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An early decision in any Windows development or porting project is to select whether file processing should be done with the C library or with the Windows functions This is not an either/or decision because the functions can be mixed with caution even when you're processing the same file The C library offers several distinct advantages, including the following
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The code will be portable to non-Windows systems Convenient line- and character-oriented functions that do not have direct Windows equivalents simplify string processing C library functions are generally easier to use than Windows functions The line and stream character-oriented functions can easily be changed to generic calls, although the portability advantage will be lost The C library will operate in a multithreaded environment, as shown in 7
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Nonetheless, there are some limitations to the C library Here are some examples
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The C library cannot manage or traverse directories, and it cannot obtain or set most file attributes The C library uses 32-bit file position in the fseek function, so, while it can read huge files sequentially, it is not possible to position anywhere in a huge file, as is required, for instance, by Program 3-1 Advanced features such as file security, memory-mapped files, file locking, asynchronous I/O, and interprocess communication are not available with the C library Some of the advanced features provide performance benefits, as shown in Appendix C
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Another possibility is to port existing UNIX code using a compatibility library Microsoft C provides a limited compatibility library with many, but not all, UNIX functions The Microsoft UNIX library includes I/ O functions, but most process management and other functions are omitted Functions are named with an underscore prefixfor example, _read, _write, _stat, and so on Decisions regarding the use and mix of C library, compatibility libraries, and the Win32/64 API should be driven by project requirements Many of the Windows advantages will be shown in the following chapters, and the performance figures in Appendix C are useful when performance is a factor
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An important issue in any system with multiple processes is coordination and synchronization of access to shared objects, such as files Windows can lock files, in whole or in part, so that no other process (running program) can access the locked file region File locks can be read-only (shared) or read-write (exclusive) Most important, the locks belong to the process Any attempt to access part of a file (using ReadFile or WriteFile) in violation of an existing lock will fail because the locks are mandatory at the process level Any attempt to obtain a conflicting lock will also fail even if the process already owns the lock File locking is a limited form of synchronization between concurrent processes and threads; synchronization is covered in much more general terms starting in 8 The most general function is LockFileEx The less general function, LockFile, can be used on Windows 9x LockFileEx is a member of the extended I/O class of functions, so the overlapped structure, used earlier to specify file position to ReadFile and WriteFile, is required to specify the 64-bit file position and range of the file region that is to be locked BOOL LockFileEx ( HANDLE hFile, DWORD dwFlags, DWORD dwReserved, DWORD nNumberOfBytesToLockLow, DWORD nNumberOfBytesToLockHigh, LPOVERLAPPED lpOverlapped)
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LockFileEx locks a byte range in an open file for either shared (multiple readers) or exclusive (one reader-writer) access
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Parameters
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hFile is the handle of an open file The handle must have GENERIC_READ or both GENERIC_READ and GENERIC_WRITE file access dwFlags determines the lock mode and whether to wait for the lock to become available LOCKFILE_EXCLUSIVE_LOCK, if set, indicates a request for an exclusive, read-write lock Otherwise, it requests a shared (read-only) lock LOCKFILE_FAIL_IMMEDIATELY, if set, specifies that the function should return immediately with FALSE if the lock cannot be acquired Otherwise, the call blocks until the lock becomes available dwReserved must be 0 The two parameters with the length of the byte range are self-explanatory lpOverlapped points to an OVERLAPPED data structure containing the start of the byte range The overlapped structure contains three data members that must be set (the others are ignored); the first two determine the start location for the locked region
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DWORD Offset (this is the correct name; not OffsetLow) DWORD OffsetHigh HANDLE hEvent should be set to 0
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A file lock is removed using a corresponding UnlockFileEx call; all the same parameters are used except dwFlags BOOL UnlockFileEx ( HANDLE hFile, DWORD dwReserved, DWORD nNumberOfBytesToLockLow, DWORD nNumberOfBytesToLockHigh, LPOVERLAPPED lpOverlapped)
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