1Tcl_TraceVar(3) Tcl Library Procedures Tcl_TraceVar(3)
2______________________________________________________________________________
6
8 Tcl_TraceVar, Tcl_TraceVar2, Tcl_UntraceVar, Tcl_UntraceVar2, Tcl_Var‐
9 TraceInfo, Tcl_VarTraceInfo2 - monitor accesses to a variable
10
12 #include <tcl.h>
13 int
15 Tcl_TraceVar(interp, varName, flags, proc, clientData)
16 int
18 Tcl_TraceVar2(interp, name1, name2, flags, proc, clientData)
19 Tcl_UntraceVar(interp, varName, flags, proc, clientData)
21 Tcl_UntraceVar2(interp, name1, name2, flags, proc, clientData)
23 ClientData
25 Tcl_VarTraceInfo(interp, varName, flags, proc, prevClientData)
26 ClientData
28 Tcl_VarTraceInfo2(interp, name1, name2, flags, proc, prevClientData)
29
31 Tcl_Interp *interp (in) Interpreter containing
32 variable.
33 const char *varName (in) Name of variable. May
35 refer to a scalar vari‐
36 able, to an array vari‐
37 able with no index, or to
38 an array variable with a
39 parenthesized index.
40 int flags (in) OR-ed combination of the
42 values TCL_TRACE_READS,
43 TCL_TRACE_WRITES,
44 TCL_TRACE_UNSETS,
45 TCL_TRACE_ARRAY,
46 TCL_GLOBAL_ONLY,
47 TCL_NAMESPACE_ONLY,
48 TCL_TRACE_RESULT_DYNAMIC
49 and
50 TCL_TRACE_RESULT_OBJECT.
51 Not all flags are used by
52 all procedures. See
53 below for more informa‐
54 tion.
55 Tcl_VarTraceProc *proc (in) Procedure to invoke when‐
57 ever one of the traced
58 operations occurs.
59 ClientData clientData (in) Arbitrary one-word value
61 to pass to proc.
62 const char *name1 (in) Name of scalar or array
64 variable (without array
65 index).
66 const char *name2 (in) For a trace on an element
68 of an array, gives the
69 index of the element.
70 For traces on scalar
71 variables or on whole
72 arrays, is NULL.
73 ClientData prevClientData (in) If non-NULL, gives last
75 value returned by
76 Tcl_VarTraceInfo or
77 Tcl_VarTraceInfo2, so
78 this call will return
79 information about next
80 trace. If NULL, this
81 call will return informa‐
82 tion about first trace.
83______________________________________________________________________________
84
86 Tcl_TraceVar allows a C procedure to monitor and control access to a
87 Tcl variable, so that the C procedure is invoked whenever the variable
88 is read or written or unset. If the trace is created successfully then
89 Tcl_TraceVar returns TCL_OK. If an error occurred (e.g. varName speci‐
90 fies an element of an array, but the actual variable is not an array)
91 then TCL_ERROR is returned and an error message is left in the inter‐
92 preter's result.
93 The flags argument to Tcl_TraceVar indicates when the trace procedure
95 is to be invoked and provides information for setting up the trace. It
96 consists of an OR-ed combination of any of the following values:
97 TCL_GLOBAL_ONLY
99 Normally, the variable will be looked up at the current level of
100 procedure call; if this bit is set then the variable will be
101 looked up at global level, ignoring any active procedures.
102 TCL_NAMESPACE_ONLY
104 Normally, the variable will be looked up at the current level of
105 procedure call; if this bit is set then the variable will be
106 looked up in the current namespace, ignoring any active proce‐
107 dures.
108 TCL_TRACE_READS
110 Invoke proc whenever an attempt is made to read the variable.
111 TCL_TRACE_WRITES
113 Invoke proc whenever an attempt is made to modify the variable.
114 TCL_TRACE_UNSETS
116 Invoke proc whenever the variable is unset. A variable may be
117 unset either explicitly by an unset command, or implicitly when
118 a procedure returns (its local variables are automatically
119 unset) or when the interpreter is deleted (all variables are
120 automatically unset).
121 TCL_TRACE_ARRAY
123 Invoke proc whenever the array command is invoked. This gives
124 the trace procedure a chance to update the array before array
125 names or array get is called. Note that this is called before
126 an array set, but that will trigger write traces.
127 TCL_TRACE_RESULT_DYNAMIC
129 The result of invoking the proc is a dynamically allocated
130 string that will be released by the Tcl library via a call to
131 ckfree. Must not be specified at the same time as
132 TCL_TRACE_RESULT_OBJECT.
133 TCL_TRACE_RESULT_OBJECT
135 The result of invoking the proc is a Tcl_Obj* (cast to a char*)
136 with a reference count of at least one. The ownership of that
137 reference will be transferred to the Tcl core for release (when
138 the core has finished with it) via a call to Tcl_DecrRefCount.
