  NAME
    trace - Monitor variable accesses and command usages
  SYNOPSIS
    trace option ?arg arg ...?
  DESCRIPTION
    trace add type name ops ?args?
      trace add command name ops command
        rename
        delete
      trace add variable name ops command
        array
        read
        write
        unset
    trace remove type name opList command
      trace remove command name opList command
      trace remove variable name opList command
    trace list type name
      trace list command name
      trace list variable name
    trace variable name ops command
    trace vdelete name ops command
    trace vinfo name
  SEE ALSO
  KEYWORDS

### NAME

trace - Monitor variable accesses and command usages

### 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, 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, three 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) namename, 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 variable, 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.

  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 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 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 list type name
  Where type is either command or variable.

  trace list 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 list 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 list 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.

### SEE ALSO

set, unset

### KEYWORDS

read, command, rename, variable, write, trace, unset

    Copyright © 1993 The Regents of the University of California.
    Copyright © 1994-1996 Sun Microsystems, Inc.
    Copyright © 2000 Ajuba Solutions.
    Copyright © 1995-1997 Roger E. Critchlow Jr.