Question

I have noticed that when I call lseek64 on my block device driver file (/dev/mybd), it always fails. (I can open, read and write on /dev/mybd ok).

However, if I can lseek64 with the same argument on /dev/sdb, which is an sata disk, it always succeed.

Does lseek requires any block device support? Or is it a pure kernel function?

Answer 1

Looking at the code, the default seek for block devices is in fs/block_dev.c:

static loff_t block_llseek(struct file *file, loff_t offset, int origin)
{
        struct inode *bd_inode = file->f_mapping->host;
        loff_t size;
        loff_t retval;

        mutex_lock(&bd_inode->i_mutex);
        size = i_size_read(bd_inode);

        retval = -EINVAL;
        switch (origin) {
                case SEEK_END:
                        offset += size;
                        break;
                case SEEK_CUR:
                        offset += file->f_pos;
                case SEEK_SET:
                        break;
                default:
                        goto out;
        }
        if (offset >= 0 && offset <= size) {
                if (offset != file->f_pos) {
                        file->f_pos = offset;
                }
                retval = offset;
        }
out:
        mutex_unlock(&bd_inode->i_mutex);
        return retval;
}

No call to the specific block device. The only particular call is to i_size_read, that just does some SMP magic.