Question

If I open a single file (using CreateFile()) two times inside a single thread such that I have two valid handles at once, will the two file handles have a shared “file pointer” (SetFilePointer()), or will the two handles have separate independent “file pointers”?

What if there are instead two concurrent threads in one process, and they each hold one handle to the same file. Will those two handles have independent file pointers?

Answer 1

Each time a thread opens a file, a new file object is created with a new set of handle-specific attributes. For example, the current byte offset attribute refers to the location in the file at which the next read or write operation using that handle will occur. Each handle to a file has a private byte offset even though the underlying file is shared. A file object is also unique to a process, except when a process duplicates a file handle to another process (by using the Windows DuplicateHandle function) or when a child process inherits a file handle from a parent process. In these situations, the two processes have separate handles that refer to the same file object.
Windows Internals 5th