2 '\" Copyright (c) 1995-1996 Sun Microsystems, Inc.
4 '\" See the file "license.terms" for information on usage and redistribution
5 '\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
7 .TH vwait n 8.0 Tcl "Tcl Built-In Commands"
8 .\" The -*- nroff -*- definitions below are for supplemental macros used
9 .\" in Tcl/Tk manual entries.
11 .\" .AP type name in/out ?indent?
12 .\" Start paragraph describing an argument to a library procedure.
13 .\" type is type of argument (int, etc.), in/out is either "in", "out",
14 .\" or "in/out" to describe whether procedure reads or modifies arg,
15 .\" and indent is equivalent to second arg of .IP (shouldn't ever be
16 .\" needed; use .AS below instead)
19 .\" Give maximum sizes of arguments for setting tab stops. Type and
20 .\" name are examples of largest possible arguments that will be passed
21 .\" to .AP later. If args are omitted, default tab stops are used.
24 .\" Start box enclosure. From here until next .BE, everything will be
25 .\" enclosed in one large box.
28 .\" End of box enclosure.
31 .\" Begin code excerpt.
36 .\" .VS ?version? ?br?
37 .\" Begin vertical sidebar, for use in marking newly-changed parts
38 .\" of man pages. The first argument is ignored and used for recording
39 .\" the version when the .VS was added, so that the sidebars can be
40 .\" found and removed when they reach a certain age. If another argument
41 .\" is present, then a line break is forced before starting the sidebar.
44 .\" End of vertical sidebar.
47 .\" Begin an indented unfilled display.
50 .\" End of indented unfilled display.
53 .\" Start of list of standard options for a Tk widget. The manpage
54 .\" argument defines where to look up the standard options; if
55 .\" omitted, defaults to "options". The options follow on successive
56 .\" lines, in three columns separated by tabs.
59 .\" End of list of standard options for a Tk widget.
61 .\" .OP cmdName dbName dbClass
62 .\" Start of description of a specific option. cmdName gives the
63 .\" option's name as specified in the class command, dbName gives
64 .\" the option's name in the option database, and dbClass gives
65 .\" the option's class in the option database.
68 .\" Print arg1 underlined, then print arg2 normally.
71 .\" Print arg1 in quotes, then arg2 normally (for trailing punctuation).
74 .\" Print an open parenthesis, arg1 in quotes, then arg2 normally
75 .\" (for trailing punctuation) and then a closing parenthesis.
77 .\" # Set up traps and other miscellaneous stuff for Tcl/Tk man pages.
81 .\" # Start an argument description
85 . ie !"\\$2"" .TP \\n()Cu
90 \&\\$1 \\fI\\$2\\fP (\\$3)
103 .\" # define tabbing values for .AP
106 .if !"\\$1"" .nr )A \\w'\\$1'u+3n
109 .if !"\\$2"" .nr )B \\w'\\$2'u+\\n()Au+3n
110 .nr )C \\n()Bu+\\w'(in/out)'u+2n
112 .AS Tcl_Interp Tcl_CreateInterp in/out
113 .\" # BS - start boxed text
114 .\" # ^y = starting y location
122 .if n \l'\\n(.lu\(ul'
125 .\" # BE - end boxed text (draw box now)
130 .ie n \l'\\n(^lu\(ul'
132 .\" Draw four-sided box normally, but don't draw top of
133 .\" box if the box started on an earlier page.
135 \h'-1.5n'\L'|\\n(^yu-1v'\l'\\n(^lu+3n\(ul'\L'\\n(^tu+1v-\\n(^yu'\l'|0u-1.5n\(ul'
138 \h'-1.5n'\L'|\\n(^yu-1v'\h'\\n(^lu+3n'\L'\\n(^tu+1v-\\n(^yu'\l'|0u-1.5n\(ul'
145 .\" # VS - start vertical sidebar
146 .\" # ^Y = starting y location
147 .\" # ^v = 1 (for troff; for nroff this doesn't matter)
151 .ie n 'mc \s12\(br\s0
154 .\" # VE - end of vertical sidebar
162 \h'|\\n(^lu+3n'\L'|\\n(^Yu-1v\(bv'\v'\\n(^tu+1v-\\n(^Yu'\h'-|\\n(^lu+3n'
169 .\" # Special macro to handle page bottom: finish off current
170 .\" # box/sidebar if in box/sidebar mode, then invoked standard
171 .\" # page bottom macro.
