trace(n)
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NAME
trace - Monitor variable accesses, command usages and command execu-
tions
SYNOPSIS
trace option ?arg arg ...?
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DESCRIPTION
This command causes Tcl commands to be executed whenever certain opera-
tions are invoked. The legal option's (which may be abbreviated) are:
trace add type name ops ?args?
Where type is command, execution, or variable.
trace add command name ops command
Arrange for command to be executed whenever command name
is modified in one of the ways given by the list ops.
Name will be resolved using the usual namespace resolu-
tion rules used by procedures. 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 command whenever the command is renamed.
Note that renaming to the empty string is consid-
ered deletion, and will not be traced with
'rename'.
delete Invoke command when the command is deleted. Com-
mands can be deleted explicitly by using the
rename command to rename the command 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 exe-
cute them.
When the trace triggers, depending on the operations
being traced, a number of arguments are appended to com-
mand so that the actual command is as follows:
command 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 com-
mand, 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 com-
mand 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 command
Arrange for command to be executed whenever command name
is executed, with traces occurring at the points indi-
cated by the list ops. Name will be resolved using the
usual namespace resolution rules used by procedures. 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 command whenever the command name is exe-
cuted, just before the actual execution takes
place.
leave Invoke command whenever the command name is exe-
cuted, just after the actual execution takes
place.
enterstep
Invoke command for every Tcl command which is exe-
cuted inside the procedure name, just before the
actual execution 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 will not result in an error and is
simply ignored.
leavestep
Invoke command for every Tcl command which is exe-
cuted inside the procedure name, just after the
actual execution takes place. Setting a leavestep
trace on a command 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 com-
mand so that the actual command is as follows:
For enter and enterstep operations:
command 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 operation), including
all arguments in their fully expanded form. Op indicates
what operation is being performed on the command execu-
tion, 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 operation), 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 enterstep or leavestep traces can lead to unintu-
itive results, since the invoked commands from one trace
can themselves lead to further command invocations for
other traces.
Command executes in the same context as the code that
invoked the traced operation: thus the command, if
invoked from a procedure, 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 nor-
mally does) then the procedure will have to use upvar or
uplevel commands if it wishes to access the local vari-
ables of the code which invoked the trace operation.
While command is executing during an execution trace,
traces on name are temporarily disabled. This allows the
command to execute name in its body without invoking any
other traces again. If an error occurs while executing
the command body, then the command name as a whole will
return that same error.
When multiple traces are set on name, then for enter and
enterstep operations, the traced commands are invoked in
the reverse order of how the traces were originally cre-
ated; 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 command
Arrange for command to be executed whenever variable name
is accessed in one of the ways given by the list 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 exists queries.
Ops indicates which operations are of interest, and is a
list of one or more of the following items:
array Invoke command 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 command whenever the variable is read.
write Invoke command whenever the variable is written.
unset 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 regis-
tered 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 read, write, or
unset 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 con-
text 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 neces-
sarily 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 vari-
able 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 vari-
ables, 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 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 remove variable com-
mand 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 command
Where type is either command, execution or variable.
trace remove command name opList command
If there is a trace set on command name with the opera-
tions and command given by opList and command, then the
trace is removed, so that command will never again be
invoked. Returns an empty string. If name doesn't
exist, the command will throw an error.
trace remove execution name opList command
If there is a trace set on command name with the opera-
tions and command given by opList and command, then the
trace is removed, so that command will never again be
invoked. Returns an empty string. If name doesn't
exist, the command will throw an error.
trace remove variable name opList command
If there is a trace set on variable name with the opera-
tions and command given by opList and command, then the
trace is removed, so that command 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 cur-
rently set on command name. Each element of the list is
itself a list containing two elements, which are the
opList and command associated with the trace. If name
doesn't have any traces set, then the result of the com-
mand will be an empty string. If name doesn't exist, the
command will throw an error.
trace info execution name
Returns a list containing one element for each trace cur-
rently set on command name. Each element of the list is
itself a list containing two elements, which are the
opList and command associated with the trace. If name
doesn't have any traces set, then the result of the com-
mand will be an empty string. If name doesn't exist, the
command will throw an error.
trace info variable name
Returns a list containing one element for each trace cur-
rently set on variable name. Each element of the list is
itself a list containing two elements, which are the
opList 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.
For backwards compatibility, three other subcommands are available:
trace variable name ops command
This is equivalent to trace add variable name ops com-
mand.
trace vdelete name ops command
This is equivalent to trace remove variable name ops com-
mand
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
SEE ALSO
set(n), unset(n)
KEYWORDS
read, command, rename, variable, write, trace, unset
Tcl 8.4 trace(n)
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