NAME
       trace - Monitor variable accesses

SYNOPSIS
       trace option ?arg arg ...?


DESCRIPTION
       This command causes Tcl commands to be executed whenever certain opera-
       tions 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  manipu-
              lated.   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 necessar-
              ily  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 differ-
              ent 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  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,  com-
              mand 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  com-
              mand 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 associ-
              ated 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
