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Tcl_TraceVar(3) Tcl Library Procedures Tcl_TraceVar(3) 





NAME


Tcl_TraceVar, Tcl_TraceVar2, Tcl_UntraceVar, Tcl_UntraceVar2,
    Tcl_VarTraceInfo, Tcl_VarTraceInfo2 - monitor accesses to a variable






SYNOPSIS


#include <tcl.h>

int
Tcl_TraceVar(interp, varName, flags, proc, clientData)

int
Tcl_TraceVar2(interp, name1, name2, flags, proc, clientData)

Tcl_UntraceVar(interp, varName, flags, proc, clientData)

Tcl_UntraceVar2(interp, name1, name2, flags, proc, clientData)

ClientData
Tcl_VarTraceInfo(interp, varName, flags, proc, prevClientData)

ClientData
Tcl_VarTraceInfo2(interp, name1, name2, flags, proc, prevClientData)






ARGUMENTS



  
Tcl_Interp *interp (in)

  
Interpreter containing variable.

  
const char *varName (in)

  
Name of variable. May refer to a scalar variable, to an array variable
      with no index, or to an array variable with a parenthesized index.

  
int flags (in)

  
OR-ed combination of the values TCL_TRACE_READS,
      TCL_TRACE_WRITES, TCL_TRACE_UNSETS, TCL_TRACE_ARRAY,
      TCL_GLOBAL_ONLY, TCL_NAMESPACE_ONLY,
      TCL_TRACE_RESULT_DYNAMIC and TCL_TRACE_RESULT_OBJECT. Not
      all flags are used by all procedures. See below for more information.

  
Tcl_VarTraceProc *proc (in)

  
Procedure to invoke whenever one of the traced operations occurs.

  
ClientData clientData (in)

  
Arbitrary one-word value to pass to proc.

  
const char *name1 (in)

  
Name of scalar or array variable (without array index).

  
const char *name2 (in)

  
For a trace on an element of an array, gives the index of the element. For
      traces on scalar variables or on whole arrays, is NULL.

  
ClientData prevClientData (in)

  
If non-NULL, gives last value returned by Tcl_VarTraceInfo or
      Tcl_VarTraceInfo2, so this call will return information about next
      trace. If NULL, this call will return information about first trace.
    
    

    

  








DESCRIPTION


Tcl_TraceVar allows a C procedure to monitor and control
    access to a Tcl variable, so that the C procedure is invoked whenever the
    variable is read or written or unset. If the trace is created successfully
    then Tcl_TraceVar returns TCL_OK. If an error occurred (e.g.
    varName specifies an element of an array, but the actual variable is
    not an array) then TCL_ERROR is returned and an error message is left
    in the interpreter's result.


The flags argument to Tcl_TraceVar indicates when
    the trace procedure is to be invoked and provides information for setting up
    the trace. It consists of an OR-ed combination of any of the following
    values:



  

  
Normally, the variable will be looked up at the current level of procedure
      call; if this bit is set then the variable will be looked up at global
      level, ignoring any active procedures.

  

  
Normally, the variable will be looked up at the current level of procedure
      call; if this bit is set then the variable will be looked up in the
      current namespace, ignoring any active procedures.

  

  
Invoke proc whenever an attempt is made to read the variable.

  

  
Invoke proc whenever an attempt is made to modify the
    variable.

  

  
Invoke proc whenever the variable is unset. A variable may be unset
      either explicitly by an unset command, or implicitly when a
      procedure returns (its local variables are automatically unset) or when
      the interpreter is deleted (all variables are automatically unset).

  

  
Invoke proc whenever the array command is invoked. This gives the
      trace procedure a chance to update the array before array names or array
      get is called. Note that this is called before an array set, but that will
      trigger write traces.

  

  
The result of invoking the proc is a dynamically allocated string
      that will be released by the Tcl library via a call to ckfree. Must
      not be specified at the same time as TCL_TRACE_RESULT_OBJECT.

  

  
The result of invoking the proc is a Tcl_Obj* (cast to a char*)
      with a reference count of at least one. The ownership of that reference
      will be transferred to the Tcl core for release (when the core has
      finished with it) via a call to Tcl_DecrRefCount. Must not be
      specified at the same time as TCL_TRACE_RESULT_DYNAMIC.




