trace(n) Tcl Built-In Commands trace(n)______________________________________________________________________________NAMEtrace - Monitor variable accesses
SYNOPSIStrace 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.
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
read, variable, write, trace, unset
Tcltrace(n)
