Add, Subtract, Multiply big numbers in C project

This project in general is about add, subtract and multiply big numbers. At the end of the program we need to compute 100!.
Build a program in C with which very large integer numbers will be entered as strings. Each character will then be converted to the corresponding digit. The calculations of the operations will be done with the methodology of the vertical acts as you had learned in the primary school. The result will be saved and printed on the computer screen as a string.
Adding example:
237462374678324834
+723674629462912321
961137004141237155

Multiply example:
823473487473
*2308230092370937
1900766284054820422538772201
The project is divided in these versions:

Version1: Add Big Numbers

Construct the int addHugeNumbers (char * a1, char * a2, char * res) which accepts three character tables. The first two tables contain the digits of the numbers to be added and in the third table the sum of the numbers will be stored as a sequence of digits. The function needs to check if the contents of the tables given contain only digits. Then make the add of these numbers, and save the result in the char * res and return the value 1, otherwise it returns 0. Build the program that will read a string from the keyboard, for example 65238340450345034503453453+233474583479475834758934578934578474623632, with the maximum length of 1000 characters, and it will use the function addHugeNumbers to add the two numbers . The result will be printed on the terminal screen.

Version2: Subtract Big Numbers

As in version1 construct the int subHugeNumbers function (char * a1, char * a2, char * res) which accepts three character tables. The first two tables contain the digits of the numbers to be subtracted and in the third table the result will be saved as a sequence of digits with also a numeric sign (if necessary). The function needs again to check if the contents of the tables given contain only digits. Then it substracts, saves the result in the res array and returns the value 1, otherwise it returns the value 0. Construct the program that will read a string from the keypad, for example, 625238340450345034503453453233474583479475-834758934578934578474623632 with a maximum length of 1000 characters and use the subHugeNumbers function to perform the subtraction. The result will then be printed on the terminal screen.

Version3: Multiply Big Numbers

As in the above two versions, we construct the int MulHugeNumbers function (char * a1, char * a2, char * res) which accepts three character tables. The first two tables contain the digits of the numbers involved in the act of multiplication, and the third table will store the result as a sequence of digits. The function needs to check if the contents of the tables given contain only digits. Then it multiplies, saves the result in the res array and returns the value 1, otherwise it returns 0. Build the program that will read two numbers from the keyboard and print their output on the terminal screen.

Version4: Compute 100!
Using the function from the previous versions, construct a program that will compute and print the 100!

Each version to be in separate folder with each own source code.
Deadline is 9th of June 20:00 CET.

Thank you.