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char * s; s = p->name(); // CORBA::string_free(s);
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When the client calls the name method, it calls a member function on a proxy object The sequence of events now is as follows The name member function on the proxy creates a request containing the name of the operation (name in this case), the object key, and the in and inout parameters for the operation (none in this case) The proxy member function writes the request to its connection to the server and immediately calls a blocking read operation on the connection (recv_len in this case) The client-side run time is now blocked until a reply arrives from the server Meanwhile, the request makes its way across the network to the server The server is blocked in its get_request operation, waiting for a request to arrive on the client connection The incoming request from the client unblocks get_request, which extracts the operation name and object key The server-side run time calls a generic invoke function, which accepts the operation name as a parameter invoke uses that name to identify the correct application member function to call and then up-calls into the application code Control has now been transferred to the application-supplied name function on the server side The name function uses string_alloc to allocate memory for the string and returns a pointer to that buffer as its return value Control is transferred back to the server-side run time, which expects to be handed a pointer to the allocated string The run time now constructs a reply containing a copy of the string and sends that reply back to the client The server-side run time calls string_free to deallocate the string (The string is no longer needed because its contents are already on their way back to the client) The server-side run time has now completed one iteration of its dispatch loop and calls get_request again, which blocks until the next client request arrives Meanwhile, the reply has made its way across the network back to the client, whose call to recv_len unblocks The return value is a byte count that specifies the length of the string to follow
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The client-side run time calls string_alloc to create a buffer containing len bytes and calls recv, which reads the string contents into the buffer The stub on the client side completes by returning a pointer to the buffer containing the string Control has now returned to the application code, which uses the string and eventually deallocates it by calling string_free It is important to note here that no memory leak occurs in either client or server On the server side, the application code calls string_alloc, and the generated code in the skeleton calls string_free after it has sent the string back to the client On the client side, the generated stub code calls string_alloc and returns a pointer to the string to the application code, which calls string_free This scenario illustrates how the application code and the ORB run time cooperate to ensure that the correct memory management activities take place for both client and server CORBA's location transparency crucially depends on these memory management rules Let us consider the preceding example once more, this time in the collocated case in which both client and server share the same address space Collocation essentially amounts to removing all the ORB-generated code (apart from some remnants that are irrelevant here), so we can imagine that we simply slide the server application code across into the client application code, deleting all the dark gray code (see Figure 77)
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Figure 77 Returning a variable-length value collocated case
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In the collocated case, the client calls the name member function on its proxy as before However, that member function is now local to the client's address space, so there is no need to go through all the intervening networking code Notice that we didn't have to change any application source code to collocate client and server Most important, memory management responsibilities are identical The server code still calls string_alloc, and the client code still calls string_free, so there is no memory leak This transparency of remote and collocated invocations is at the heart of the memory management rules for variable-length parameters If you keep the preceding pictures in mind, you will find it much easier to understand why variable-length parameters are passed the way they are Note that arguments similar to those for return values also apply to inout and out parameters The point is that the sending side allocates a variable-length value, and the receiving side deallocates it again 7147 Parameter Passing for Strings and Wide Strings Given the discussion in the preceding section, it is not hard to work out how strings must be passed Here is the IDL for an operation that passes string parameters in all possible directions:
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interface Foo { string string_op( in string inout string out string ); };
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