Program 5-7 atouEL: File Conversion with Explicit Linking in .NET

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Program 5-7 atouEL: File Conversion with Explicit Linking
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/* 5 atou Explicit Link version */ #include "EvryThngh" int _tmain (int argc, LPTSTR argv []) { /* Declare variable Asc2Un to be a function */ BOOL (*Asc2Un)(LPCTSTR, LPCTSTR, BOOL); DWORD LocFileIn, LocFileOut, LocDLL, DashI; HINSTANCE hDLL; FARPROC pA2U; LocFileIn = Options (argc, argv, _T ("i"), &DashI, NULL); LocFileOut = LocFileIn + 1; LocDLL = LocFileOut + 1; /* /* Test for existing file && DashI is omitted */ Load the ASCII-to-Unicode function */
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hDLL = LoadLibrary (argv [LocDLL]); if (hDLL == NULL) ReportError (_T ("Failed loading DLL"), 1, TRUE); /* Get the entry point address */
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pA2U = GetProcAddress (hDLL, "Asc2Un"); if (pA2U == NULL) ReportError (_T ("Failed to find entry point"), 2, TRUE); /* Cast the pointer A typedef could be used here */ Asc2Un = (BOOL (*)(LPCTSTR, LPCTSTR, BOOL)) pA2U; /* Call the function */ Asc2Un (argv [LocFileIn], argv [LocFileOut], FALSE); FreeLibrary (hDLL); return 0; }
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Building the Asc2Un DLLs
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This program was tested with the two file conversion functions, which must be built as DLLs with different names but identical entry points There is only one entry point in this case The only significant change in the source code is the addition of a storage modifier, _declspec (dllexport), to export the function
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The DLL Entry Point
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Optionally, you can specify an entry point for every DLL you create, and this entry point is normally invoked automatically every time a process attaches or detaches the DLL LoadLibraryEx, however, allows you to prevent entry point execution For implicitly linked (load-time) DLLs, process attachment and detachment occur when the process starts and terminates In the case of explicitly linked DLLs, LoadLibrary, LoadLibraryEx, and FreeLibrary cause the attachment and detachment calls The entry point is also invoked when new threads ( 7) are created or terminated by the process The DLL entry point, DllMain, is introduced here but will not be fully exploited until 12 (Program 12-4), where it provides a convenient way for threads to manage resources and so-called Thread Local Storage (TLS) in a thread-safe DLL BOOL DllMain ( HINSTANCE hDll, DWORD Reason, LPVOID Reserved)
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The hDll value corresponds to the instance obtained from LoadLibrary Reserved, if NULL, indicates that the process attachment was caused by LoadLibrary; otherwise, it was caused by implicit load-time linking Likewise, FreeLibrary gives a NULL value for process detachment Reason will have one of four values: DLL_PROCESS_ATTACH, DLL_THREAD_ATTACH, DLL_THREAD_DETACH, and DLL_PROCESS_DETACH DLL entry point functions are normally written as switch statements and return TRUE to indicate correct operation The system serializes calls to DllMain so that only one thread at a time can execute it (threads are thoroughly discussed starting in 7) This serialization is essential because DllMain must perform initializations that must be completed without interruption As a consequence, however, it is recommended that there not be any blocking calls, such as I/O or wait functions (see 8) within the entry point, because they would prevent other threads from entering LoadLibrary and LoadLibraryEx, in particular, should never be called from a DLL entry point as that would create additional DLL entry point calls DisableThreadLibraryCalls will disable thread attachment/detachment calls for a specified DLL instance Disabling the thread calls can be helpful when threads do not require any unique resources during initialization
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DLL Version Management
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A common problem with DLLs concerns difficulties that occur as a library is upgraded with new symbols and features are added A major DLL advantage is that multiple applications can share a single implementation This power, however, leads to compatibility complications, such as the following
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New functions may be added, invalidating the offsets that implicitly linked applications assume when they link with a LIB file Explicit linking avoids this problem A new version may change behavior, causing problems to existing applications that have not been updated Applications that depend on new DLL functionality sometimes link with older DLL versions
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DLL version compatibility problems, popularly referred to as "DLL hell," can be irreconcilable if only one version of the DLL is to be maintained in a single directory However, it is not necessarily simple to provide distinct version-specific directories for different versions There are several solutions
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Use the DLL version number as part of the DLL and LIB file names, usually as a suffix For example, Utility_3_0DLL and Utility_3_0LIB are used on the examples on the book's Web site and with all the projects to correspond with the book version number By using either explicit or implicit linking, applications can then determine their version requirements and access files with distinct names This solution is commonly used with UNIX applications Microsoft introduced the concept of side-by-side DLLs or assemblies and components This solution requires adding a manifest, written in XML, to the application so as to define the DLL requirements This topic is beyond the book's scope, but additional information can be found on the Microsoft developer Web site The NET Framework provides additional support for side-by-side execution
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The first approach, including the version number as part of the file name, is used in the example projects To provide additional support so that applications can determine the DLL information, DllGetVersion is implemented in all the DLLs; many Microsoft DLLs also provide this callback function as a standard method to obtain version information dynamically The function takes the following form: HRESULT CALLBACK DllGetVersion( DLLVERSIONINFO *pdvi )
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Information about the DLL is returned in the DLLVERSIONINFO structure, which contains DWORD fields for cbSize (the structure size), dwMajorVersion, dwMinorVersion, dwBuildNumber, and dwPlatformID The last field, dwPlatformID, can be set to DLLVER_PLATFORM_NT if the DLL cannot run on Windows 9x or to DLLVER_PLATFORM_WINDOWS if there are no restrictions The cbSize field should be set to sizeof (DLLVERSIONINFO) The normal return value is NOERROR Utility_3_0 implements DllGetVersion
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