Provided by: tcl8.4-doc_8.4.20-7_all 
NAME
trace - Monitor variable accesses, command usages and command executions
SYNOPSIS
trace option ?arg arg ...?
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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 command 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 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 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 operations 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 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 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 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. 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 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 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 unintuitive 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 normally does) then the procedure
will have to use upvar or uplevel commands if it wishes to access the local variables 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 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 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 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
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
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 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 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 operations 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 operations 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 operations 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 currently 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 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 elements, 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
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 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 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(3tcl), unset(3tcl)
KEYWORDS
read, command, rename, variable, write, trace, unset
Tcl 8.4 trace(3tcl)