c - Creating big file on Windows

Question

I need to create big relatively big (1-8 GB) files. What is the fastest way to do so on Windows using C or C++ ? I need to create them on the fly and the speed is really an issue. File will be used for storage emulation i.e will be access randomly in different offsets and i need that all storage will be preallocate but not initialized, currently we are writing all storage with dummy data and it's taking too long.

Thanks.

Answer 1

Use the Win32 API, CreateFile, SetFilePointerEx, SetEndOfFile, and CloseHandle. In that same order.

The trick is in the SetFilePointerEx function. From MSDN:

Note that it is not an error to set the file pointer to a position beyond the end of the file. The size of the file does not increase until you call the SetEndOfFile, WriteFile, or WriteFileEx function.

Windows explorer actually does this same thing when copying a file from one location to another. It does this so that the disk does not need to re-allocate the file for a fragmented disk.