15.4. File Permissions

In this section, we describe the permission scheme applied to files and directories. Although we talk about permissions here mainly as they apply to regular files and directories, the rules that we describe apply to all types of files, including devices, FIFOs, and UNIX domain sockets. Furthermore, the System V and POSIX interprocess communication objects (shared memory, semaphores, and message queues) also have permission masks, and the rules that apply for these objects are similar to those for files.

15.4.1. Permissions on Regular Files

As noted in Section 15.1, the bottom 12 bits of the st_mode field of the stat structure define the permissions for a file. The first 3 of these bits are special bits known as the set-user-ID, set-group-ID, and sticky bits (labeled U, G, and T, respectively, in Figure 15-1). We say more about these bits in Section 15.4.5. The remaining 9 bits form the mask defining the permissions that are granted to various categories of users accessing the file. The file permissions mask divides the world into three categories: