{"id":23491,"date":"2023-04-19T08:55:35","date_gmt":"2023-04-19T08:55:35","guid":{"rendered":"https:\/\/www.booksofall.com\/id\/?post_type=product&p=23491"},"modified":"2023-04-19T08:55:35","modified_gmt":"2023-04-19T08:55:35","slug":"unix-lecture-notes-chapter-04","status":"publish","type":"product","link":"https:\/\/www.booksofall.com\/id\/unix-lecture-notes-chapter-04\/","title":{"rendered":"UNIX Lecture Notes – Chapter 04"},"content":{"rendered":"

4.1 Overview<\/h2>\n

This chapter begins by examining how a program can exercise some control over the connections that it makes with disk \u001cfiles. It describes the data structures used by the kernel to manage open \u001cfiles, and explains how processes can interact with these fi\u001cles. It then explores device fi\u001cles and how they are used and structured, and device drivers<\/a> and their roles and structures. From there it looks at block and character device fi\u001cle differences, after which it turns to terminals. The remainder of the chapter is then devoted to controlling terminals.<\/p>\n

4.2 Open Files<\/h2>\n

The programs that we have studied so far operate on disk files, which are fi\u001cles that reside, of course, on disks. You are by now well-acquainted with the general model of fi\u001cle I\/O: open the fi\u001cle, access it, and close it. Remember that this model works for any kind of \u001cfile, not just disk fi\u001cles. Here though, we are interested only in disk fi\u001cles.<\/p>\n

In this model of fi\u001cle I\/O<\/a>, opening a fi\u001cle returns a reference to an object that can be used to access the fi\u001cle, either for reading or writing. When you use the open() system call, you get a \u001cle descriptor in return; when you use the fopen() C standard I\/O library call, you get a FILE pointer in return. Either way, you are getting a scalar object (i.e., a small integer or a pointer) that is associated with a hidden (kernel) data structure that allows you to access the \u001cfile. Here, we will explore how much control we can exercise over the way in which the program is connected to the fi\u001cle.<\/p>\n

When a process opens a disk \u001cfile using the open() system call, the kernel creates a data structure that represents the connection between the process and the \u001cfile. The returned \u001cfile descriptor can be used by the process to access the fi\u001cle. A fi\u001cle descriptor is simply an index into a per-process table that the kernel uses to locate that specific data structure. Different versions of UNIX call this data structure different things, and the data structure may have slightly di\u001bfferent members from one UNIX variant to another, but the basic members and purpose of the structure are invariant: its most important member is the fi\u001cle pointer, and its purpose is to store the position in the fi\u001cle from which the next operation will take place (whether it is a read or a write.)<\/p>\n

\"\"<\/p>\n

This data structure is called a \u001cfile entry in BSD<\/a> UNIX and a fi\u001cle structure in Linux. I will call it a fi\u001cle structure in these notes. The connection between a process and a \u001cfile is completely characterized by the information contained in the fi\u001cle structure. Aside from the \u001cfile pointer, the kinds of information it typically contains include:<\/p>\n