trace(n) Tcl Built-In Commands trace(n)
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. 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
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 necessarily 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 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 com-
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 exam-
ple). 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.
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 con-
taining 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