File Descriptors

ExOS uses a VFS like interface between high-level file operations like read, write, close, etc and the underlying filesystem object that a fd represents. Each underlying object specifies an array of function pointers that implement the specified operation allowing new filesystem objects to be transparently used.

Several of these file object methods deserve particular attention because they do not correspond to typical VFS-like operations:

• select_pred: This returns a wakeup predicate that will evaluate to true when the object is ready for an operation. The top level fd code collects wakeup predicates from all fd objects that a select is being done on and concatenates them together to form one large predicate that the process blocks on.
• lookup: This implements a partial namei. It maps a single pathname component from one file object to another. The source and destination file objects may be different types of file objects if the pathname component corresponds to crossing a mount point.
• bmap: Translates a logical block number to a physical block number. Both block numbers are only meaningful to the underlying fd object. For example, an NFS fd object might implement this function as the identity function since NFS has no concept of physical block. This function is used when programs wish to bypass the normal filesystem operations and want to read/write an object's blocks directly.
• mount: Associates a mapping from one file object to another. Subsequent lookup operations at this point will return the new file object.
• close0: Close operation called on the underlying object every time close is called on the fd. The close method is only called when the last reference to an fd is closed.