139 Must not be specified at the same time as
140 TCL_TRACE_RESULT_DYNAMIC.
141 Whenever one of the specified operations occurs on the variable, proc
143 will be invoked. It should have arguments and result that match the
144 type Tcl_VarTraceProc:
145 typedef char *Tcl_VarTraceProc(
147 ClientData clientData,
148 Tcl_Interp *interp,
149 char *name1,
150 char *name2,
151 int flags);
152 The clientData and interp parameters will have the same values as those
154 passed to Tcl_TraceVar when the trace was created. ClientData typi‐
155 cally points to an application-specific data structure that describes
156 what to do when proc is invoked. Name1 and name2 give the name of the
157 traced variable in the normal two-part form (see the description of
158 Tcl_TraceVar2 below for details). Flags is an OR-ed combination of
159 bits providing several pieces of information. One of the bits
160 TCL_TRACE_READS, TCL_TRACE_WRITES, TCL_TRACE_ARRAY, or TCL_TRACE_UNSETS
161 will be set in flags to indicate which operation is being performed on
162 the variable. The bit TCL_GLOBAL_ONLY will be set whenever the vari‐
163 able being accessed is a global one not accessible from the current
164 level of procedure call: the trace procedure will need to pass this
165 flag back to variable-related procedures like Tcl_GetVar if it attempts
166 to access the variable. The bit TCL_NAMESPACE_ONLY will be set when‐
167 ever the variable being accessed is a namespace one not accessible from
168 the current level of procedure call: the trace procedure will need to
169 pass this flag back to variable-related procedures like Tcl_GetVar if
170 it attempts to access the variable. The bit TCL_TRACE_DESTROYED will
171 be set in flags if the trace is about to be destroyed; this informa‐
172 tion may be useful to proc so that it can clean up its own internal
173 data structures (see the section TCL_TRACE_DESTROYED below for more
174 details). Lastly, the bit TCL_INTERP_DESTROYED will be set if the
175 entire interpreter is being destroyed. When this bit is set, proc must
176 be especially careful in the things it does (see the section
177 TCL_INTERP_DESTROYED below). The trace procedure's return value should
178 normally be NULL; see ERROR RETURNS below for information on other
179 possibilities.
180 Tcl_UntraceVar may be used to remove a trace. If the variable speci‐
182 fied by interp, varName, and flags has a trace set with flags, proc,
183 and clientData, then the corresponding trace is removed. If no such
184 trace exists, then the call to Tcl_UntraceVar has no effect. The same
185 bits are valid for flags as for calls to Tcl_TraceVar.
186 Tcl_VarTraceInfo may be used to retrieve information about traces set
188 on a given variable. The return value from Tcl_VarTraceInfo is the
189 clientData associated with a particular trace. The trace must be on
190 the variable specified by the interp, varName, and flags arguments
191 (only the TCL_GLOBAL_ONLY and TCL_NAMESPACE_ONLY bits from flags is
192 used; other bits are ignored) and its trace procedure must the same as
193 the proc argument. If the prevClientData argument is NULL then the
194 return value corresponds to the first (most recently created) matching
195 trace, or NULL if there are no matching traces. If the prevClientData
196 argument is not NULL, then it should be the return value from a previ‐
197 ous call to Tcl_VarTraceInfo. In this case, the new return value will
198 correspond to the next matching trace after the one whose clientData
199 matches prevClientData, or NULL if no trace matches prevClientData or
200 if there are no more matching traces after it. This mechanism makes it
201 possible to step through all of the traces for a given variable that
202 have the same proc.
203
205 The procedures Tcl_TraceVar2, Tcl_UntraceVar2, and Tcl_VarTraceInfo2
206 are identical to Tcl_TraceVar, Tcl_UntraceVar, and Tcl_VarTraceInfo,
207 respectively, except that the name of the variable consists of two
208 parts. Name1 gives the name of a scalar variable or array, and name2
209 gives the name of an element within an array. When name2 is NULL,
210 name1 may contain both an array and an element name: if the name con‐
211 tains an open parenthesis and ends with a close parenthesis, then the
212 value between the parentheses is treated as an element name (which can
213 have any string value) and the characters before the first open paren‐
214 thesis are treated as the name of an array variable. If name2 is NULL
215 and name1 does not refer to an array element it means that either the
216 variable is a scalar or the trace is to be set on the entire array
217 rather than an individual element (see WHOLE-ARRAY TRACES below for
218 more information).
219
221 During read, write, and array traces, the trace procedure can read,
222 write, or unset the traced variable using Tcl_GetVar2, Tcl_SetVar2, and
223 other procedures. While proc is executing, traces are temporarily dis‐
224 abled for the variable, so that calls to Tcl_GetVar2 and Tcl_SetVar2
225 will not cause proc or other trace procedures to be invoked again.
226 Disabling only occurs for the variable whose trace procedure is active;
227 accesses to other variables will still be traced. However, if a vari‐
228 able is unset during a read or write trace then unset traces will be
229 invoked.
230 During unset traces the variable has already been completely expunged.
232 It is possible for the trace procedure to read or write the variable,
233 but this will be a new version of the variable. Traces are not dis‐
234 abled during unset traces as they are for read and write traces, but
235 existing traces have been removed from the variable before any trace
236 procedures are invoked. If new traces are set by unset trace proce‐
237 dures, these traces will be invoked on accesses to the variable by the
238 trace procedures.
