Tcl_TraceVar(3) Tcl Library Procedures Tcl_TraceVar(3)
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NAME
Tcl_TraceVar, Tcl_TraceVar2, Tcl_UntraceVar, Tcl_UntraceVar2, Tcl_VarTraceInfo, Tcl_VarTraceInfo2 -monitor Tcl_VarTraceInfo2monitor
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 vari-able variable
able 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 opera-tions operations
tions 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_VarTrace-Info Tcl_VarTraceInfo
Info or Tcl_VarTraceInfo2, so this call will return
information about next trace. If NULL, this call will
return information about first trace.
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DESCRIPTION
Tcl_TraceVar allows a C procedure to monitor and control access to a Tcl variable, so that the C pro-cedure procedure
cedure is invoked whenever the variable is read or written or unset. If the trace is created suc-cessfully successfully
cessfully then Tcl_TraceVar returns TCL_OK. If an error occurred (e.g. varName specifies an element
of an array, but the actual variable isn't 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:
TCL_GLOBAL_ONLY
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.
TCL_NAMESPACE_ONLY
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 pro-cedures. procedures.
cedures.
TCL_TRACE_READS
Invoke proc whenever an attempt is made to read the variable.
TCL_TRACE_WRITES
Invoke proc whenever an attempt is made to modify the variable.
TCL_TRACE_UNSETS
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).
TCL_TRACE_ARRAY
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. |
TCL_TRACE_RESULT_DYNAMIC ||
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. |
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 opera-tion operation
tion 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 pro-cedure procedure
cedure 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 isn't 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 ele-ment element
ment 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 vari-able variable
able using Tcl_GetVar2, Tcl_SetVar2, and other procedures. While proc is executing, traces are tem-porarily temporarily
porarily 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 invoca-tions invocations
tions 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. leastrecently-created.
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 fur-ther further
ther 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 pro-cedure procedure
cedure 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 is a partially-formed result in the
interpreter's result area. If the trace procedure does anything that could damage this result (such
as calling Tcl_Eval) then it must save the original values of the interpreter's result and freeProc
fields and restore them 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 isn't 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 par-tially partially
tially deleted. All that trace procedures should do under these circumstances is to clean up and
free their own internal data structures.
BUGS
Tcl doesn't 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.
KEYWORDS
clientData, trace, variable
Tcl 7.4 Tcl_TraceVar(3)
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