Linux Device Drivers Development

Steps to llseek

1. Use the switch statement to check every possible whence case, since they are limited, and adjust newpos accordingly:

switch( whence ){ 
    case SEEK_SET:/* relative from the beginning of file */ 
        newpos = offset; /* offset become the new position */ 
        break; 
    case SEEK_CUR: /* relative to current file position */ 
        newpos = file->f_pos + offset; /* just add offset to the current position */ 
        break; 
    case SEEK_END: /* relative to end of file */ 
        newpos = filesize + offset; 
        break; 
    default: 
        return -EINVAL; 
} 

2. Check whether newpos is valid:

if ( newpos < 0 ) 
    return -EINVAL; 

3. Update f_pos with the new position:

filp->f_pos = newpos; 

4. Return the new file-pointer position:

return newpos; 

The following is an example of a user program ...