1Tcl_TraceCommand(3) Tcl Library Procedures Tcl_TraceCommand(3)
2______________________________________________________________________________
6
8 Tcl_CommandTraceInfo, Tcl_TraceCommand, Tcl_UntraceCommand - monitor
9 renames and deletes of a command
10
12 #include <tcl.h>
13 ClientData
15 Tcl_CommandTraceInfo(interp, cmdName, flags, proc, prevClientData)
16 int
18 Tcl_TraceCommand(interp, cmdName, flags, proc, clientData)
19 void
21 Tcl_UntraceCommand(interp, cmdName, flags, proc, clientData)
22
24 Tcl_Interp *interp (in) Interpreter contain‐
25 ing the command.
26 const char *cmdName (in) Name of command.
28 int flags (in) OR'ed collection of
30 the values
31 TCL_TRACE_RENAME and
32 TCL_TRACE_DELETE.
33 Tcl_CommandTraceProc *proc (in) Procedure to call
35 when specified opera‐
36 tions occur to cmd‐
37 Name.
38 ClientData clientData (in) Arbitrary argument to
40 pass to proc.
41 ClientData prevClientData (in) If non-NULL, gives
43 last value returned
44 by Tcl_CommandTrace‐
45 Info, so this call
46 will return informa‐
47 tion about next
48 trace. If NULL, this
49 call will return
50 information about
51 first trace.
52_________________________________________________________________
53
55 Tcl_TraceCommand allows a C procedure to monitor operations performed
56 on a Tcl command, so that the C procedure is invoked whenever the com‐
57 mand is renamed or deleted. If the trace is created successfully then
58 Tcl_TraceCommand returns TCL_OK. If an error occurred (e.g. cmdName
59 specifies a non-existent command) then TCL_ERROR is returned and an
60 error message is left in the interpreter's result.
61 The flags argument to Tcl_TraceCommand indicates when the trace proce‐
63 dure is to be invoked. It consists of an OR'ed combination of any of
64 the following values:
65 TCL_TRACE_RENAME
67 Invoke proc whenever the command is renamed.
68 TCL_TRACE_DELETE
70 Invoke proc when the command is deleted.
71 Whenever one of the specified operations occurs to the command, proc
73 will be invoked. It should have arguments and result that match the
74 type Tcl_CommandTraceProc:
75 typedef void Tcl_CommandTraceProc(
76 ClientData clientData,
77 Tcl_Interp *interp,
78 const char *oldName,
79 const char *newName,
80 int flags);
81 The clientData and interp parameters will have the same values as those
82 passed to Tcl_TraceCommand when the trace was created. ClientData typ‐
83 ically points to an application-specific data structure that describes
84 what to do when proc is invoked. OldName gives the name of the command
85 being renamed, and newName gives the name that the command is being
86 renamed to (or an empty string or NULL when the command is being
87 deleted.) Flags is an OR'ed combination of bits potentially providing
88 several pieces of information. One of the bits TCL_TRACE_RENAME and
89 TCL_TRACE_DELETE will be set in flags to indicate which operation is
90 being performed on the command. The bit TCL_TRACE_DESTROYED will be
91 set in flags if the trace is about to be destroyed; this information
92 may be useful to proc so that it can clean up its own internal data
93 structures (see the section TCL_TRACE_DESTROYED below for more
94 details). Lastly, the bit TCL_INTERP_DESTROYED will be set if the
95 entire interpreter is being destroyed. When this bit is set, proc must
96 be especially careful in the things it does (see the section
97 TCL_INTERP_DESTROYED below).
98 Tcl_UntraceCommand may be used to remove a trace. If the command spec‐
100 ified by interp, cmdName, and flags has a trace set with flags, proc,
101 and clientData, then the corresponding trace is removed. If no such
102 trace exists, then the call to Tcl_UntraceCommand has no effect. The
103 same bits are valid for flags as for calls to Tcl_TraceCommand.
104 Tcl_CommandTraceInfo may be used to retrieve information about traces
106 set on a given command. The return value from Tcl_CommandTraceInfo is
107 the clientData associated with a particular trace. The trace must be
108 on the command specified by the interp, cmdName, and flags arguments
109 (note that currently the flags are ignored; flags should be set to 0
110 for future compatibility) and its trace procedure must the same as the
111 proc argument. If the prevClientData argument is NULL then the return
112 value corresponds to the first (most recently created) matching trace,
113 or NULL if there are no matching traces. If the prevClientData argu‐
114 ment is not NULL, then it should be the return value from a previous
115 call to Tcl_CommandTraceInfo. In this case, the new return value will
116 correspond to the next matching trace after the one whose clientData
117 matches prevClientData, or NULL if no trace matches prevClientData or
118 if there are no more matching traces after it. This mechanism makes it
119 possible to step through all of the traces for a given command that
120 have the same proc.
121
123 During rename traces, the command being renamed is visible with both
124 names simultaneously, and the command still exists during delete traces
125 (if TCL_INTERP_DESTROYED is not set). However, there is no mechanism
126 for signaling that an error occurred in a trace procedure, so great
127 care should be taken that errors do not get silently lost.
128
130 It is possible for multiple traces to exist on the same command. When
131 this happens, all of the trace procedures will be invoked on each
132 access, in order from most-recently-created to least-recently-created.
133 Attempts to delete the command during a delete trace will fail
134 silently, since the command is already scheduled for deletion anyway.
135 If the command being renamed is renamed by one of its rename traces,
136 that renaming takes precedence over the one that triggered the trace
137 and the collection of traces will not be reexecuted; if several traces
138 rename the command, the last renaming takes precedence.
139
141 In a delete callback to proc, the TCL_TRACE_DESTROYED bit is set in
142 flags.
143
145 When an interpreter is destroyed, unset traces are called for all of
146 its commands. The TCL_INTERP_DESTROYED bit will be set in the flags
147 argument passed to the trace procedures. Trace procedures must be
148 extremely careful in what they do if the TCL_INTERP_DESTROYED bit is
149 set. It is not safe for the procedures to invoke any Tcl procedures on
150 the interpreter, since its state is partially deleted. All that trace
151 procedures should do under these circumstances is to clean up and free
152 their own internal data structures.
153
155 Tcl does not do any error checking to prevent trace procedures from
156 misusing the interpreter during traces with TCL_INTERP_DESTROYED set.
157
159 clientData, trace, command
160Tcl 7.4 Tcl_TraceCommand(3)