Understanding the Linux Kernel, 3rd Edition

Implementations of VFS System Calls

For the sake of brevity, we cannot discuss the implementation of all the VFS system calls listed in Table 12-1. However, it could be useful to sketch out the implementation of a few system calls, in order to show how VFS’s data structures interact.

Let’s reconsider the example proposed at the beginning of this chapter: a user issues a shell command that copies the MS-DOS file /floppy/TEST to the Ext2 file /tmp/test. The command shell invokes an external program such as cp, which we assume executes the following code fragment:

    inf = open("/floppy/TEST", O_RDONLY, 0);
    outf = open("/tmp/test", O_WRONLY | O_CREAT | O_TRUNC, 0600);
    do {
        len = read(inf, buf, 4096);
        write(outf, buf, len);
    } while (len);
    close(outf);
    close(inf);

Actually, the code of the real cp program is more complicated, because it must also check for possible error codes returned by each system call. In our example, we focus our attention on the “normal” behavior of a copy operation.

The open( ) System Call

The open( ) system call is serviced by the sys_open( ) function, which receives as its parameters the pathname filename of the file to be opened, some access mode flags flags, and a permission bit mask mode if the file must be created. If the system call succeeds, it returns a file descriptor—that is, the index assigned to the new file in the current->files->fd array of pointers to file objects; otherwise, it returns -1.

In our example, open( ) is invoked twice; the first time to open ...