Whenever one of the specified operations occurs on the variable,
    proc will be invoked. It should have arguments and result that match
    the type Tcl_VarTraceProc:






typedef char *Tcl_VarTraceProc(


        ClientData clientData,


        Tcl_Interp *interp,


        char *name1,


        char *name2,


        int flags);




The clientData and interp parameters will have the
    same values as those passed to Tcl_TraceVar when the trace was
    created. ClientData typically points to an application-specific data
    structure that describes what to do when proc is invoked.
    Name1 and name2 give the name of the traced variable in the
    normal two-part form (see the description of Tcl_TraceVar2 below for
    details). Flags is an OR-ed combination of bits providing several
    pieces of information. One of the bits TCL_TRACE_READS,
    TCL_TRACE_WRITES, TCL_TRACE_ARRAY, or TCL_TRACE_UNSETS
    will be set in flags to indicate which operation is being performed
    on the variable. The bit TCL_GLOBAL_ONLY will be set whenever the
    variable being accessed is a global one not accessible from the current
    level of procedure call: the trace procedure will need to pass this flag
    back to variable-related procedures like Tcl_GetVar if it attempts to
    access the variable. The bit TCL_NAMESPACE_ONLY will be set whenever
    the variable being accessed is a namespace one not accessible from the
    current level of procedure call: the trace procedure will need to pass this
    flag back to variable-related procedures like Tcl_GetVar if it
    attempts to access the variable. The bit TCL_TRACE_DESTROYED will be
    set in flags if the trace is about to be destroyed; this information
    may be useful to proc so that it can clean up its own internal data
    structures (see the section TCL_TRACE_DESTROYED below for more
    details). Lastly, the bit TCL_INTERP_DESTROYED will be set if the
    entire interpreter is being destroyed. When this bit is set, proc
    must be especially careful in the things it does (see the section
    TCL_INTERP_DESTROYED below). The trace procedure's return value
    should normally be NULL; see ERROR RETURNS below for information on
    other possibilities.


Tcl_UntraceVar may be used to remove a trace. If the
    variable specified by interp, varName, and flags has a
    trace set with flags, proc, and clientData, then the
    corresponding trace is removed. If no such trace exists, then the call to
    Tcl_UntraceVar has no effect. The same bits are valid for
    flags as for calls to Tcl_TraceVar.


Tcl_VarTraceInfo may be used to retrieve information about
    traces set on a given variable. The return value from
    Tcl_VarTraceInfo is the clientData associated with a
    particular trace. The trace must be on the variable specified by the
    interp, varName, and flags arguments (only the
    TCL_GLOBAL_ONLY and TCL_NAMESPACE_ONLY bits from flags
    is used; other bits are ignored) and its trace procedure must the same as
    the proc argument. If the prevClientData argument is NULL then
    the return value corresponds to the first (most recently created) matching
    trace, or NULL if there are no matching traces. If the prevClientData
    argument is not NULL, then it should be the return value from a previous
    call to Tcl_VarTraceInfo. In this case, the new return value will
    correspond to the next matching trace after the one whose clientData
    matches prevClientData, or NULL if no trace matches
    prevClientData or if there are no more matching traces after it. This
    mechanism makes it possible to step through all of the traces for a given
    variable that have the same proc.






TWO-PART NAMES


The procedures Tcl_TraceVar2, Tcl_UntraceVar2, and
    Tcl_VarTraceInfo2 are identical to Tcl_TraceVar,
    Tcl_UntraceVar, and Tcl_VarTraceInfo, respectively, except
    that the name of the variable consists of two parts. Name1 gives the
    name of a scalar variable or array, and name2 gives the name of an
    element within an array. When name2 is NULL, name1 may contain
    both an array and an element name: if the name contains an open parenthesis
    and ends with a close parenthesis, then the value between the parentheses is
    treated as an element name (which can have any string value) and the
    characters before the first open parenthesis are treated as the name of an
    array variable. If name2 is NULL and name1 does not refer to
    an array element it means that either the variable is a scalar or the trace
    is to be set on the entire array rather than an individual element (see
    WHOLE-ARRAY TRACES below for more information).






ACCESSING VARIABLES DURING TRACES


During read, write, and array traces, the trace procedure can
    read, write, or unset the traced variable using Tcl_GetVar2,
    Tcl_SetVar2, and other procedures. While proc is executing,
    traces are temporarily disabled for the variable, so that calls to
    Tcl_GetVar2 and Tcl_SetVar2 will not cause proc or
    other trace procedures to be invoked again. Disabling only occurs for the
    variable whose trace procedure is active; accesses to other variables will
    still be traced. However, if a variable is unset during a read or write
    trace then unset traces will be invoked.


During unset traces the variable has already been completely
    expunged. It is possible for the trace procedure to read or write the
    variable, but this will be a new version of the variable. Traces are not
    disabled during unset traces as they are for read and write traces, but
    existing traces have been removed from the variable before any trace
    procedures are invoked. If new traces are set by unset trace procedures,
    these traces will be invoked on accesses to the variable by the trace
    procedures.






