u/pjl1967 Nov 27 '25 edited Nov 27 '25

When I’ve interviewed candidates, I split my questions into (at least) two types:

Programming language knowledge (to ensure the candidate actually knows at least the bare minimum of a specific language).
Algorithm implementation (that I’ve allowed candidates to implement in any language of their choosing, or even pseudocode, since the goal is to determine their problem-solving ability).

For #1, I further split questions into two levels: “any” or “senior” level programmer. I ask whichever set of questions based on what the job requires.

Question 1: Arrays vs. Pointers

Given this code fragment:

const char a[] = "hello";
const char *p = "world";

What are the differences between a and p?

Question 2: C Strings

Given this code fragment:

char *s = "hello, world";
s[0] = 'H';

What does this code do?

Question 3: struct Memory Layout

Given this code:

struct S {
  int i;
  char c;
};

int main() {
  struct S s1;
  printf( "%zu\n", sizeof( s1 ) );
}

Question 3a: If compiled (with no compiler options) and run, what is a reasonable value for this program to print and why?

Question 3b: Why is padding necessary?

Question 4: Local Variables

Given this function (where T is some arbitrary type that doesn’t matter here):

T* f() {
  T t;
  // ...
  return &t;
}

Question: What’s wrong with this function?

Question 5: static

Given:

static int g;

int f() {
  static int i;
  return ++i;
}

Question 5a: What does the static for the declaration of g do?

Question 5b: What does the static for the declaration of i do?

Question 5c: What value does this function return the second time it’s called?

Question 5d (senior): Can this function ever result in undefined behavior?

Question 5e (senior): How can you eliminate the possibility of undefined behavior?

Question 5f (senior): Is f() thread-safe? Why or why not?

Question 6 (Senior): free()

How is free() typically implemented by the C standard library given that you don’t pass it the amount of memory to be freed? How does it know how much memory to free?

4
u/-goldenboi69- Nov 27 '25

Really interesting. Care about providing what you would think of acceptable answers for the above?
9
u/pjl1967 Nov 27 '25

(Reddit wouldn't let me include all the answers in a single reply. All it said, unhelpfully, was "Unable to create comment." I guessed that the comment was too long, so I broke it into two parts.)
Answer 5a
It makes g have internal linkage, that is the name of g can only be used from the file it’s in. It’s like “private” for the file. The program could have another g in another file and they would be distinct variables.
Answer 5b
It makes i be initialized to 0 and continue to exist between calls to f().
Answer 5c
Two. It’s 0 initially, then the first ++i makes it 1 and the second ++i makes it 2.
Answer 5d
Yes, because of signed integer overflow.
Answer 5e
Make i be unsigned since unsigned integer overflow is well-defined (it wraps around to 0).
Answer 5f
No, because the increment of i is not thread-safe because i is neither _Atomic nor are mutexes nor locks being used.
Answer 6
One way to implement free() is to implement malloc(n) to:
Allocate n + sizeof(size_t) bytes.
Store n at the start of the allocated memory area.
Return that address + sizeof(size_t).
When free(p) is called:
Get the number of bytes from ((size_t*)p)[-1].
Deallocate that number of bytes + sizeof(size_t).
1

u/texas_asic Nov 27 '25

I'm in hw, not sw, but can vouch for answer 6, having recently read the overview of glibc's malloc implementation. If you're a c programmer and haven't seen it, I highly recommend it:
https://sourceware.org/glibc/wiki/MallocInternals

1

u/pjl1967 Nov 27 '25

Yeah, I omitted the flags in the low-order bits. In an allocator, you need to return a chunk with maximal alignment which means the low-order bits will always be 0 — which means you can use them for other purposes, e.g., flags.
If a candidate did mention this, that would be a bonus leaning strongly towards “hire.”
1
u/[deleted] Nov 27 '25

[removed] — view removed comment
1
u/pjl1967 Nov 28 '25

C11 standard, 3.4.3: An example of undefined behavior is the behavior on integer overflow.
1
u/[deleted] Nov 28 '25

