Provided by: tcl8.4-doc_8.4.20-8_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