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Diffstat (limited to 'doc/vwait.n')
| -rw-r--r-- | doc/vwait.n | 276 |
1 files changed, 20 insertions, 256 deletions
diff --git a/doc/vwait.n b/doc/vwait.n index e595a74..f516d46 100644 --- a/doc/vwait.n +++ b/doc/vwait.n @@ -3,7 +3,7 @@ '\" '\" See the file "license.terms" for information on usage and redistribution '\" of this file, and for a DISCLAIMER OF ALL WARRANTIES. -'\" +'\" .TH vwait n 8.0 Tcl "Tcl Built-In Commands" .so man.macros .BS @@ -12,127 +12,45 @@ vwait \- Process events until a variable is written .SH SYNOPSIS \fBvwait\fR \fIvarName\fR -.sp -\fBvwait\fR ?\fIoptions\fR? ?\fIvarName ...\fR? .BE + .SH DESCRIPTION .PP This command enters the Tcl event loop to process events, blocking the application if no events are ready. It continues processing -events until some event handler sets the value of the global variable +events until some event handler sets the value of variable \fIvarName\fR. Once \fIvarName\fR has been set, the \fBvwait\fR command will return as soon as the event handler that modified -\fIvarName\fR completes. The \fIvarName\fR argument is always interpreted as -a variable name with respect to the global namespace, but can refer to any -namespace's variables if the fully-qualified name is given. -.PP -In the second more complex command form \fIoptions\fR allow for finer -control of the wait operation and to deal with multiple event sources. -\fIOptions\fR can be made up of -.TP -\fB\-\-\fR -. -Marks the end of options. All following arguments are handled as -variable names. -.TP -\fB\-all\fR -. -All conditions for the wait operation must be met to complete the -wait operation. Otherwise (the default) the first event completes -the wait. -.TP -\fB\-extended\fR -. -An extended result in list form is returned, see below for explanation. -.TP -\fB\-nofileevents\fR -. -File events are not handled in the wait operation. -.TP -\fB\-noidleevents\fR -. -Idle handlers are not invoked during the wait operation. -.TP -\fB\-notimerevents\fR -. -Timer handlers are not serviced during the wait operation. -.TP -\fB\-nowindowevents\fR -. -Events of the windowing system are not handled during the wait operation. -.TP -\fB\-readable\fR \fIchannel\fR -. -\fIChannel\fR must name a Tcl channel open for reading. If \fIchannel\fR -is or becomes readable the wait operation completes. -.TP -\fB\-timeout\fR \fImilliseconds\fR -. -The wait operation is constrained to \fImilliseconds\fR. -.TP -\fB\-variable\fR \fIvarName\fR -. -\fIVarName\fR must be the name of a global variable. Writing or -unsetting this variable completes the wait operation. -.TP -\fB\-writable\fR \fIchannel\fR -. -\fIChannel\fR must name a Tcl channel open for writing. If \fIchannel\fR -is or becomes writable the wait operation completes. -.PP -The result returned by \fBvwait\fR is for the simple form an empty -string. If the \fI\-timeout\fR option is specified, the result is the -number of milliseconds remaining when the wait condition has been -met, or -1 if the wait operation timed out. -.PP -If the \fI\-extended\fR option is specified, the result is made up -of a Tcl list with an even number of elements. Odd elements -take the values \fBreadable\fR, \fBtimeleft\fR, \fBvariable\fR, -and \fBwritable\fR. Even elements are the corresponding variable -and channel names or the remaining number of milliseconds. -The list is ordered by the occurrences of the event(s) with the -exception of \fBtimeleft\fR which always comes last. +\fIvarName\fR completes. \fIvarName\fR must be globally scoped +(either with a call to \fBglobal\fR for the \fIvarName\fR, or with +the full namespace path specification). .PP In some cases the \fBvwait\fR command may not return immediately -after \fIvarName\fR et.al. is set. This happens if the event handler +after \fIvarName\fR is set. This can happen if the event handler that sets \fIvarName\fR does not complete immediately. For example, if an event handler sets \fIvarName\fR and then itself calls \fBvwait\fR to wait for a different variable, then it may not return for a long time. During this time the top-level \fBvwait\fR is blocked waiting for the event handler to complete, so it cannot -return either. (See the \fBNESTED VWAITS BY EXAMPLE\fR below.) -.PP -To be clear, \fImultiple \fBvwait\fI calls will nest and will not happen in -parallel\fR. The outermost call to \fBvwait\fR will not return until all the -inner ones do. It is recommended that code should never nest \fBvwait\fR -calls (by avoiding putting them in event callbacks) but when that is not -possible, care should be taken to add interlock variables to the code to -prevent all reentrant calls to \fBvwait\fR that are not \fIstrictly\fR -necessary. Be aware that the synchronous modes of operation of some Tcl -packages (e.g.,\ \fBhttp\fR) use \fBvwait\fR internally; if using the event -loop, it is best to use the asynchronous