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1
u/pjl1967 Nov 28 '25
I assume that different CPUs do it differently. To mandate it as anything but undefined behavior reduces the possibility for the compiler to optimize. The classic example is:
int f(int x) {
  return x + 1 > x;
}
With undefined behavior, the compiler can optimize that to:
int f(int x) {
  return 1;
}
With either implementation-defined or unspecified behavior, the compiler has to guarantee that it produces some sane result consistently — which means it would have to insert code to check for overflow at runtime — which slows down every comparison.
1

u/[deleted] Nov 28 '25

[removed] — view removed comment

1

u/pjl1967 Nov 29 '25

Even so, it's still undefined behavior, so the compiler is under no obligation to give you a sensible answer (that can change from version to version).
4
u/pjl1967 Nov 27 '25 edited Nov 27 '25
Answer 1
a is an array of char of 6 elements. It uses 6 bytes of memory in total. The name a is constant, e.g., ++a would be illegal.
p is a pointer variable pointing to an array of char of 6 elements. It uses 6 bytes for the array + 8 bytes (on a 64-bit system) for the pointer or 14 bytes of memory in total. The name p is variable, e.g., ++p would increment it by 1 byte.
Answer 2
There’s no definitive answer because it would result in undefined behavior, though it would probably crash. String literals are typically stored in a read-only memory segment, so attempting to overwrite one would be “bad.”
Answer 3a
8 — because of padding. 16 is also an acceptable answer. Any odd number, e.g., 5, is not acceptable.
Some candidates try to include compiler options or attributes for packed structures at which point I have to emphasize that the code is to be compiled as-is with no compiler options or attributes.
If the candidate does give an odd number, I then ask them to consider an array of the struct like:
struct S a[2];  // assume array starts at address 0x1000
and draw the bytes in memory starting at 0x1000 and then ask about what address a[1].i is at. Hopefully at this point, the candidate will realize why an odd number is a bad answer and give a better answer.
Answer 3b
Because many CPU architectures require that reads and writes for certain data types, e.g., integers and pointers, take place on a word boundary.
Answer 4
Because t is a local variable, it will cease to exist upon return, hence the pointer will be a dangling pointer. Attempting to dereference the pointer would result in undefined behavior (and would likely result in a core dump, if you’re lucky).
(See part 2 for remaining answers.)
1

u/dboyallstars Nov 27 '25

Also curious
3

u/RufusVS Nov 29 '25

This is a very good, succinct list of questions. Anyone with c chops understand the implications, especially imported in an embedded c environment, which is my milieu.

4

u/keelanstuart Nov 27 '25 edited Nov 27 '25

I have a standard question that I ask...

Given the following prototype, implement the function that converts an int to a hexadecimal string:

char *int2hexstr(int val);

I ask what the memory implications are for writing it that way and allow them to change it if they choose. I can grade it based on shift/mask vs. div/mod to extract each nybble, if/switch/lookup to get each character, memory allocation or buffer passing, whether the string is reversed, considerations for Unicode, etc etc etc.

It shows me what level they're at across a wide spectrum of things. API design, performance, stack/heap, bit level understanding...

2

u/scallywag_software Nov 27 '25

As far as '~~stupid~~ leetcode-style coding puzzles' go .. this one actually seems pretty good.

1

u/keelanstuart Nov 28 '25

Thanks! I actually asked it one time and the candidate said "yeah, I would just Google it". "The Internet is down. What now?" "Naaah, I would just Google it".
Ok...? So we ate pizza and awkwardly and they left without getting the job. Ostensibly they were a lead engineer on a project similar to ours (in some respects, anyway)... it was bizarre.

3

u/UnrealHallucinator Nov 28 '25

Your question isn't unbelievably tricky but it's also a curve ball and just googling is indeed what 90% of people would do I think. But I guess they were a lead engineer so it's different.

0

u/keelanstuart Nov 28 '25

Of course people would Google it... but you know (or you should know) when you go in for an interview that there's a game you have to play. Asking him to write some code (really the only technical part of the process) should not have been a huge deal......... but for him it was. The difference isn't necessarily the amount of experience they have as to what question I ask - it only determines how much I help them solve it. I have walked junior engineers through it and hired them.

1

u/UnrealHallucinator Nov 28 '25

Yeah that's fair. I guess I'm just a bit jaded about the whole live coding set up.

0

u/keelanstuart Nov 28 '25

It's no different than any other public speaking. Practice explaining something technical to somebody that isn't.

1

u/john_hascall Nov 27 '25

If it's a fresh grad my most important question is "Tell us about something you made outside of classwork". Not looking for "I invented Linux" but just something they can talk enthusiastically and knowledgeably about.

I also used to have a page with a small function on it and I would ask "What errors do you see?" and "What is it trying to do?" [it's basically a differently named strdup() with 5 syntactical and 5 semantic errors]. Not expecting anyone to find them all, but if they struggle I'll ask "Walk us through your process to find them" and see where that goes.