178 .\" Draw three-sided box if this is the box's first page,
179 .\" draw two sides but no top otherwise.
180 .ie !\\n(^b-1 \h'-1.5n'\L'|\\n(^yu-1v'\l'\\n(^lu+3n\(ul'\L'\\n(^tu+1v-\\n(^yu'\h'|0u'\c
181 .el \h'-1.5n'\L'|\\n(^yu-1v'\h'\\n(^lu+3n'\L'\\n(^tu+1v-\\n(^yu'\h'|0u'\c
184 .nr ^x \\n(^tu+1v-\\n(^Yu
185 \kx\h'-\\nxu'\h'|\\n(^lu+3n'\ky\L'-\\n(^xu'\v'\\n(^xu'\h'|0u'\c
198 .\" # DS - begin display
204 .\" # DE - end display
210 .\" # SO - start of list of standard options
212 'ie '\\$1'' .ds So \\fBoptions\\fR
213 'el .ds So \\fB\\$1\\fR
214 .SH "STANDARD OPTIONS"
220 .\" # SE - end of list of standard options
225 See the \\*(So manual entry for details on the standard options.
227 .\" # OP - start of full description for a single option
232 Command-Line Name: \\fB\\$1\\fR
233 Database Name: \\fB\\$2\\fR
234 Database Class: \\fB\\$3\\fR
238 .\" # CS - begin code excerpt
244 .\" # CE - end code excerpt
249 .\" # UL - underline word
253 .\" # QW - apply quotation marks to word
255 .ie '\\*(lq'"' ``\\$1''\\$2
256 .\"" fix emacs highlighting
257 .el \\*(lq\\$1\\*(rq\\$2
259 .\" # PQ - apply parens and quotation marks to word
261 .ie '\\*(lq'"' (``\\$1''\\$2)\\$3
262 .\"" fix emacs highlighting
263 .el (\\*(lq\\$1\\*(rq\\$2)\\$3
265 .\" # QR - quoted range
267 .ie '\\*(lq'"' ``\\$1''\\-``\\$2''\\$3
268 .\"" fix emacs highlighting
269 .el \\*(lq\\$1\\*(rq\\-\\*(lq\\$2\\*(rq\\$3
271 .\" # MT - "empty" string
276 '\" Note: do not modify the .SH NAME line immediately below!
278 vwait \- Process events until a variable is written
280 \fBvwait\fR \fIvarName\fR
284 This command enters the Tcl event loop to process events, blocking
285 the application if no events are ready. It continues processing
286 events until some event handler sets the value of the global variable
287 \fIvarName\fR. Once \fIvarName\fR has been set, the \fBvwait\fR
288 command will return as soon as the event handler that modified
289 \fIvarName\fR completes. The \fIvarName\fR argument is always interpreted as
290 a variable name with respect to the global namespace, but can refer to any
291 namespace's variables if the fully-qualified name is given.
293 In some cases the \fBvwait\fR command may not return immediately
294 after \fIvarName\fR is set. This happens if the event handler
295 that sets \fIvarName\fR does not complete immediately. For example,
296 if an event handler sets \fIvarName\fR and then itself calls
297 \fBvwait\fR to wait for a different variable, then it may not return
298 for a long time. During this time the top-level \fBvwait\fR is
299 blocked waiting for the event handler to complete, so it cannot
300 return either. (See the \fBNESTED VWAITS BY EXAMPLE\fR below.)
