Standard ML of New Jersey

The POLL signature


signature POLL

The Unsafe.Poll structure provides an interface to automatic software polling. The pollEvent flag is watched (with user-specified frequency), and when it becomes true, a user-specified function is invoked. This is all done without hardware interrupts; it is useful where the operating system does not provide an interface to hardware signals, or where the cost of hardware signal-handling is too high.

Polling does not slow down user code very much, because the polling code is integrated with the "heap exhausted?" tests that are used to invoke the garbage collector.

In SML/NJ version 110, polling works on the MIPS, Sparc, and Intel x86 architectures.

Soft polling is used to support garbage collection for the MP thread library; fully follow only steps 1 and 2 for MP. For custom application, follow all steps.

1) build a runtime system for polling by adding "-DSOFT_POLL" to the appropriate mk.<arch$GT;-<os$GT; make script in src/runtime/objs (see, for example, mk.x86-linux-poll) then, do a "make clean" followed by a "make -f mk.<arch$GT;-<os$GT;"

2) use the resulting runtime (src/runtime/objs/run.<arch>-<os>) for the following steps

3) start the compiler and set it to emit polling checks:

 Compiler.Control.CG.pollChecks := true;
For MP support: One may now want to build an entire compiler that checks for poll events: if retargeting, after the retarget do
 <ARCH>VisComp.Control.CG.pollChecks := true;
 Compiler.Control.CG.pollChecks := true;

4) polling is now available in the freshly compiled compiler to use, install a polling handler, e.g.:

 Unsafe.Poll.setHandler (SOME(fn x => (print "in handler\n"; x)));
 Unsafe.Poll.setHandler (SOME(fn x => (print "in handler\n";
			      Unsafe.Poll.pollEvent := false;

5) set a polling frequency: Unsafe.Poll.setFreq (SOME 50); Frequency (SOME x) sets the polling interval to approximately x*1000 instructions; frequency NONE disables polling.

6) the installed poll handler will be called after Unsafe.Poll.pollEvent := true; occurs.


exception BadPollFreq
val pollEvent : bool ref
val setHandler : (unit cont -> unit cont) option -> unit
val inqHandler : unit -> (unit cont -> unit cont) option
val setFreq : int option -> unit
val inqFreq : unit -> int option


exception BadPollFreq

When this variable becomes true (and the frequency is not NONE), the hander will be invoked. pollEvent is accessible to the runtime system and may (of course) be set asynchronously by other threads.

setHandler a
, install f as the event handler. When pollEvent becomes true, the current continuation k will be saved, pollEvent will be set to false, and f(k) will be called. No polling will occur during the execution of f. When f returns some continuation k', then polling will resume and k' will be invoked.


, disable polling.

inqHandler ()
Returns the current event handler.

setFreq a
a=(SOME i)
, sets the polling frequency to
; that is, approximately every
instructions, the pollEvent variable will be checked.


, disables polling.

inqFreq ()
Returns the current frequency value.

See Also

Posix.Signal, SMLofNJ.SysInfo.hasSoftwarePolling

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Last Modified October 28, 1997
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Copyright © 1998 Bell Labs, Lucent Technologies