Fetching, modifying information on binary file?

Question

I have a binary file whose size is "x+a+y" bytes. I have some questions;

• Without exhaustive reading x bytes, How can I jump to start point of a bytes ?
• Without taking all of the information reside in a line and then doing required modification and finally writing new information to the same line, How can I just change "a bytes" of the data in place whose starting address is shown as P?
• Can I put a pointer to a starting point of "a bytes" that is to P ?

Question about how data is stored in pile file;

• Why is all of the primitive data type ( such as int ) stored in pile file preferred to be unsigned ?

• Why is offset always chosen as unsigned int ? eX: x,a,y are known value

  ___________________________________________________________
  |                             |     |                     |
  |                             |     |                     |
  |----------> x <--------------|> a <|-------> y <---------|
                                ^
                                ^
                                P
  

Platform linux

Note: If I do dublicate question, please inform me so that I can delete this question. Feel free when you command smth

Answer 1

There is a method in ifstream called seekg that moves the read-position to a specific place in the file. See http://en.cppreference.com/w/cpp/io/basic_istream/seekg.