Provided by: tcl8.5-doc_8.5.19-1_all 
NAME
trace - Monitor variable accesses, command usages and command executions
SYNOPSIS
trace option ?arg arg ...?
________________________________________________________________________________________________________________
DESCRIPTION
This command causes Tcl commands to be executed whenever certain operations 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 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:
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 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 commandPrefix 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 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 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. CommandPrefix is
invoked just before the actual execution of the Tcl command being reported 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 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. CommandPrefix is
invoked just after the actual execution 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 being traced, a number of arguments
are appended to commandPrefix 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 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:
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 unintuitive 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 commandPrefix, 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 commandPrefix 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 commandPrefix is executing during an execution trace, traces on name are temporarily
disabled. This allows the commandPrefix to execute name in its body without 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 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 commandPrefix
Arrange for commandPrefix 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 commandPrefix 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 commandPrefix 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 commandPrefix whenever the variable is read.
write Invoke commandPrefix whenever the variable is written.
unset Invoke commandPrefix 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 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 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.
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 access 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 returned 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 returned
by the trace command (this mechanism can be used to implement read-only variables, for
example). For write traces, commandPrefix 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, 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 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 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 operations 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 operations 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 operations 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 currently 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 currently 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 currently 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 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
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)