Tcl_TraceVar(3Tcl)Tcl_TraceVar(3Tcl)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.
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. If |
the name references an |
element of an array, then |
it must be in writable |
memory: Tcl will make |
temporary modifications to |
it while looking up the |
name.
int flags (in) OR-ed combination of the
values TCL_TRACE_READS,
TCL_TRACE_WRITES, and
TCL_TRACE_UNSETS, and
TCL_GLOBAL_ONLY. Not all
flags are used by all
procedures. See below for
more information.
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Tcl_TraceVar(3Tcl)Tcl_TraceVar(3Tcl)
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.
char *name1 (in) Name of scalar or array
variable (without array
index).
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 isn't an array)
then TCL_ERROR is returned and an error message is left in interp-
>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_TRACE_READS
Invoke proc whenever an attempt is made to read the variable.
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Tcl_TraceVar(3Tcl)Tcl_TraceVar(3Tcl)
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).
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, 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_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
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Tcl_TraceVar(3Tcl)Tcl_TraceVar(3Tcl)
TCL_GLOBAL_ONLY bit 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 has already been
separated by the caller into two parts. Name1 gives the name of a scalar
variable or array, and name2 gives the name of an element within an
array. If name2 is NULL 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 and write 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 |
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Tcl_TraceVar(3Tcl)Tcl_TraceVar(3Tcl)
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 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. 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).
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Tcl_TraceVar(3Tcl)Tcl_TraceVar(3Tcl)
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 partially 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.
KEYWORDS
clientData, trace, variable
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