Author: nanxiao

From LDD3, “char drivers” section:

loff_t f_pos;

The current reading or writing position. loff_t is a 64-bit value on all platforms ( long long in gcc terminology). The driver can read this value if it needs to know the current position in the file but should not normally change it; read and write should update a position using the pointer they receive as the last argument instead of acting on filp->f_pos directly. The one exception to this rule is in the llseek method, the purpose of which is to change the file position.

Why “read and write should update a position using the pointer they receive as the last argument instead of acting on filp->f_pos directly“? After checking the kernel code(the version is 3.0), I get the answer.

Use read system call as an example, and others are similar. Firstly, check read code (fs/read_write.c):

SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count)
{
    struct file *file;
    ssize_t ret = -EBADF;
    int fput_needed;

    file = fget_light(fd, &fput_needed);
    if (file) {
        loff_t pos = file_pos_read(file);
        ret = vfs_read(file, buf, count, &pos);
        file_pos_write(file, pos);
        fput_light(file, fput_needed);
    }

    return ret;
}

The core part is the following part:

loff_t pos = file_pos_read(file);
ret = vfs_read(file, buf, count, &pos);
file_pos_write(file, pos);

file_pos_read is very simple, just one statement:

static inline loff_t file_pos_read(struct file *file)
{
    return file->f_pos;
}

It returns the current file position.

Then let we see the vfs_read:

ssize_t vfs_read(struct file *file, char __user *buf, size_t count, loff_t *pos)
{
    ssize_t ret;

    if (!(file->f_mode & FMODE_READ))
        return -EBADF;
    if (!file->f_op || (!file->f_op->read && !file->f_op->aio_read))
        return -EINVAL;
    if (unlikely(!access_ok(VERIFY_WRITE, buf, count)))
        return -EFAULT;

    ret = rw_verify_area(READ, file, pos, count);
    if (ret >= 0) {
        count = ret;
        if (file->f_op->read)
            ret = file->f_op->read(file, buf, count, pos);
        else
            ret = do_sync_read(file, buf, count, pos);
        if (ret > 0) {
            fsnotify_access(file);
            add_rchar(current, ret);
        }
        inc_syscr(current);
    }

    return ret;
}

Exclude a lot of condition checks, the skeleton is just like this:

if (file->f_op->read)
    ret = file->f_op->read(file, buf, count, pos);
else
    ret = do_sync_read(file, buf, count, pos);

If the driver provides the read function, use it, else call do_sync_read. No matter which function is used, the new file position should be updated in the memory which pos points to.

Finally, it is file_pos_write‘s function to update the new position:

static inline void file_pos_write(struct file *file, loff_t pos)
{
    file->f_pos = pos;
}

From the above analysis, we can see that it’s no need for every device driver update the file position, and file_pos_read/write will do this uniformly.Other functions are similar, so we can answer the question posted at the beginning of the article now.