1Tcl_AsyncCreate(3) Tcl Library Procedures Tcl_AsyncCreate(3)
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8 Tcl_AsyncCreate, Tcl_AsyncMark, Tcl_AsyncInvoke, Tcl_AsyncDelete,
9 Tcl_AsyncReady - handle asynchronous events
10
12 #include <tcl.h>
13 Tcl_AsyncHandler
15 Tcl_AsyncCreate(proc, clientData)
16 Tcl_AsyncMark(async)
18 int
20 Tcl_AsyncInvoke(interp, code)
21 Tcl_AsyncDelete(async)
23 int
25 Tcl_AsyncReady()
26
28 Tcl_AsyncProc *proc (in) Procedure to invoke to handle
29 an asynchronous event.
30 ClientData clientData (in) One-word value to pass to
32 proc.
33 Tcl_AsyncHandler async (in) Token for asynchronous event
35 handler.
36 Tcl_Interp *interp (in) Tcl interpreter in which com‐
38 mand was being evaluated when
39 handler was invoked, or NULL
40 if handler was invoked when
41 there was no interpreter
42 active.
43 int code (in) Completion code from command
45 that just completed in
46 interp, or 0 if interp is
47 NULL.
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49
52 These procedures provide a safe mechanism for dealing with asynchronous
53 events such as signals. If an event such as a signal occurs while a
54 Tcl script is being evaluated then it isn't safe to take any substan‐
55 tive action to process the event. For example, it isn't safe to evalu‐
56 ate a Tcl script since the interpreter may already be in the middle of
57 evaluating a script; it may not even be safe to allocate memory, since
58 a memory allocation could have been in progress when the event
59 occurred. The only safe approach is to set a flag indicating that the
60 event occurred, then handle the event later when the world has returned
61 to a clean state, such as after the current Tcl command completes.
62 Tcl_AsyncCreate, Tcl_AsyncDelete, and Tcl_AsyncReady are thread sensi‐
64 tive. They access and/or set a thread-specific data structure in the
65 event of a core built with --enable-threads. The token created by
66 Tcl_AsyncCreate contains the needed thread information it was called
67 from so that calling Tcl_AsyncMark(token) will only yield the origin
68 thread into the asynchronous handler.
69 Tcl_AsyncCreate creates an asynchronous handler and returns a token for
71 it. The asynchronous handler must be created before any occurrences of
72 the asynchronous event that it is intended to handle (it is not safe to
73 create a handler at the time of an event). When an asynchronous event
74 occurs the code that detects the event (such as a signal handler)
75 should call Tcl_AsyncMark with the token for the handler. Tcl_Async‐
76 Mark will mark the handler as ready to execute, but it will not invoke
77 the handler immediately. Tcl will call the proc associated with the
78 handler later, when the world is in a safe state, and proc can then
79 carry out the actions associated with the asynchronous event. Proc
80 should have arguments and result that match the type Tcl_AsyncProc:
81 typedef int Tcl_AsyncProc(
82 ClientData clientData,
83 Tcl_Interp *interp,
84 int code);
85 The clientData will be the same as the clientData argument passed to
86 Tcl_AsyncCreate when the handler was created. If proc is invoked just
87 after a command has completed execution in an interpreter, then interp
88 will identify the interpreter in which the command was evaluated and
89 code will be the completion code returned by that command. The com‐
90 mand's result will be present in the interpreter's result. When proc
91 returns, whatever it leaves in the interpreter's result will be
92 returned as the result of the command and the integer value returned by
93 proc will be used as the new completion code for the command.
94 It is also possible for proc to be invoked when no interpreter is
96 active. This can happen, for example, if an asynchronous event occurs
97 while the application is waiting for interactive input or an X event.
98 In this case interp will be NULL and code will be 0, and the return
99 value from proc will be ignored.
100 The procedure Tcl_AsyncInvoke is called to invoke all of the handlers
102 that are ready. The procedure Tcl_AsyncReady will return non-zero
103 whenever any asynchronous handlers are ready; it can be checked to
104 avoid calls to Tcl_AsyncInvoke when there are no ready handlers. Tcl
105 calls Tcl_AsyncReady after each command is evaluated and calls
106 Tcl_AsyncInvoke if needed. Applications may also call Tcl_AsyncInvoke
107 at interesting times for that application. For example, Tcl's event
108 handler calls Tcl_AsyncReady after each event and calls Tcl_AsyncInvoke
109 if needed. The interp and code arguments to Tcl_AsyncInvoke have the
110 same meaning as for proc: they identify the active interpreter, if
111 any, and the completion code from the command that just completed.
112 Tcl_AsyncDelete removes an asynchronous handler so that its proc will
114 never be invoked again. A handler can be deleted even when ready, and
115 it will still not be invoked.
116 If multiple handlers become active at the same time, the handlers are
118 invoked in the order they were created (oldest handler first). The
119 code and the interpreter's result for later handlers reflect the values
120 returned by earlier handlers, so that the most recently created handler
121 has last say about the interpreter's result and completion code. If
122 new handlers become ready while handlers are executing, Tcl_AsyncInvoke
123 will invoke them all; at each point it invokes the highest-priority
124 (oldest) ready handler, repeating this over and over until there are no
125 longer any ready handlers.
126
129 It is almost always a bad idea for an asynchronous event handler to
130 modify the interpreter's result or return a code different from its
131 code argument. This sort of behavior can disrupt the execution of
132 scripts in subtle ways and result in bugs that are extremely difficult
133 to track down. If an asynchronous event handler needs to evaluate Tcl
134 scripts then it should first save the interpreter's result plus the
135 values of the variables errorInfo and errorCode (this can be done, for
136 example, by storing them in dynamic strings). When the asynchronous
137 handler is finished it should restore the interpreter's result, error‐
138 Info, and errorCode, and return the code argument.
139
142 asynchronous event, handler, signal
143Tcl 7.0 Tcl_AsyncCreate(3)