Linux Device Drivers, Second Edition

The file Structure

struct file, defined in <linux/fs.h>, is the second most important data structure used in device drivers. Note that a file has nothing to do with the FILEs of user-space programs. A FILE is defined in the C library and never appears in kernel code. A struct file, on the other hand, is a kernel structure that never appears in user programs.

The file structure represents an open file. (It is not specific to device drivers; every open file in the system has an associated struct file in kernel space.) It is created by the kernel on open and is passed to any function that operates on the file, until the last close. After all instances of the file are closed, the kernel releases the data structure. An open file is different from a disk file, represented by struct inode.

In the kernel sources, a pointer to struct file is usually called either file or filp (“file pointer”). We’ll consistently call the pointer filp to prevent ambiguities with the structure itself. Thus, file refers to the structure and filp to a pointer to the structure.

The most important fields of struct file are shown here. As in the previous section, the list can be skipped on a first reading. In the next section though, when we face some real C code, we’ll discuss some of the fields, so they are here for you to refer to.

mode_t f_mode;

The file mode identifies the file as either readable or writable (or both), by means of the bits FMODE_READ and FMODE_WRITE. You might want to check ...