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Interesting C Interview Questions and Answers

40 points by giZm0 14y ago

deleted 14y ago
[deleted]
mitjak 14y ago
I've studied most of these as part of my introductory C course in university. Are they really that interesting?
- steve-howard 14y ago
  Considering that tons and tons of people apply for programming jobs who don't know how to program at all, they're at least borderline useful. Additionally there are not many classes with C at all these days.
  - tjoff 14y ago
    Additionally there are not many classes with C at all these days. Depends on what you are studying. While I never had any specific "learn C" courses it is by far the most used language in other courses (OS programming, parallel programming, compiler construction, network programming, micro-controllers, graphics and game programming, security courses etc.). No other language comes close (a lot of the time a C++ compiler is used but the C subset is what ultimately has been used).
- danielvinson 14y ago
  I took a total of 2 C classes as part of my CS degree and never learned any of these...
  - jlgreco 14y ago
    If nothing else, you should be aware of the issues that using fixed size buffers with no bounds checking causes. So at least know enough to know not to use 'gets' and 'strcpy'.
- throwaway54-762 14y ago
  None of these are especially interesting C questions... Most rely on assumptions about the x86 architecture and GCC (stack grows downwards, ordering of stack variables in memory (not registers), ...). #3 notes a warning for main not returning int, but doesn't point out that without stdlib.h, malloc() and free() will be declared implicitly with incorrect types. (Edit: this produces a warning on 4.7 GCC and Clang 3.0.) Almost all of these questions have something wrong with them other than what the author is pointing out. I question the author's familiarity with C.
  - peteri 14y ago
    Also the casts to (char *) from malloc suggest that the author is a C++ programmer.
    - eswangren 14y ago
      I left the same comment before I saw yours. Whoever wrote this does not know C, they think they do because the use C++.
    - pfedor 14y ago
      There's a discussion in "The Practice of Programming" of which is a better style when programming in straight C. The argument for casting was that could make porting to C++ easier. The argument against it went like that (hope I'm not botching it): if the appropriate header file was not included and consequently malloc() was implicitly assumed to return an int, then without a cast assigning the return value of malloc to say char* would likely trigger a warning, but the cast will silence the warning. This could be a real bug if the convention for returning an int was different from returning a pointer (say if one was returned via a register and the other wasn't.)
      - peteri 14y ago
        Or if you were programming on an 8086 in large memory model where int is 16 bit and a pointer is 32 bit (16 bit segment, 16 bit offset) This makes me wonder if anyone ever did a compiler where malloc returned a segment + offset (I think that gives a 48 bit pointer) for a 386 class machine.
kstenerud 14y ago
Question 10 is a bad question. The host environment is under no obligation to provide you with pointers to writable memory in argv. Furthermore, this is veering off from C knowledge into C trivia. Question 12 is even worse, as the processing order is implementation defined. On my compiler, it prints out 60..40..60. C questions should be designed to show whether or not the candidate can write robust, professional quality code, not to test esoteric knowledge that even the interviewer can't get right. I was asked questions like these in an interview once. It was such a massive red flag to me that I went with another company (the final straw was arguing over the size of int, which he insisted was always 32 bits).
- genbattle 14y ago
  I think alot of these examples use esoteric or just plain "bad" C code just in order to create pits for programmers to fall into. In real life you would avoid writing code like this precisely because of these problems. The author even mentions for at least one of the examples that he had to disable or ignore compiler errors/warnings to run the code. Sure, it can be important to know what these problems are and how to identify them if you should have to read through some terrible code written by someone else. Still, I think the author goes a bit far; it should be more important that you hire a programmer who doesn't produce this sort of code in the first place. In the end I think there is no "best way" to technically test programmers; you're better off covering as many bases as possible with a bit of programming, a bit of documentation/explanation, some debugging, etc.
- eridius 14y ago
  Wow, question 12 is so bad that they completely ignore the C standard and make up rules (arguments evaluated left-to-right). There's no sequence point between function arguments.
- tedunangst 14y ago
  Further to point one, it's under no obligation to provide you with writable memory of the size you'd like. strncpy(argv[0], "NewName", 7); WTF does 7 come from, the source or the destination?
joezydeco 14y ago
Don't give me hints in the title of the question! That kind of wrecked the challenge.
jdoliner 14y ago
Well, Though the above code is not freeing up the memory allocated to ‘ptr’ but still this would not cause a memory leak as after the processing is done the program exits. Well this is always true, so I guess that means there's no such thing as memory leaks in C. Checkmate garbage collectors.
- omra 14y ago
  Generally modern operating systems free any memory left that the program is not using, but is still accessible. However, if you overwrite an old pointer, there is no way that the operating system could catch that. To illustrate the difference, I wrote two quick programs. This one has memory that the OS can recover: #include <stdlib.h> int main(void) { char *a = malloc(10); return 0; } This one has memory that the OS cannot recover: #include <stdlib.h> int main(void) { char *a = malloc(10); a = malloc(10); return 0; }
  - mikeash 14y ago
    So if you run your second example a billion times or so, your OS will run out of RAM? No, that's not how it works....
  - Jare 14y ago
    To get rid of the hard memory leak in the second example, just call free(a) twice before returning. They don't call it pointer magic for nothing! /sarcasm
  - humbledrone 14y ago
    The OS always recovers all of the memory from a process that has terminated (barring horrible kernel bugs), and it doesn't need to look through the process image for pointers that need to be freed. When a process allocates memory, the kernel doesn't give it a physical memory address. It gives it a virtual address, which, when accessed, will be mapped to some physical address. The kernel has to keep track of which virtual addresses map to which physical addresses for each process, and it stores these mappings in a data structure called a "page table". At all times, the kernel knows what memory segments are in use by a given process. So, when a process exits, the kernel can just look at its page table to see what memory had been allocated to it. Provided that no other processes shared the same memory, it can free it.
mikeash 14y ago
I wonder why there are so many articles about C getting passed around where the author just doesn't know the language that well, or at the very least ventures well outside the boundaries of his knowledge. Where are the good articles on C, and why don't they ever show up on HN?
- diggan 14y ago
  The question is why it's on 12th place on the index. Don't people read articles before they press the little arrow?
romac 14y ago
Here is another interesting C/Objective-C Q&A: http://www.eosgarden.com/en/articles/objc-quizz/ http://www.eosgarden.com/en/articles/objc-quizz/
eswangren 14y ago
The author apparently doesn't know that casting the return value of malloc in C is redundant and can actually hide an error. I stopped reading when I saw that, this was not written by someone who knows C very well.
JBionics 14y ago
For those of you interested, the first stage in the SpaceX interview process, at least for the flight software group, is a series of questions like these, with A-G multiple choice answers. You had to identify the class of bug. The G choice was always "No, looks good". It's timed.
lurker14 14y ago
Is there a reason for any modern compiler to support linking to functions like gets() without setting an "--use-unsafe-libraries" flag?

