tracevar.3

tcl是工具命令语言

.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 consists of 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.
.VS 8.1
When \fIname2\fR is NULL, 
\fIname1\fR 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 \fIname2\fR is NULL and \fIname1\fR does not refer
to an array element 
.VE
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, write, and array 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 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 \fBTcl_SetVar\fR
and \fBTcl_SetVar2\fR.
.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,
.VS 8.4
unless (\fIexactly\fR 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 \fBckfree\fR) or a
Tcl_Obj* (cast to char* and to be released with
\fBTcl_DecrRefCount\fR) containing the error message.
.VE 8.4
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.
.PP
Array traces are not yet integrated with the Tcl "info exists" command,
nor is there Tcl-level access to array traces.

.SH KEYWORDS
clientData, trace, variable