239
241 When read tracing has been specified for a variable, the trace proce‐
242 dure will be invoked whenever the variable's value is read. This
243 includes set Tcl commands, $-notation in Tcl commands, and invocations
244 of the Tcl_GetVar and Tcl_GetVar2 procedures. Proc is invoked just
245 before the variable's value is returned. It may modify the value of
246 the variable to affect what is returned by the traced access. If it
247 unsets the variable then the access will return an error just as if the
248 variable never existed.
249 When write tracing has been specified for a variable, the trace proce‐
251 dure will be invoked whenever the variable's value is modified. This
252 includes set commands, commands that modify variables as side effects
253 (such as catch and scan), and calls to the Tcl_SetVar and Tcl_SetVar2
254 procedures). Proc will be invoked after the variable's value has been
255 modified, but before the new value of the variable has been returned.
256 It may modify the value of the variable to override the change and to
257 determine the value actually returned by the traced access. If it
258 deletes the variable then the traced access will return an empty
259 string.
260 When array tracing has been specified, the trace procedure will be
262 invoked at the beginning of the array command implementation, before
263 any of the operations like get, set, or names have been invoked. The
264 trace procedure can modify the array elements with Tcl_SetVar and
265 Tcl_SetVar2.
266 When unset tracing has been specified, the trace procedure will be
268 invoked whenever the variable is destroyed. The traces will be called
269 after the variable has been completely unset.
270
272 If a call to Tcl_TraceVar or Tcl_TraceVar2 specifies the name of an
273 array variable without an index into the array, then the trace will be
274 set on the array as a whole. This means that proc will be invoked
275 whenever any element of the array is accessed in the ways specified by
276 flags. When an array is unset, a whole-array trace will be invoked
277 just once, with name1 equal to the name of the array and name2 NULL;
278 it will not be invoked once for each element.
279
281 It is possible for multiple traces to exist on the same variable. When
282 this happens, all of the trace procedures will be invoked on each
283 access, in order from most-recently-created to least-recently-created.
284 When there exist whole-array traces for an array as well as traces on
285 individual elements, the whole-array traces are invoked before the
286 individual-element traces. If a read or write trace unsets the vari‐
287 able then all of the unset traces will be invoked but the remainder of
288 the read and write traces will be skipped.
289
291 Under normal conditions trace procedures should return NULL, indicating
292 successful completion. If proc returns a non-NULL value it signifies
293 that an error occurred. The return value must be a pointer to a static
294 character string containing an error message, unless (exactly one of)
295 the TCL_TRACE_RESULT_DYNAMIC and TCL_TRACE_RESULT_OBJECT flags is set,
296 which specify that the result is either a dynamic string (to be
297 released with ckfree) or a Tcl_Obj* (cast to char* and to be released
298 with Tcl_DecrRefCount) containing the error message. If a trace proce‐
299 dure returns an error, no further traces are invoked for the access and
300 the traced access aborts with the given message. Trace procedures can
301 use this facility to make variables read-only, for example (but note
302 that the value of the variable will already have been modified before
303 the trace procedure is called, so the trace procedure will have to
304 restore the correct value).
305 The return value from proc is only used during read and write tracing.
307 During unset traces, the return value is ignored and all relevant trace
308 procedures will always be invoked.
309
311 A trace procedure can be called at any time, even when there is a par‐
312 tially formed result in the interpreter's result area. If the trace
313 procedure does anything that could damage this result (such as calling
314 Tcl_Eval) then it must save the original values of the interpreter's
315 result and freeProc fields and restore them before it returns.
316
318 It is legal to set a trace on an undefined variable. The variable will
319 still appear to be undefined until the first time its value is set. If
320 an undefined variable is traced and then unset, the unset will fail
321 with an error (“no such variable”), but the trace procedure will still
322 be invoked.
323
325 In an unset callback to proc, the TCL_TRACE_DESTROYED bit is set in
326 flags if the trace is being removed as part of the deletion. Traces on
327 a variable are always removed whenever the variable is deleted; the
328 only time TCL_TRACE_DESTROYED is not set is for a whole-array trace
329 invoked when only a single element of an array is unset.
330
332 When an interpreter is destroyed, unset traces are called for all of
333 its variables. The TCL_INTERP_DESTROYED bit will be set in the flags
334 argument passed to the trace procedures. Trace procedures must be
335 extremely careful in what they do if the TCL_INTERP_DESTROYED bit is
336 set. It is not safe for the procedures to invoke any Tcl procedures on
337 the interpreter, since its state is partially deleted. All that trace
338 procedures should do under these circumstances is to clean up and free
339 their own internal data structures.
340
342 Tcl does not do any error checking to prevent trace procedures from
343 misusing the interpreter during traces with TCL_INTERP_DESTROYED set.
344 Array traces are not yet integrated with the Tcl info exists command,
346 nor is there Tcl-level access to array traces.
347
349 trace(n)
350
352 clientData, trace, variable
353Tcl 7.4 Tcl_TraceVar(3)