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feature level differences between them that we' ll cover later, such as SRWLs being nonrecursive As with the C R IT I CA L_S E C T ION, a SRWL instance is a simple structure, S RW LOCK, that can be allocated anywhere you choose SRWLs are new to Vista, so you'll have to define a _W I N 3 2_WINNT version of exe6ee or greater before importing W i n d ows h to use them Before using a S RW LOCK instance, you have to initialize it with a call to
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I n i t i a l i z e S RW L o c k Because SRWLs don't use any dynamically allocated
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events or memory internally, there is no need to delete them later on, and initialization ensures the right bit pattern is contained in memory
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VOID WINAPI I n it i a l i zeSRWLoc k ( PSRWLOCK SRWLoc k ) ;
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Once you have initialized the lock, threads can then begin acquiring in exclusive (write) or shared (read) mode with the Ac q u i reSRWLoc k E x c l u s ive and Ac q u i r e S R W L o c k S h a red functions, respectively Both accept a single argument of type P S RW LOCK, which is a type definition for S RW LOCK *, and have no return value The corresponding functions R e l e a s e S RW L oc k E xc l u s iv e and R e l e a s eS RW L o c k S h a red release the lock in the specified mode
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VOI D VOI D VOID VOI D WI NAP I WINAPI WINAPI WINAPI Ac q u i reSRWLo c k E x c l u sive ( PSRWLOCK SRWLoc k ) ; Ac q u i reSRWLo c k S h a red ( PSRW LOC K SRWLoc k ) ; Relea seSRWLoc k E x c l u s ive ( PSRWLOCK SRWLoc k ) ; Relea seSRWLoc kSha red ( PSRWLOCK SRWLoc k ) ;
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Attempted lock acquisitions will block i f the lock is held by another thread in a mode that is incompatible at the time of the attempted acquisi tion: that is, if the thread is owned exclusively, all attempts block; if it is owned in shared mode, exclusive attempts block Blocking is done with a nonalertable wait, and waiters are released in a roughly FIFO order, although the lock is unfair and will permit concurrent acquisition attempts to succeed When the lock is released and both readers and writers are wait ing, the lock will prefer to wake up waiting writer threads first When there are no writers, all waiting reader threads are awakened Acquiring a SRWL in shared or exclusive mode will never fail due to low resource conditions, and, hence, there is no alternative API to pre-allocate internal data structures Once a SRWL has been initialized, it's ready to use The secret to SRWL's ability to work in low resource conditions is the
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R e a d e r , W r i t e r Locks ( RW Ls)
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same secret to critical sections working in low resource conditions: keyed events The substantial performance improvements made to keyed events in Windows Vista has made it possible to use them as the sole waiting mech anism for SRWLs In fact, you might want to consider using SRWLs with exclusive-mode-only acquisitions and releases over Win32 critical sections, due to their lightweight nature For small amounts of contention, a SRWL will actually outperform a critical region Unlike critical sections, SRWLs don't support nonblocking acquire APIs, such as T ryAc q u i r e S R W L o c k E x c l u s i ve, for example This would be a nice feature, but it has not yet been made available SRWLs also use a spin-wait for a constant number of spins that is neither configurable nor dynamic, but that has been chosen for good average case performance, much like CLR monitors Also note that Vista SRWLs do not support changing the lock mode after the lock has been acquired For example, "upgrading" from shared to exclusive or "downgrading" from exclusive to shared are fairly common fea tures for RWLs, but (due to its lightweight nature), the Vista lock doesn't support either Here's an example of using one such lock
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