CALLBACK TIMING


When read tracing has been specified for a variable, the trace
    procedure will be invoked whenever the variable's value is read. This
    includes set Tcl commands, $-notation in Tcl commands, and
    invocations of the Tcl_GetVar and Tcl_GetVar2 procedures.
    Proc is invoked just before the variable's value is returned. It may
    modify the value of the variable to affect what is returned by the traced
    access. If it unsets the variable then the access will return an error just
    as if the variable never existed.


When write tracing has been specified for a variable, the trace
    procedure will be invoked whenever the variable's value is modified. This
    includes set commands, commands that modify variables as side effects
    (such as catch and scan), and calls to the Tcl_SetVar
    and Tcl_SetVar2 procedures). Proc will be invoked after the
    variable's value has been modified, but before the new value of the variable
    has been returned. It may modify the value of the variable to override the
    change and to determine the value actually returned by the traced access. If
    it deletes the variable then the traced access will return an empty
  string.


When array tracing has been specified, the trace procedure will be
    invoked at the beginning of the array command implementation, before any of
    the operations like get, set, or names have been invoked. The trace
    procedure can modify the array elements with Tcl_SetVar and
    Tcl_SetVar2.


When unset tracing has been specified, the trace procedure will be
    invoked whenever the variable is destroyed. The traces will be called after
    the variable has been completely unset.






WHOLE-ARRAY TRACES


If a call to Tcl_TraceVar or Tcl_TraceVar2 specifies
    the name of an array variable without an index into the array, then the
    trace will be set on the array as a whole. This means that proc will
    be invoked whenever any element of the array is accessed in the ways
    specified by flags. When an array is unset, a whole-array trace will
    be invoked just once, with name1 equal to the name of the array and
    name2 NULL; it will not be invoked once for each element.






MULTIPLE TRACES


It is possible for multiple traces to exist on the same variable.
    When this happens, all of the trace procedures will be invoked on each
    access, in order from most-recently-created to least-recently-created. When
    there exist whole-array traces for an array as well as traces on individual
    elements, the whole-array traces are invoked before the individual-element
    traces. If a read or write trace unsets the variable then all of the unset
    traces will be invoked but the remainder of the read and write traces will
    be skipped.






ERROR RETURNS


Under normal conditions trace procedures should return NULL,
    indicating successful completion. If proc returns a non-NULL value it
    signifies that an error occurred. The return value must be a pointer to a
    static character string containing an error message, unless (exactly
    one of) the TCL_TRACE_RESULT_DYNAMIC and
    TCL_TRACE_RESULT_OBJECT flags is set, which specify that the result
    is either a dynamic string (to be released with ckfree) or a Tcl_Obj*
    (cast to char* and to be released with Tcl_DecrRefCount) containing
    the error message. If a trace procedure returns an error, no further traces
    are invoked for the access and the traced access aborts with the given
    message. Trace procedures can use this facility to make variables read-only,
    for example (but note that the value of the variable will already have been
    modified before the trace procedure is called, so the trace procedure will
    have to restore the correct value).


The return value from proc is only used during read and
    write tracing. During unset traces, the return value is ignored and all
    relevant trace procedures will always be invoked.






RESTRICTIONS


A trace procedure can be called at any time, even when there are
    partially formed results stored in the interpreter. If the trace procedure
    does anything that could damage this result (such as calling
    Tcl_Eval) then it must use the Tcl_SaveInterpState and related
    routines to save and restore the original state of the interpreter before it
    returns.






UNDEFINED VARIABLES


It is legal to set a trace on an undefined variable. The variable
    will still appear to be undefined until the first time its value is set. If
    an undefined variable is traced and then unset, the unset will fail with an
    error (“no such variable”), but the trace procedure will still
    be invoked.






TCL_TRACE_DESTROYED FLAG


In an unset callback to proc, the
    TCL_TRACE_DESTROYED bit is set in flags if the trace is being
    removed as part of the deletion. Traces on a variable are always removed
    whenever the variable is deleted; the only time TCL_TRACE_DESTROYED
    is not set is for a whole-array trace invoked when only a single element of
    an array is unset.






TCL_INTERP_DESTROYED


When an interpreter is destroyed, unset traces are called for all
    of its variables. The TCL_INTERP_DESTROYED bit will be set in the
    flags argument passed to the trace procedures. Trace procedures must
    be extremely careful in what they do if the TCL_INTERP_DESTROYED bit
    is set. It is not safe for the procedures to invoke any Tcl procedures on
    the interpreter, since its state is partially deleted. All that trace
    procedures should do under these circumstances is to clean up and free their
    own internal data structures.






BUGS


Tcl does not do any error checking to prevent trace procedures
    from misusing the interpreter during traces with TCL_INTERP_DESTROYED
    set.


Array traces are not yet integrated with the Tcl info
    exists command, nor is there Tcl-level access to array traces.






SEE ALSO


trace(n)






KEYWORDS


clientData, trace, variable







  

    7.4
    Tcl