340 lines
14 KiB
Groff
340 lines
14 KiB
Groff
'\"
|
|
'\" Copyright (c) 1989-1993 The Regents of the University of California.
|
|
'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
|
|
'\"
|
|
'\" See the file "license.terms" for information on usage and redistribution
|
|
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
|
|
'\"
|
|
'\" SCCS: @(#) TraceVar.3 1.26 96/08/26 12:59:52
|
|
'\"
|
|
.so man.macros
|
|
.TH Tcl_TraceVar 3 7.4 Tcl "Tcl Library Procedures"
|
|
.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
|
|
.sp
|
|
int
|
|
\fBTcl_TraceVar(\fIinterp, varName, flags, proc, clientData\fB)\fR
|
|
.sp
|
|
int
|
|
\fBTcl_TraceVar2(\fIinterp, name1, name2, flags, proc, clientData\fB)\fR
|
|
.sp
|
|
\fBTcl_UntraceVar(\fIinterp, varName, flags, proc, clientData\fB)\fR
|
|
.sp
|
|
\fBTcl_UntraceVar2(\fIinterp, name1, name2, flags, proc, clientData\fB)\fR
|
|
.sp
|
|
ClientData
|
|
\fBTcl_VarTraceInfo(\fIinterp, varName, flags, proc, prevClientData\fB)\fR
|
|
.sp
|
|
ClientData
|
|
\fBTcl_VarTraceInfo2(\fIinterp, name1, name2, flags, proc, prevClientData\fB)\fR
|
|
.SH ARGUMENTS
|
|
.AS Tcl_VarTraceProc prevClientData
|
|
.AP Tcl_Interp *interp in
|
|
Interpreter containing variable.
|
|
.AP 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.
|
|
.AP 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.
|
|
.AP Tcl_VarTraceProc *proc in
|
|
Procedure to invoke whenever one of the traced operations occurs.
|
|
.AP ClientData clientData in
|
|
Arbitrary one-word value to pass to \fIproc\fR.
|
|
.AP char *name1 in
|
|
Name of scalar or array variable (without array index).
|
|
.AP 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.
|
|
.AP ClientData prevClientData in
|
|
If non-NULL, gives last value returned by \fBTcl_VarTraceInfo\fR or
|
|
\fBTcl_VarTraceInfo2\fR, so this call will return information about
|
|
next trace. If NULL, this call will return information about first
|
|
trace.
|
|
.BE
|
|
|
|
.SH DESCRIPTION
|
|
.PP
|
|
\fBTcl_TraceVar\fR 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 \fBTcl_TraceVar\fR returns
|
|
TCL_OK. If an error occurred (e.g. \fIvarName\fR 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 \fIinterp->result\fR.
|
|
.PP
|
|
The \fIflags\fR argument to \fBTcl_TraceVar\fR 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:
|
|
.TP
|
|
\fBTCL_GLOBAL_ONLY\fR
|
|
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.
|
|
.TP
|
|
\fBTCL_TRACE_READS\fR
|
|
Invoke \fIproc\fR whenever an attempt is made to read the variable.
|
|
.TP
|
|
\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).
|
|
.PP
|
|
Whenever one of the specified operations occurs on the variable,
|
|
\fIproc\fR will be invoked.
|
|
It should have arguments and result that match the type
|
|
\fBTcl_VarTraceProc\fR:
|
|
.CS
|
|
typedef char *Tcl_VarTraceProc(
|
|
ClientData \fIclientData\fR,
|
|
Tcl_Interp *\fIinterp\fR,
|
|
char *\fIname1\fR,
|
|
char *\fIname2\fR,
|
|
int \fIflags\fR);
|
|
.CE
|
|
The \fIclientData\fR and \fIinterp\fR parameters will
|
|
have the same values as those passed to \fBTcl_TraceVar\fR when the
|
|
trace was created.
|
|
\fIClientData\fR typically points to an application-specific
|
|
data structure that describes what to do when \fIproc\fR
|
|
is invoked.
|
|
\fIName1\fR and \fIname2\fR give the name of the traced variable
|
|
in the normal two-part form (see the description of \fBTcl_TraceVar2\fR
|
|
below for details).
|
|
\fIFlags\fR 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 \fIflags\fR 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 \fBTcl_GetVar\fR if it
|
|
attempts to access the variable.
|
|
The bit TCL_TRACE_DESTROYED 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 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, \fIproc\fR 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.
|
|
.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.
|
|
If no such trace exists, then the call to \fBTcl_UntraceVar\fR
|
|
has no effect.
|
|
The same bits are valid for \fIflags\fR as for calls to \fBTcl_TraceVar\fR.
|
|
.PP
|
|
\fBTcl_VarTraceInfo\fR may be used to retrieve information about
|
|
traces set on a given variable.
