as rwua.

EXAMPLES
       Print  a  message whenever either of the global variables foo and bar are updated, even if
       they have a different local name at the time (which can be done with the upvar command):
	      proc tracer {varname args} {
		  upvar #0 $varname var
		  puts "$varname was updated to be \"$var\""
	      }
	      trace add variable foo write "tracer foo"
	      trace add variable bar write "tracer bar"

       Ensure that the global variable foobar always contains the product of the global variables
       foo and bar:
	      proc doMult args {
		  global foo bar foobar
		  set foobar [expr {$foo * $bar}]
	      }
	      trace add variable foo write doMult
	      trace add variable bar write doMult

       Print a trace of what commands are executed during the processing of a Tcl procedure:
	      proc x {} { y }
	      proc y {} { z }
	      proc z {} { puts hello }
	      proc report args {puts [info level 0]}
	      trace add execution x enterstep report
	      x
		-> report y enterstep
		  report z enterstep
		  report {puts hello} enterstep
		  hello

SEE ALSO
       set(n), unset(n)

KEYWORDS
       read, command, rename, variable, write, trace, unset

Tcl					       8.4					 trace(n)
Unix & Linux Commands & Man Pages : ©2000 - 2018 Unix and Linux Forums


All times are GMT -4. The time now is 12:34 PM.