Tcl Built-In Commands trace(1T)
NAME
trace - Monitor variable accesses, command usages and com-
mand executions
SYNOPSIS
trace option ?arg arg ...?
DESCRIPTION
This command causes Tcl commands to be executed whenever
certain operations 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 com-
mand name is modified in one of the ways given by
the list ops. Name will be resolved using the
usual namespace resolution rules used by pro-
cedures. 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 considered deletion, and will not
be traced with 'rename'.
delete
Invoke command when the command is deleted.
Commands can be deleted explicitly by using
the rename command to rename the command to
an empty string. Commands are also deleted
when the interpreter is deleted, but traces
will not be invoked because there is no
interpreter in which to execute them.
When the trace triggers, depending on the opera-
tions being traced, a number of arguments are
appended to command so that the actual command is
as follows:
command oldName newName op
OldName and newName give the traced command's
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Tcl Built-In Commands trace(1T)
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 command, 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 command 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 com-
mand name is executed, with traces occurring at
the points indicated 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
executed, just before the actual execution
takes place.
leave
Invoke command whenever the command name is
executed, just after the actual execution
takes place.
enterstep
Invoke command for every Tcl command which is
executed 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
executed inside the procedure name, just
after the actual execution takes place.
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Tcl Built-In Commands trace(1T)
Setting a leavestep trace on a command will
not result in an error and is simply ignored.
When the trace triggers, depending on the opera-
tions being traced, a number of arguments are
appended to command 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 execution, 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 ques-
tion. 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 indi-
cates 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
unintuitive results, since the invoked commands
from one trace can themselves lead to further com-
mand invocations for other traces.
Command executes in the same context as the code
that invoked the traced operation: thus the com-
mand, 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 normally 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 opera-
tion.
While command is executing during an execution
trace, traces on name are temporarily disabled.
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Tcl Built-In Commands trace(1T)
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 com-
mands are invoked in the reverse order of how the
traces were originally created; 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 vari-
able name is accessed in one of the ways given by
the list ops. Name may refer to a normal vari-
able, 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 vari-
able 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 writ-
ten.
unset
Invoke command whenever the variable is
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Tcl Built-In Commands trace(1T)
unset. Variables can be unset explicitly
with the unset command, or implicitly when
procedures return (all of their local vari-
ables 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 read, write, or unset as
defined above.
Command executes in the same context as the code
that invoked the traced operation: if the vari-
able was accessed as part of a Tcl procedure, then
command will have access to the same local vari-
ables 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 opera-
tion. 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
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Tcl Built-In Commands trace(1T)
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 origi-
nal 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 dis-
abled. 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 unde-
fined 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 return-
ing 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 command described below, until the vari-
able 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.
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Tcl Built-In Commands trace(1T)
trace remove command name opList command
If there is a trace set on command name with the
operations and command given by opList and com-
mand, 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
operations and command given by opList and com-
mand, 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
operations and command given by opList and com-
mand, 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 currently set on command name. Each element
of the list is itself a list containing two ele-
ments, which are the opList and command associated
with the trace. If name doesn't have any traces
set, then the result of the command 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 currently set on command name. Each element
of the list is itself a list containing two ele-
ments, which are the opList and command associated
with the trace. If name doesn't have any traces
set, then the result of the command 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 currently set on variable name. Each ele-
ment of the list is itself a list containing two
elements, which are the opList and command associ-
ated with the trace. If name doesn't exist or
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Tcl Built-In Commands trace(1T)
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
command.
trace vdelete name ops command
This is equivalent to trace remove variable name
ops command
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 sim-
ply 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(1T), unset(1T)
KEYWORDS
read, command, rename, variable, write, trace, unset
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Tcl Built-In Commands trace(1T)
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
Availability SUNWTcl
Interface Stability Uncommitted
NOTES
Source for Tcl is available on http:/opensolaris.org.
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