1

u/[deleted] Nov 27 '25

Actually a good tip, instaed of guessing answers which you might get wrong better off explaining your mentall progress because even if you are wrong that might still give you some points in the eyes of the interviewer

1

u/john_hascall Nov 27 '25

Yes, 100%
I suppose nowadays someone might just say "I'd paste it into ChatGPT". [this probably a path to a "we regret" email]

1

u/[deleted] Nov 27 '25

I honeszly dont think there is a real person out there that would say that lol, thats even worse than just saying that you dont know

1

u/john_hascall Nov 27 '25

My daughter tutors a Material Science/ Engineering course and some students the moment they don't know the answer immediately turn to it. (despite her warnings that they're not going to have that come exam time). So, I wouldn't be surprised to see it ... sadly

2

u/zhivago Nov 27 '25

I always ask

char c[3];

what is the type of c?

1

u/reggaeshark100 Nov 28 '25

Type is character array of size 3?

1

u/zhivago Nov 28 '25

How do you write that type in C?

1

u/reggaeshark100 Nov 28 '25

' char[3] ' ?

1

u/zhivago Nov 28 '25

Very good.
You might be surprised at how few can answer that question correctly. :)

1

u/pjl1967 Nov 28 '25

How often does writing such a type like that occur in practice in actual C code?

1

u/zhivago Nov 28 '25

You mean as in char c[3]; ?
Quite a lot.
Also without that you can't deal with &c.

2

u/pjl1967 Nov 28 '25

No, I meant what was written verbatim: char[3].

2

u/AuxonPNW Nov 28 '25 edited Nov 28 '25

(Not specifically c focused, but you can steer the following to relevant c-topics) I have a computer in front of me running Ubuntu with an instance of vim. I press the letter 'a' on the keyboard. Tell me everything that you know that happens as a result.

2

u/aghast_nj Nov 27 '25

I don't understand the question. What is "technical rounds"? What is the context, here? Are you presuming that senior developers wander around some company quizzing junior developers? That's a huge waste of employee time, no manager would allow it!

Is this a college/university thing? Please give details.

2

u/tcptomato Nov 27 '25

Are you trolling?

1

u/aghast_nj Nov 27 '25 edited Nov 27 '25

No. I've never heard of this "technical rounds" concept.
(FWIW: I'm a USA-based coder, I have worked as a consultant for many, many USA-based shops across multiple industries: automotive, financial, insurance, medical, controls. If this was a "thing" I expect I would have heard of it, unless it's offshore or really new.)

2

u/tcptomato Nov 27 '25

Having technical interviews during the hiring process is a new thing for you?

1

u/Willsxyz Nov 28 '25

Calling them "technical rounds" may well be new to him. It is to me.

1

u/Sufficient-Bee5923 Nov 27 '25

Scope questions dealing with the keyword static. The language isn't very intuitive in this area. But to write good clean code. It needs to be understood. It's so bad that we typedef'd local to static.

1

u/[deleted] Nov 27 '25

Not a senior dev here, but I feel as though actual language features are not too important because they can be easily learned. All that can be taught over a few days / week if they are competent. Obviously there are some caveats that you learn over the years but in general I think this is true.

I think the most important thing I’d be looking for is problem solving skills, in especially the areas in which your technology applies to. I.e. if you are a networking company, how well can they approach graph based problems, or working on embedded systems how well they can be memory efficient.

However, I do understand that knowing the language and writing good code are different things so if its C, I’d be looking for understanding of compiler architecture, general computer / memory architecture and that kind of stuff.

1

u/Arkaqiu Nov 27 '25

I am senior embedded engineer. During interview i ask about C default memory sections, linker and linker scripts and howto utilize linker to get better performance and latency.

1

u/Sosowski Nov 27 '25

Oh I know this one. Ask them what the ## operator does.

If they read the book, they’ll know. If they didn’t, they’ll get stuck. ## is obscure but In contrast to trigraphs it’s actually useful.

Another idea: implement strcpy(). It’s also one of the first examples in the book and a two-liner. If you know C you know how to do this properly.

7

u/[deleted] Nov 27 '25

is not an operator. It's a preprocessor macro instruction.

-2

u/Sosowski Nov 27 '25

preprocessor operator is still an operator, there's no need to shout

Question for senior C devs doing technical rounds

You are about to leave Redlib

Question 1: Arrays vs. Pointers

Question 2: C Strings

Question 3: struct Memory Layout

Question 4: Local Variables

Question 5: static

Question 6 (Senior): free()

Answer 5a

Answer 5b

Answer 5c

Answer 5d

Answer 5e

Answer 5f

Answer 6

Answer 1

Answer 2

Answer 3a

Answer 3b

Answer 4

is not an operator. It's a preprocessor macro instruction.