Ordinary Interview/screening questions

These are generic questions that I have come accross.

List of common Interview questions+answers for candidates, as well as recruiters. I regularly update and come back to it.

Answers are short and generic. For the whole picture, read the right book or check documentation.

The rating is just my personal preference.

What is an OS (Operating System) ?

An operating system (OS) is system software that manages computer hardware and software resources, and provides common services for computer programs.

Source: https://en.wikipedia.org/wiki/Operating_system

Rating: ★★★★★

Why do we need Operating Systems ?

For providing abstraction to the underlying hardware. An E-Mail server only works by utilising abstraction layers from a kernel. The kernel handles the tacky hardware communications, while userspace enjoys only having to issue a send() to send data.

Rating: ★★★☆☆

List CPU architectures

Possible answers: RISC-V, ARM, X86, etc..

Rating: ☆☆☆☆☆

What is the difference between Authentication and Authorization ?

Authenticating is proving who you say you are is true. Authorization is whether you are allowed to perform an action.

For example, at the airport, you might be able to “prove” who you are, but that doesn’t entitle you to board any plane you want. You’ll only be authorized to board your plane, but not others.

Rating: ★★★★☆

How are files protected from unauthorized changes on a computer ?

Depending on the operating system, it is implemented by some kind of underlying access control policy. Each file has an owner and a policy to enact when other users interact with the file.

Alice owns file1. Bob cannot open this file, but Alice may want to give read-only access to Bob, or even decide to let him edit it too. Bob cannot give himself access to file1 unless Alice allows him to.

Rating: ★★★★☆

What is the difference between a function call and a syscall ?

A function call involves just incremeting/decrementing some register pointer to go read another part of the process’s memory. A syscall involves making use of an interrupt, giving control to the kernel, and back. This is way slower as it requires synchronisation and kernel-to-user space data copying.

Rating: ★★★★★

What is the difference between the stack vs the heap ?

The stack is staticaly allocated memory to the process, while the heap is dynamically allocated, on request of the process.

The amount of memory on the stack cannot be changed, while memory on the heap can be decreased or increased. Processes must do their diligence and free() memory they are not using anymore, to avoid running OOM.

Rating: ★★★★★

What is the difference between a pointer and a reference ?

A reference is a C++ abstraction of a raw pointer. A reference is an “always dereferenced” pointer, which can never be null and always points to valid memory. Pointer-arithmetic is not allowed with references.

Rating: ★★★★★

What is the use of a copy-constructor ?

A copy constructor allows for classes/structs to implement their own copying mechanisms. This is most useful for data structures with pointers, where naive member copying is not possible.

Rating: ★★★★☆

What is the difference between TCP and UDP ?

TCP guaranties that a packet is received, as well as in the right order. It can also “adapt” to congestion by gradually resending smaller and smaller packets, then re-assembling at the destination.

UDP has none of these. It may as well never arrive, neither the client or server would know. Extra userspace code is required for this, but then why not use TCP ? ;)

Due to this, “bare UDP” is considered faster but “lossy”, and is more suitable for low-latency “loss-forgiving” contexts. Examples include VPNs, Live video streaming.

Rating: ★★★★☆

How does symmetric encryption work ?

Alice wants to exchange a secret message m with Bob. They both have a “key” k. Alice can encrypt the message m with k, transfer to Bob and he can decrypt it using k.

Encryption/decryption can be performed by ciphers such as AES.

Rating: ★★★★★

Explain what is SSL

SSL is the predecessor to TLS and was the basis for the HTTPS protocol.

Rating: ★★★☆☆

What is an SQL injection ?

An SQL injection is an attack that can be performed on systems that do not validate or escape user input, preventing from reaching SQL’s expansion + evaluation.

It could just be something like typing the following in a username field : where 1=1; -- to bypass login and login randomly.

Rating: ★★★★☆

What to do if we have code that iterates over a large number of elements ?

Parallelise. If you are executing the same operation on all items, use SIMD instructions to perform this. Consider offloading to GPU (though memory transfer can be costly). The graal would just be using vectorizing instructions from the CPU, and avoiding this data transfer, though, this is hardware dependent.

Rating: ★★★★★

What is a virtual class for ?

Basically for generic programming where you have a Dog and Cat class, derived from an virtual Animal. You can then call non-member function void bark(Animal&) with derived classes and it will dynamically call the right functions for each type. This is because the “correct” member functions are stored on a table, the infamous vTable. Another way to achieve this, is composition, which is more usable than inheritance.

Rating: ★★★★☆

What are the steps to the TCP 3-way handshake ?

SYN -> SYN-ACK -> ACK. From client to server.

Rating: ★★★★☆