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.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 \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, TCL_PARSE_PART1, 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 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.

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).

As a special case, if the flag TCL_PARSE_PART1 is specified,

\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 the flag TCL_PARSE_PART1 is given,

\fIname2\fR should be NULL since the array and element names

are taken from \fIname1\fR.

.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