|
|
The return value from \fBTcl_VarTraceInfo\fR is the \fIclientData\fR
|
|
associated with a particular trace.
|
|
The trace must be on the variable specified by the \fIinterp\fR,
|
|
\fIvarName\fR, and \fIflags\fR arguments (only the TCL_GLOBAL_ONLY
|
|
bit from \fIflags\fR is used; other bits are ignored) and its trace procedure
|
|
must the same as the \fIproc\fR argument.
|
|
If the \fIprevClientData\fR 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 \fIprevClientData\fR argument isn't NULL, then it should
|
|
be the return value from a previous call to \fBTcl_VarTraceInfo\fR.
|
|
In this case, the new return value will correspond to the next
|
|
matching trace after the one whose \fIclientData\fR matches
|
|
\fIprevClientData\fR, or NULL if no trace matches \fIprevClientData\fR
|
|
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 \fIproc\fR.
|
|
|
|
.SH "TWO-PART NAMES"
|
|
.PP
|
|
The procedures \fBTcl_TraceVar2\fR, \fBTcl_UntraceVar2\fR, and
|
|
\fBTcl_VarTraceInfo2\fR are identical to \fBTcl_TraceVar\fR,
|
|
\fBTcl_UntraceVar\fR, and \fBTcl_VarTraceInfo\fR, respectively,
|
|
except that the name of the variable has already been
|
|
separated by the caller into two parts.
|
|
\fIName1\fR gives the name of a scalar variable or array,
|
|
and \fIname2\fR gives the name of an element within an
|
|
array.
|
|
If \fIname2\fR 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).
|
|
|
|
.SH "ACCESSING VARIABLES DURING TRACES"
|
|
.PP
|
|
During read and write traces, the
|
|
trace procedure can read, write, or unset the traced
|
|
variable using \fBTcl_GetVar2\fR, \fBTcl_SetVar2\fR, and
|
|
other procedures.
|
|
While \fIproc\fR is executing, traces are temporarily disabled
|
|
for the variable, so that calls to \fBTcl_GetVar2\fR and
|
|
\fBTcl_SetVar2\fR will not cause \fIproc\fR 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.
|
|
.PP
|
|
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.
|
|
|
|
.SH "CALLBACK TIMING"
|
|
.PP
|
|
When read tracing has been specified for a variable, the trace
|
|
procedure will be invoked whenever the variable's value is
|
|
read. This includes \fBset\fR Tcl commands, \fB$\fR-notation
|
|
in Tcl commands, and invocations of the \fBTcl_GetVar\fR
|
|
and \fBTcl_GetVar2\fR procedures.
|
|
\fIProc\fR 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.
|
|
.PP
|
|
When write tracing has been specified for a variable, the
|
|
trace procedure will be invoked whenever the variable's value
|
|
is modified. This includes \fBset\fR commands,
|
|
commands that modify variables as side effects (such as
|
|
\fBcatch\fR and \fBscan\fR), and calls to the \fBTcl_SetVar\fR
|
|
and \fBTcl_SetVar2\fR procedures).
|
|
\fIProc\fR 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.
|
|
.PP
|
|
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.
|
|
|
|
.SH "WHOLE-ARRAY TRACES"
|
|
.PP
|
|
If a call to \fBTcl_TraceVar\fR or \fBTcl_TraceVar2\fR 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 \fIproc\fR will be invoked whenever any
|
|
element of the array is accessed in the ways specified by
|
|
\fIflags\fR.
|
|
When an array is unset, a whole-array trace will be invoked
|
|
just once, with \fIname1\fR equal to the name of the array
|
|
and \fIname2\fR NULL; it will not be invoked once for each
|
|
element.
|
|
|
|
.SH "MULTIPLE TRACES"
|
|
.PP
|
|
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.
|
|
|
|
.SH "ERROR RETURNS"
|
|
.PP
|
|
Under normal conditions trace procedures should return NULL, indicating
|
|
successful completion.
|
|
If \fIproc\fR 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).
|
|
.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
|
|
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 \fBTcl_Eval\fR) then it must save the original values of
|
|
the interpreter's \fBresult\fR and \fBfreeProc\fR fields and restore
|
|
them before it returns.
|
|
|
|
.SH "UNDEFINED VARIABLES"
|
|
.PP
|
|
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.
|
|
|
|
.SH "TCL_TRACE_DESTROYED FLAG"
|
|
.PP
|
|
In an unset callback to \fIproc\fR, the TCL_TRACE_DESTROYED 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 TCL_TRACE_DESTROYED isn't 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 TCL_INTERP_DESTROYED 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 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.
|
|
|
|
.SH BUGS
|
|
.PP
|
|
Tcl doesn't do any error checking to prevent trace procedures
|
|
from misusing the interpreter during traces with TCL_INTERP_DESTROYED
|
|
set.
|
|
|
|
.SH KEYWORDS
|
|
clientData, trace, variable
|