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As mentioned earlier, under some circumstances the initialization of a critical section may attempt to allocate a kernel object This allocation may fail due to low resources, leading to an E R RO R_OUT _O F _M EMORY exception being thrown Critical sections are quite different in this regard from most of the Win32 library because most other APIs will return F A L S E or an error code to indicate allocation failure rather than using an exception This is slightly annoying, because many native programmers prefer return codes to exceptions and, therefore, have to treat this as a special case or perform some translation Worse, many don' t realize it can happen, leading to reli ability holes (ie, due to unhand led exceptions in very rare and hard-to test-for circumstances) In Vista, the new I n i t i a l i z eC r i t i c a l S e c t i o n E x A P I conforms t o Win32 standards and, instead, returns F A L S E t o indicate failure
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Woes of Lazy Alloca tion And, as also already mentioned, subsequent calls to E n t e r C r it i c a l S e c t i o n and L e a v e C r it i c a l S e c t i o n on Windows 2000 also can throw SEH E R ROR_OUT_O F _M EMORY exceptions as well The rea son is subtle The kernel team made a change in the move to Windows 2000 so that critical sections would lazily allocate the kernel object the first time it was needed (ie, when a thread needs to wait) versus the previous behav ior of always allocating one during section initialization The reason that lazy allocation was preferred is that kernel objects are heavyweight; allocating one for initialized, but unused, critical sections increases the cost of each section itself and hence the overall pressure on the system, includ ing some consumption of nonpageable kernel memory Particularly around the Windows 2000 time frame, many more people were writing multi threaded code primarily for server SMP programs It's relatively common now to have hundreds or thousands of critical sections in a single process And many critical sections are used only occasionally (or never at all), meaning that the auto-reset event often isn't used Requiring that kernel resources always be allocated up front became a rather large scalability lim itation But the addition of lazy initialization suddenly meant that the first time thread tried to enter a critical section already owned by another thread (with a failed spin wait) required the auto-reset kernel event to be allocated on the spot This allocation can fail What's worse, you can't recover from this exception On most OSs, the C R I T ICA L_S ECTION data structure is left in a corrupt and unusable state And it gets worse L e a veC r i t i c a l S e c t i o n also can fail under some even more obscure circumstances: if E nt e r C r it i c a l S e c t i o n fails, throwing an out of memory exception, a subsequent call to L e a v e C r i t i c a l S e c t i o n would notice the damaged state and respond b y attempting t o allocate the event This too could fail, causing even more corruption and confusion Dealing with this condition effectively means that any call to enter or leave a critical section on Windows 2000 must be wrapped inside a try/catch block, which is unrealistic A slight mitigation to this issue was made available in Windows 2000: a flag could be passed to the I n it i a l i z e C r i t i c a l S e c t ionAn d S p i n C o u n t API to request that Windows pre allocates the event To pre-allocate the event at initialization time with this function, turn on the high-bit of the dwS p i n C o u n t argument
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