I am pretty much beginner to C and creating a user login system. For that I have made an multi-dimensional array and when a person will register its name, his/her username gets appended to the array but it does not save changes to that array. So, when I re-run that program again and enter that same username i had used for registration, that username is not accepted.
What I basically want is that when I register any username, the program appends that username to the array in which all usernames are stored as well as saves that username in the array so that it can get recognised afterwards.
So, is there any solution for this?
A practical approach to your problem is to use some database library, like sqlite. Or some indexed file library like GNU gdbm or Tokyo Cabinet.
You certainly will need some operating system specific API to manage time. For Linux, see time(7). For Windows, study the Win32 API.
If you want to manage millions of users you could use some relational database server, e.g. PostGreSQL. Then read some SQL tutorial.
You could decide to keep the user related data in JSON format.
Read also Modern C and look into this C reference website. See also the documentation of your C compiler (e.g. GCC, use it as gcc -Wall -Wextra -g) and of your debugger (e.g. GDB).
PS. Don't ignore legal issues, e.g. GDPR in Europe.