I want to make a telephone directory in C language. It should have the following features:

Records have to be saved in a file(tel.txt) and all the features should have access to the file.

The University needs a Telephone Directory System that allows its Telephone Operator (TO) to carry out the following:

1. Search employee telephone number. The TO needs to search staff telephone number by employee name or id as a search key. The system should read the tel.txt file and return the telephone number searched.

2. Assign telephone numbers to new employees. The system should provide a feature for the TO to add telephone number for new employees into the tel.txt file. The system should allow the TO to store employee id, employee name, telephone number and department name for each of the new employee into the tel.txt file. There should not be any duplicate entry and the tel.txt file has to be created (by the system) if it does not exist.

3. Update telephone number of the existing employees. The system should allow the TO to key-in employee id or name and update their telephone numbers.

Produce a list of all employees and their telephone numbers department-wise. You are expected to use structures to store data read from the tel.txt file and sort them department-wise using array and/or linked-list.