trace(3tcl) Tcl Built-In Commands trace(3tcl)
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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 options (which may be abbreviated) are:
trace add type name ops ?args?
Where type is command, execution, or variable.
trace add command name ops commandPrefix
Arrange for commandPrefix to be executed (with additional
arguments) whenever command name is modified in one of
the ways given by the list ops. Name will be resolved us-
ing the usual namespace resolution rules used by com-
mands. 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 commandPrefix whenever the traced command
is renamed. Note that renaming to the empty
string is considered deletion, and will not be
traced with “rename”.
delete Invoke commandPrefix when the traced command is
deleted. Commands can be deleted explicitly by us-
ing 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 in-
voked because there is no interpreter in which to
execute them.
When the trace triggers, depending on the operations be-
ing traced, a number of arguments are appended to com-
mandPrefix so that the actual command is as follows:
commandPrefix 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 commandPrefix
Arrange for commandPrefix to be executed (with additional
arguments) whenever command 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 commands. 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 commandPrefix whenever the command name is
executed, just before the actual execution takes
place.
leave Invoke commandPrefix whenever the command name is
executed, just after the actual execution takes
place.
enterstep
Invoke commandPrefix for every Tcl command which
is executed from the start of the execution of the
procedure name until that procedure finishes. Com-
mandPrefix is invoked just before the actual exe-
cution of the Tcl command being reported takes
place. For example if we have “proc foo {} { puts
"hello" }”, then an enterstep trace would be in-
voked just before “puts "hello"” is executed.
Setting an enterstep trace on a command name that
does not refer to a procedure will not result in
an error and is simply ignored.
leavestep
Invoke commandPrefix for every Tcl command which
is executed from the start of the execution of the
procedure name until that procedure finishes. Com-
mandPrefix is invoked just after the actual execu-
tion of the Tcl command being reported takes
place. Setting a leavestep trace on a command
name that does not refer to a procedure will not
result in an error and is simply ignored.
When the trace triggers, depending on the operations be-
ing traced, a number of arguments are appended to com-
mandPrefix so that the actual command is as follows:
For enter and enterstep operations:
commandPrefix 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 “in-
valid command” error will occur.
For leave and leavestep operations:
commandPrefix 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.
CommandPrefix executes in the same context as the code
that invoked the traced operation: thus the commandPre-
fix, if invoked from a procedure, will have access to the
same local variables as code in the procedure. This con-
text may be different than the context in which the trace
was created. If commandPrefix invokes a procedure (which
it normally does) then the procedure will have to use up-
var or uplevel commands if it wishes to access the local
variables of the code which invoked the trace operation.
While commandPrefix is executing during an execution
trace, traces on name are temporarily disabled. This al-
lows the commandPrefix to execute name in its body with-
out invoking any other traces again. If an error occurs
while executing the commandPrefix, 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 commandPrefix
Arrange for commandPrefix 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 variable, an ele-
ment 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-
Prefix 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 vis-
ible 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 commandPrefix whenever the variable is ac-
cessed 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 commandPrefix whenever the variable is
read.
write Invoke commandPrefix whenever the variable is
written.
unset Invoke commandPrefix whenever the variable is un-
set. Variables can be unset explicitly with the
unset command, or implicitly when procedures re-
turn (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
commandPrefix so that the actual command is as follows:
commandPrefix 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.
CommandPrefix 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 commandPrefix
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 commandPrefix
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 ac-
cess is made through a variable defined with the upvar
command.
For read and write traces, commandPrefix can modify the
variable to affect the result of the traced operation.
If commandPrefix modifies the value of a variable during
a read or write trace, then the new value will be re-
turned as the result of the traced operation. The return
value from commandPrefix 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 re-
turned by the trace command (this mechanism can be used
to implement read-only variables, for example). For
write traces, commandPrefix is invoked after the vari-
able's value has been changed; it can write a new value
into the variable to override the original value speci-
fied in the write operation. To implement read-only
variables, commandPrefix will have to restore the old
value of the variable.
While commandPrefix is executing during a read or write
trace, traces on the variable are temporarily disabled.
This means that reads and writes invoked by commandPrefix
will occur directly, without invoking commandPrefix (or
any other traces) again. However, if commandPrefix un-
sets 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 re-
turn, then the trace will be invoked in the variable con-
text 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 vari-
able, the trace will be invoked. Any errors in unset
traces are ignored.
If there are multiple traces on a variable they are in-
voked in order of creation, most-recent first. If one
trace returns an error, then no further traces are in-
voked 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 un-
til 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 commandPrefix
Where type is either command, execution or variable.
trace remove command name opList commandPrefix
If there is a trace set on command name with the opera-
tions and command given by opList and commandPrefix, then
the trace is removed, so that commandPrefix will never
again be invoked. Returns an empty string. If name
does not exist, the command will throw an error.
trace remove execution name opList commandPrefix
If there is a trace set on command name with the opera-
tions and command given by opList and commandPrefix, then
the trace is removed, so that commandPrefix will never
again be invoked. Returns an empty string. If name
does not exist, the command will throw an error.
trace remove variable name opList commandPrefix
If there is a trace set on variable name with the opera-
tions and command given by opList and commandPrefix, then
the trace is removed, so that commandPrefix 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 commandPrefix associated with the trace. If
name does not have any traces set, then the result of the
command will be an empty string. If name does not 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 commandPrefix associated with the trace. If
name does not have any traces set, then the result of the
command will be an empty string. If name does not 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 commandPrefix associated with the trace. If
name does not exist or does not 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
Print a trace of what commands are executed during the processing of a
Tcl procedure:
proc x {} { y }
proc y {} { z }
proc z {} { puts hello }
proc report args {puts [info level 0]}
trace add execution x enterstep report
x
→ report y enterstep
report z enterstep
report {puts hello} enterstep
hello
SEE ALSO
set(3tcl), unset(3tcl)
KEYWORDS
read, command, rename, variable, write, trace, unset
Tcl 8.4 trace(3tcl)
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