Tcl_TraceVar(3tcl) Tcl Library Procedures Tcl_TraceVar(3tcl)
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NAME
Tcl_TraceVar, Tcl_TraceVar2, Tcl_UntraceVar, Tcl_UntraceVar2, Tcl_Var-
TraceInfo, 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 vari-
able, to an array vari-
able 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_OB-
JECT. Not all flags are
used by all procedures.
See below for more infor-
mation.
Tcl_VarTraceProc *proc (in) Procedure to invoke when-
ever 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 ar-
rays, is NULL.
ClientData prevClientData (in) If non-NULL, gives last
value returned by
Tcl_VarTraceInfo or
Tcl_VarTraceInfo2, so
this call will return in-
formation about next
trace. If NULL, this
call will return informa-
tion 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 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 speci-
fies 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 inter-
preter'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 proce-
dures.
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 un-
set) or when the interpreter is deleted (all variables are auto-
matically 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_RE-
SULT_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_DY-
NAMIC.
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 typi-
cally 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 vari-
able 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 when-
ever 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 informa-
tion 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 de-
tails). Lastly, the bit TCL_INTERP_DESTROYED will be set if the entire
interpreter is being destroyed. When this bit is set, proc must be es-
pecially careful in the things it does (see the section TCL_INTERP_DE-
STROYED 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 speci-
fied 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 re-
turn 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 previ-
ous 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 con-
tains 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 paren-
thesis 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 dis-
abled 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 vari-
able 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 dis-
abled 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 proce-
dures, 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 proce-
dure will be invoked whenever the variable's value is read. This in-
cludes 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 vari-
able 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 proce-
dure 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 in-
voked 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_Set-
Var2.
When unset tracing has been specified, the trace procedure will be in-
voked 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 ar-
ray 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 ac-
cess, 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 in-
dividual-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 re-
leased with ckfree) or a Tcl_Obj* (cast to char* and to be released
with Tcl_DecrRefCount) containing the error message. If a trace proce-
dure 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 re-
store 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 par-
tially formed results stored in the interpreter. If the trace proce-
dure 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 re-
turns.
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 in-
voked 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 ex-
tremely 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 pro-
cedures 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(3tcl)
KEYWORDS
clientData, trace, variable
Tcl 7.4 Tcl_TraceVar(3tcl)
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