302 To be clear, \fImultiple \fBvwait\fI calls will nest and will not happen in
303 parallel\fR. The outermost call to \fBvwait\fR will not return until all the
304 inner ones do. It is recommended that code should never nest \fBvwait\fR
305 calls (by avoiding putting them in event callbacks) but when that is not
306 possible, care should be taken to add interlock variables to the code to
307 prevent all reentrant calls to \fBvwait\fR that are not \fIstrictly\fR
308 necessary. Be aware that the synchronous modes of operation of some Tcl
309 packages (e.g.,\ \fBhttp\fR) use \fBvwait\fR internally; if using the event
310 loop, it is best to use the asynchronous callback-based modes of operation of
311 those packages where available.
314 Run the event-loop continually until some event calls \fBexit\fR.
315 (You can use any variable not mentioned elsewhere, but the name
316 \fIforever\fR reminds you at a glance of the intent.)
322 Wait five seconds for a connection to a server socket, otherwise
323 close the socket and continue running the script:
326 # Initialise the state
327 after 5000 set state timeout
328 set server [socket -server accept 12345]
330 global state connectionInfo
332 set connectionInfo $args
335 # Wait for something to happen
338 # Clean up events that could have happened
340 after cancel set state timeout
342 # Do something based on how the vwait finished...
345 puts "no connection on port 12345"
348 puts "connection: $connectionInfo"
349 puts [lindex $connectionInfo 0] "Hello there!"
354 A command that will wait for some time delay by waiting for a namespace
355 variable to be set. Includes an interlock to prevent nested waits.
358 namespace eval example {
362 if {$v ne "waiting"} {
364 after $delay [namespace code {set v done}]
365 \fBvwait\fR [namespace which -variable v]
372 When running inside a \fBcoroutine\fR, an alternative to using \fBvwait\fR is
373 to \fByield\fR to an outer event loop and to get recommenced when the variable
374 is set, or at an idle moment after that.
377 coroutine task apply {{} {
378 # simulate [after 1000]
379 after 1000 [info coroutine]
382 # schedule the setting of a global variable, as normal
383 after 2000 {set var 1}
385 # simulate [\fBvwait\fR var]
386 proc updatedVar {task args} {
388 trace remove variable ::var write "updatedVar $task"
390 trace add variable ::var write "updatedVar [info coroutine]"
394 .SS "NESTED VWAITS BY EXAMPLE"
396 This example demonstrates what can happen when the \fBvwait\fR command is
397 nested. The script will never finish because the waiting for the \fIa\fR
398 variable never finishes; that \fBvwait\fR command is still waiting for a
399 script scheduled with \fBafter\fR to complete, which just happens to be
400 running an inner \fBvwait\fR (for \fIb\fR) even though the event that the
401 outer \fBvwait\fR was waiting for (the setting of \fIa\fR) has occurred.
420 If you run the above code, you get this output:
428 The script will never print
430 until after it has printed
432 because of the nesting of \fBvwait\fR commands, and yet \fIb\fR will not be
433 set until after the outer \fBvwait\fR returns, so the script has deadlocked.
434 The only ways to avoid this are to either structure the overall program in
435 continuation-passing style or to use \fBcoroutine\fR to make the continuations
436 implicit. The first of these options would be written as:
441 trace add variable b write {apply {args {
443 trace remove variable ::b write \e
444 [lrange [info level 0] 0 1]
454 trace add variable a write {apply {args {
456 trace remove variable a write [lrange [info level 0] 0 1]
464 The second option, with \fBcoroutine\fR and some helper procedures, is done
468 # A coroutine-based wait-for-variable command
469 proc waitvar globalVar {
470 trace add variable ::$globalVar write \e
471 [list apply {{v c args} {
472 trace remove variable $v write \e
473 [lrange [info level 0] 0 3]
475 }} ::$globalVar [info coroutine]]
478 # A coroutine-based wait-for-some-time command
480 after $ms [info coroutine]
484 coroutine task-1 eval {
491 coroutine task-2 eval {
498 coroutine task-3 eval {
508 asynchronous I/O, event, variable, wait