Tcl Source Code

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.TH Tcl_TraceVar 3 7.4 Tcl "Tcl Library Procedures"
.so man.macros
.BS
.SH NAME
Tcl_TraceVar, Tcl_TraceVar2, Tcl_UntraceVar, Tcl_UntraceVar2, Tcl_VarTraceInfo, Tcl_VarTraceInfo2 \- monitor accesses to a variable
.SH SYNOPSIS
.nf
\fB#include <tcl.h>\fR

\fBTCL_TRACE_WRITES\fR
Invoke \fIproc\fR whenever an attempt is made to modify the variable.
.TP
\fBTCL_TRACE_UNSETS\fR
Invoke \fIproc\fR whenever the variable is unset.
A variable may be unset either explicitly by an \fBunset\fR command,
or implicitly when a procedure returns (its local variables are
automatically unset) or when the interpreter is deleted (all
variables are automatically unset).
.TP
\fBTCL_TRACE_ARRAY\fR
Invoke \fIproc\fR 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.

procedure call:  the trace procedure will need to pass this flag
back to variable-related procedures like \fBTcl_GetVar\fR if it
attempts to access the variable.
The bit \fBTCL_TRACE_DESTROYED\fR will be set in \fIflags\fR if the trace is
about to be destroyed;  this information may be useful to \fIproc\fR
so that it can clean up its own internal data structures (see
the section \fBTCL_TRACE_DESTROYED\fR below for more details).
Lastly, the bit \fBTCL_INTERP_DESTROYED\fR will be set if the entire
interpreter is being destroyed.
When this bit is set, \fIproc\fR must be especially careful in
the things it does (see the section \fBTCL_INTERP_DESTROYED\fR below).
The trace procedure's return value should normally be NULL;  see
\fBERROR RETURNS\fR below for information on other possibilities.
.PP
\fBTcl_UntraceVar\fR may be used to remove a trace.
If the variable specified by \fIinterp\fR, \fIvarName\fR, and \fIflags\fR
has a trace set with \fIflags\fR, \fIproc\fR, and
\fIclientData\fR, then the corresponding trace is removed.

.PP
The return value from \fIproc\fR 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.
.SH "RESTRICTIONS"
.PP









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 \fBTcl_Eval\fR) then it must use the \fBTcl_SaveInterpState\fR
and related routines to save and restore the original state of
the interpreter before it returns.
.SH "UNDEFINED VARIABLES"

In an unset callback to \fIproc\fR, the \fBTCL_TRACE_DESTROYED\fR bit
is set in \fIflags\fR 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 \fBTCL_TRACE_DESTROYED\fR is not set is for
a whole-array trace invoked when only a single element of an
array is unset.
.SH "TCL_INTERP_DESTROYED"
.PP
When an interpreter is destroyed, unset traces are called for
all of its variables.
The \fBTCL_INTERP_DESTROYED\fR bit will be set in the \fIflags\fR
argument passed to the trace procedures.
Trace procedures must be extremely careful in what they do if
the \fBTCL_INTERP_DESTROYED\fR 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.
.SH BUGS
.PP
Tcl does not do any error checking to prevent trace procedures
from misusing the interpreter during traces with \fBTCL_INTERP_DESTROYED\fR
set.
.PP
Array traces are not yet integrated with the Tcl \fBinfo exists\fR command,
nor is there Tcl-level access to array traces.
.SH "SEE ALSO"
trace(n)
.SH KEYWORDS
clientData, trace, variable

#define TCL_NAMESPACE_ONLY	 2
#define TCL_APPEND_VALUE	 4
#define TCL_LIST_ELEMENT	 8
#define TCL_TRACE_READS		 0x10
#define TCL_TRACE_WRITES	 0x20
#define TCL_TRACE_UNSETS	 0x40
#define TCL_TRACE_DESTROYED	 0x80


#define TCL_INTERP_DESTROYED	 0x100


#define TCL_LEAVE_ERR_MSG	 0x200
#define TCL_TRACE_ARRAY		 0x800
#ifndef TCL_REMOVE_OBSOLETE_TRACES
/* Required to support old variable/vdelete/vinfo traces. */
#define TCL_TRACE_OLD_STYLE	 0x1000
#endif
/* Indicate the semantics of the result of a trace. */

 * TCL_TRACE_ANY_EXEC		- OR'd combination of all EXEC flags.
 * TCL_TRACE_EXEC_IN_PROGRESS   - The callback function on this trace is
 *				  currently executing. Therefore we don't let
 *				  further traces execute.
 * TCL_TRACE_EXEC_DIRECT	- This execution trace is triggered directly
 *				  by the command being traced, not because of
 *				  an internal trace.
 * The flags 'TCL_TRACE_DESTROYED' and 'TCL_INTERP_DESTROYED' may also be used
 * in command execution traces.
 */

#define TCL_TRACE_ENTER_DURING_EXEC	4
#define TCL_TRACE_LEAVE_DURING_EXEC	8
#define TCL_TRACE_ANY_EXEC		15
#define TCL_TRACE_EXEC_IN_PROGRESS	0x10
#define TCL_TRACE_EXEC_DIRECT		0x20

    part1 = TclGetString(part1Ptr);
    part2 = part2Ptr? TclGetString(part2Ptr) : NULL;

    return TclCallVarTraces(iPtr, arrayPtr, varPtr, part1, part2, flags,
	    leaveErrMsg);
}




int
TclCallVarTraces(
    Interp *iPtr,		/* Interpreter containing variable. */
    Var *arrayPtr,	/* Pointer to array variable that contains the
				 * variable, or NULL if the variable isn't an
				 * element of an array. */
    Var *varPtr,		/* Variable whose traces are to be invoked. */