Linux Device Drivers Development

Steps to read

1. Prevent from reading beyond the file size, and return end-of-file:

if (*pos >= filesize) 
  return 0; /* 0 means EOF */ 

2. The number of bytes read can't go beyond the file size. Adjust count appropriately:

if (*pos + count > filesize) 
    count = filesize - (*pos); 

3. Find the location from which you will start the read:

void *from = pos_to_address (*pos); /* convert pos into valid address */ 

4. Copy the data into the user space buffer and return an error on failure:

sent = copy_to_user(buf, from, count); 
if (sent) 
    return -EFAULT; 

5. Advance the file's current position according to the number of bytes read, and return the number of bytes copied:

*pos += count; 
return count; 

The following is an example of a driver read() file operation, ...