callback-based modes of operation of -those packages where available. +return either. .SH EXAMPLES -.PP Run the event-loop continually until some event calls \fBexit\fR. (You can use any variable not mentioned elsewhere, but the name \fIforever\fR reminds you at a glance of the intent.) -.PP .CS \fBvwait\fR forever .CE .PP Wait five seconds for a connection to a server socket, otherwise close the socket and continue running the script: -.PP .CS # Initialise the state after 5000 set state timeout set server [socket -server accept 12345] proc accept {args} { - global state connectionInfo - set state accepted - set connectionInfo $args + global state connectionInfo + set state accepted + set connectionInfo $args } # Wait for something to happen @@ -144,172 +62,18 @@ after cancel set state timeout # Do something based on how the vwait finished... switch $state { - timeout { - puts "no connection on port 12345" - } - accepted { - puts "connection: $connectionInfo" - puts [lindex $connectionInfo 0] "Hello there!" - } -} -.CE -.PP -A command that will wait for some time delay by waiting for a namespace -variable to be set. Includes an interlock to prevent nested waits. -.PP -.CS -namespace eval example { - variable v done - proc wait {delay} { - variable v - if {$v ne "waiting"} { - set v waiting - after $delay [namespace code {set v done}] - \fBvwait\fR [namespace which -variable v] - } - return $v - } -} -.CE -.PP -When running inside a \fBcoroutine\fR, an alternative to using \fBvwait\fR is -to \fByield\fR to an outer event loop and to get recommenced when the variable -is set, or at an idle moment after that. -.PP -.CS -coroutine task apply {{} { - # simulate [after 1000] - after 1000 [info coroutine] - yield - - # schedule the setting of a global variable, as normal - after 2000 {set var 1} - - # simulate [\fBvwait\fR var] - proc updatedVar {task args} { - after idle $task - trace remove variable ::var write "updatedVar $task" - } - trace add variable ::var write "updatedVar [info coroutine]" - yield -}} -.CE -.SS "NESTED VWAITS BY EXAMPLE" -.PP -This example demonstrates what can happen when the \fBvwait\fR command is -nested. The script will never finish because the waiting for the \fIa\fR -variable never finishes; that \fBvwait\fR command is still waiting for a -script scheduled with \fBafter\fR to complete, which just happens to be -running an inner \fBvwait\fR (for \fIb\fR) even though the event that the -outer \fBvwait\fR was waiting for (the setting of \fIa\fR) has occurred. -.PP -.CS -after 500 { - puts "waiting for b" - \fBvwait\fR b - puts "b was set" -} -after 1000 { - puts "setting a" - set a 10 + timeout { + puts "no connection on port 12345" + } + accepted { + puts "connection: $connectionInfo" + puts [lindex $connectionInfo 0] "Hello there!" + } } -puts "waiting for a" -\fBvwait\fR a -puts "a was set" -puts "setting b" -set b 42 -.CE -.PP -If you run the above code, you get this output: -.PP -.CS -waiting for a -waiting for b -setting a .CE -.PP -The script will never print -.QW "a was set" -until after it has printed -.QW "b was set" -because of the nesting of \fBvwait\fR commands, and yet \fIb\fR will not be -set until after the outer \fBvwait\fR returns, so the script has deadlocked. -The only ways to avoid this are to either structure the overall program in -continuation-passing style or to use \fBcoroutine\fR to make the continuations -implicit. The first of these options would be written as: -.PP -.CS -after 500 { - puts "waiting for b" - trace add variable b write {apply {args { - global a b - trace remove variable ::b write \e - [lrange [info level 0] 0 1] - puts "b was set" - set ::done ok - }}} -} -after 1000 { - puts "setting a" - set a 10 -} -puts "waiting for a" -trace add variable a write {apply {args { - global a b - trace remove variable a write [lrange [info level 0] 0 1] - puts "a was set" - puts "setting b" - set b 42 -}}} -\fBvwait\fR done -.CE -.PP -The second option, with \fBcoroutine\fR and some helper procedures, is done -like this: -.PP -.CS -# A coroutine-based wait-for-variable command -proc waitvar globalVar { - trace add variable ::$globalVar write \e - [list apply {{v c args} { - trace remove variable $v write \e - [lrange [info level 0] 0 3] - after 0 $c - }} ::$globalVar [info coroutine]] - yield -} -# A coroutine-based wait-for-some-time command -proc waittime ms { - after $ms [info coroutine] - yield -} -coroutine task-1 eval { - puts "waiting for a" - waitvar a - puts "a was set" - puts "setting b" - set b 42 -} -coroutine task-2 eval { - waittime 500 - puts "waiting for b" - waitvar b - puts "b was set" - set done ok -} -coroutine task-3 eval { - waittime 1000 - puts "setting a" - set a 10 -} -\fBvwait\fR done -.CE .SH "SEE ALSO" global(n), update(n) + .SH KEYWORDS -asynchronous I/O, event, variable, wait -'\" Local Variables: -'\" mode: nroff -'\" fill-column: 78 -'\" End: +event, variable, wait |