3.10. File SharingThe UNIX System supports the sharing of open files among different processes. Before describing the dup function, we need to describe this sharing. To do this, we'll examine the data structures used by the kernel for all I/O.The following description is conceptual. It may or may not match a particular implementation. Refer to Bach [1986] for a discussion of these structures in System V. McKusick et al. [1996] describes these structures in 4.4BSD. McKusick and Neville-Neil [2005] cover FreeBSD 5.2. For a similar discussion of Solaris, see Mauro and McDougall [2001].Section 3.14) Figure 3.6. Kernel data structures for open filesKleiman 1986]. The v-node propagated through various vendor implementations as support for Sun's Network File System (NFS) was added. The first release from Berkeley to provide v-nodes was the 4.3BSD Reno release, when NFS was added.In SVR4, the v-node replaced the file systemindependent i-node of SVR3. Solaris is derived from SVR4 and thus uses v-nodes.Instead of splitting the data structures into a v-node and an i-node, Linux uses a file systemindependent i-node and a file systemdependent i-node.If two independent processes have the same file open, we could have the arrangement shown in Figure 3.7. We assume here that the first process has the file open on descriptor 3 and that the second process has that same file open on descriptor 4. Each process that opens the file gets its own file table entry, but only a single v-node table entry is required for a given file. One reason each process gets its own file table entry is so that each process has its own current offset for the file. Figure 3.7. Two independent processes with the same file openSection 3.11.) It is possible for more than one file descriptor entry to point to the same file table entry, as we'll see when we discuss the dup function in Section 3.12. This also happens after a fork when the parent and the child share the same file table entry for each open descriptor (Section 8.3).Note the difference in scope between the file descriptor flags and the file status flags. The former apply only to a single descriptor in a single process, whereas the latter apply to all descriptors in any process that point to the given file table entry. When we describe the fcntl function in Section 3.14, we'll see how to fetch and modify both the file descriptor flags and the file status flags.Everything that we've described so far in this section works fine for multiple processes that are reading the same file. Each process has its own file table entry with its own current file offset. Unexpected results can arise, however, when multiple processes write to the same file. To see how to avoid some surprises, we need to understand the concept of atomic operations. |
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