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The function signature is very similar to MsgWa it F o rMu l t i p l e Ob j e c t s The dwF l a g s argument may contain 0 o r more o f the flags COWAI T_WAITA L L (OxOl ) or COWAIT_A L E RTAB L E (Ox02) As you may well imagine, the first specifies that a wait-all (rather than the default of wait-any) is desired, and the latter ensures that pending APCs are dispatched by the as kernel This function encapsulates poorly documented, mysterious logic that will auto matically pump certain classes of messages Specifically, when the wait occurs on a Single Threaded Apartment (STA), COM RPC messages are processed, and only a subset of the possible windowing messages are processed, via the M s gWa it F o rMu lt i p l e O b j e c t s E x function When called from a thread in a different apartment type, the call simply passes through to the W a i t F o rM u l t i p l eO b j e ct s E x API
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When to Pump Messages Deciding when to pump messages is seldom straightforward Not doing so, in the best case, is completely harmless (if a message never arrives during the wait) In the worst case, it can cause a deadlock that brings the program to its knees Somewhere in the middle fall performance issues, which can vary between minor impacts to throughput (in the case of, say, COM on the server) or GUI responsiveness, and major impacts that destroy a server ' s performance or give users the impression that their GUI is hung, causing them to kill the application, possibly indi rectly corrupting data in the process At the same time, pumping causes reentrancy Reentrancy is caused when some logically unrelated piece of work enters on top of the existing callstack If you pump messages during a blocking operation, this code seems to execute "in the middle" of the wait If there is any thread specific state established at the time this reentrancy occurs, application behavior can go haywire, often leading to state corruption For example, if a mutex is held when reentrancy occurs, it will be accidentally shared between the code that was active before the reentrancy and the reentrant code itself, due to mutex recursion The decision to pump and risk reentrancy must be made carefully and must include consideration and precautions to ensure that application state invariants are prepared to handle the possibility of reentrancy The decision of whether to pump is often also informed by the length of a blocking operation If you're doing GUI programming, you really ought to avoid all blocking on the GUI thread (as already noted) In some
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circumstances, however, the overhead required t o marshal work t o a separate thread versus a short expected wait time may mean that staying on the GUI thread and doing a little pumping is appropriate (Beware! This is a slippery slope!) These cases really ought to be rare Often what seems like a short wait time can turn out to be forever under unexpected circum stances, such as trying to resolve a DNS entry when your user 's network cable has just become unplugged Most GUI frameworks will automatically pump messages when modal dialog boxes are shown With COM it's sel dom so straightforward, because the sole purpose of sending and pumping for messages is for cross-thread synchronization And so, in order to avoid deadlocks, pumping is typically inescapable For sophisticated applications, choosing when to pump on a case-by case basis is reasonable, but for most applications, deciding to always (or never) pump messages on threads with message queues can simplify your life quite a bit A popular approach is to pump COM messages, but not GUI messages, as we saw with the CoWa i t F o rMu l t i pleHa n d l e s API This at least homogenizes the categories of failures you are apt to see in your code base, and lets you opt-in specific call sites after the fact in response to testing and bugs The CLR similarly chooses to always pump messages when it's on a GUI or COM STA thread, as in CoWa it F o rMu l t i p l e Ha n d l e s, which brings us to the next topic: how the CLR waits
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Managed Code Now we turn to the way in which managed code interoperates with Windows kernel synchronization Everything mentioned here is, effec tively, a thin veneer over everything we just discussed in the context of native code
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