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 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
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
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
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
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