Tcl/Tk Documentation > TclLib > TraceVar
- Tcl_TraceVar, Tcl_TraceVar2, Tcl_UntraceVar, Tcl_UntraceVar2, Tcl_VarTraceInfo, Tcl_VarTraceInfo2 - monitor accesses to a variable
- TWO-PART NAMES
- ACCESSING VARIABLES DURING TRACES
- CALLBACK TIMING
- WHOLE-ARRAY TRACES
- MULTIPLE TRACES
- ERROR RETURNS
- UNDEFINED VARIABLES
- TCL_TRACE_DESTROYED FLAG
Tcl_TraceVar(interp, varName, flags, proc, clientData)
Tcl_TraceVar2(interp, name1, name2, flags, proc, clientData)
Tcl_UntraceVar(interp, varName, flags, proc, clientData)
Tcl_UntraceVar2(interp, name1, name2, flags, proc, clientData)
Tcl_VarTraceInfo(interp, varName, flags, proc, prevClientData)
Tcl_VarTraceInfo2(interp, name1, name2, flags, proc, prevClientData)
- 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.
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.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.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.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.Tcl_Eval) then it must save the original values of the interpreter's result and freeProc fields and restore them before it returns.
Array traces are not yet integrated with the Tcl info exists command, nor is there Tcl-level access to array traces.clientData, trace, variable