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In the SumC example, arguments are passed by value, but native code often requires data structures to be passed by reference to avoid the cost of copying the entire structure and passing only a pointer to the native data. The ZeroC function resets a complex number whose pointer is passed as an argument: void CINTEROPDLL_API ZeroC(Complex* c) { c->re = 0; c->im = 0; } The F# declaration for the function is the same as the C prototype: [<DllImport("CInteropDLL")>] extern void ZeroC(Complex* c) Now you need a way to obtain a pointer given a value of type Complex in F#. You can use the && operator that is used to indicate a pass by reference and that results in passing the pointer to the structure expected by the C function: let mutable c4 = CInterop.SumC(c1, c2) printf "c4 = SumC(c1, c2) = %f + %fi\n" c4.re c4.im CInterop.ZeroC(&&c4) printf "c4 = %f + %fi\n" c4.re c4.im In C and C++, the notion of objects (or struct instances) and the classes of memory are orthogonal: an object can be allocated on the stack or on the heap and share the same declaration. In .NET, this is not the case; objects are instances of classes and are allocated on the heap, and value types are stored in the stack or wrapped into objects in the heap. Is it possible to pass objects to native functions through PInvoke The main issue with objects is that the heap is managed by the garbage collector, and one possible strategy for garbage collection is copy collection, which is a technique that moves objects in the heap when a collection

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For example, imagine our website has 1,000,000 users, and for each user we have to connect as a separate database account Say we have a connection pool of 100 connections The connections in this pool are good for use only for the 100 users whose connections exist in the pool currently The moment a new user tries to connect, the connection pool cannot serve that user, since each user requires connection to a different account Thus, one of the connections will have to be aged out and a new connection established for this user Clearly, the purpose of connection pool is defeated if you need to tear down and create a connection every time a new user whose connection is not currently in the pool tries to connect.

occurs Thus, the base address in memory of an object may change over time, which can be a serious problem if the pointer to the object has been marshalled to a native function through a PInvoke invocation The CLR provides an operation called pinning that allows pinning an object and preventing it from moving during a garbage collection Pinned pointers are assigned to local variables, and pinning is released when the function performing the pinning exits It is important to understand the scope of pinning, since if the native code stores the pointer somewhere before returning, the pointer may become invalid but still usable from native code Now let s define an object type for Complex and marshal F# objects to a C function The goal is to marshal the F# object to the ZeroC function.

Contrast this with a scenario where the 1,000,000 users can be mapped into 10 database users based on different privileges each user requires Now the likelihood of a new user getting a hit in the connection pool is very high, thus allowing us to share the 100 connections among a much larger end-user population So how do we map these many users to a manageable number of database users We will look at this aspect in detail in the next chapter Let s now take a brief look at what the term connection caching means before we discuss Oracle9i connection pooling and caching, followed by Oracle 10g implicit connection caching..

But perhaps the most useful thing to do with the captured rendered output is to pack the HTML into the body of an e-mail message and send the web page to somebody via Simple Mail Transfer Protocol (SMTP). This is what we re doing on Intertech Training s website. Figure 3-20 shows a page displaying a course outline.

At the core, connection caching and connection pooling refer to the same concept: pooling physical database connections to be shared across multiple client sessions. Connection caching, usually implemented in the middle tier, refers to the concept of creating a cache of physical connections using the connection pooling framework provided by the JDBC driver. In Oracle9i, we need to create our own cache on top of the connection pooling framework provided by Oracle JDBC drivers. We can use a sample connection cache provided by Oracle as well. In Oracle 10g, the connection cache can be enabled implicitly at the data source level itself, thus obviating the need to maintain or manage our own cache. The next section discusses in detail the Oracle9i implementation of connection pooling and caching.

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