40 points by giZm0 14y ago

deleted 14y ago

[deleted]

mitjak 14y ago

I've studied most of these as part of my introductory C course in university. Are they really that interesting?

steve-howard 14y ago
Considering that tons and tons of people apply for programming jobs who don't know how to program at all, they're at least borderline useful. Additionally there are not many classes with C at all these days.
- tjoff 14y ago
  Additionally there are not many classes with C at all these days. Depends on what you are studying. While I never had any specific "learn C" courses it is by far the most used language in other courses (OS programming, parallel programming, compiler construction, network programming, micro-controllers, graphics and game programming, security courses etc.). No other language comes close (a lot of the time a C++ compiler is used but the C subset is what ultimately has been used).
danielvinson 14y ago
I took a total of 2 C classes as part of my CS degree and never learned any of these...
- jlgreco 14y ago
  If nothing else, you should be aware of the issues that using fixed size buffers with no bounds checking causes. So at least know enough to know not to use 'gets' and 'strcpy'.
throwaway54-762 14y ago
None of these are especially interesting C questions... Most rely on assumptions about the x86 architecture and GCC (stack grows downwards, ordering of stack variables in memory (not registers), ...). #3 notes a warning for main not returning int, but doesn't point out that without stdlib.h, malloc() and free() will be declared implicitly with incorrect types. (Edit: this produces a warning on 4.7 GCC and Clang 3.0.) Almost all of these questions have something wrong with them other than what the author is pointing out. I question the author's familiarity with C.
- peteri 14y ago
  Also the casts to (char *) from malloc suggest that the author is a C++ programmer.
  - eswangren 14y ago
    I left the same comment before I saw yours. Whoever wrote this does not know C, they think they do because the use C++.
  - pfedor 14y ago
    There's a discussion in "The Practice of Programming" of which is a better style when programming in straight C. The argument for casting was that could make porting to C++ easier. The argument against it went like that (hope I'm not botching it): if the appropriate header file was not included and consequently malloc() was implicitly assumed to return an int, then without a cast assigning the return value of malloc to say char* would likely trigger a warning, but the cast will silence the warning. This could be a real bug if the convention for returning an int was different from returning a pointer (say if one was returned via a register and the other wasn't.)
    - peteri 14y ago
      Or if you were programming on an 8086 in large memory model where int is 16 bit and a pointer is 32 bit (16 bit segment, 16 bit offset) This makes me wonder if anyone ever did a compiler where malloc returned a segment + offset (I think that gives a 48 bit pointer) for a 386 class machine.

kstenerud 14y ago

Question 10 is a bad question. The host environment is under no obligation to provide you with pointers to writable memory in argv. Furthermore, this is veering off from C knowledge into C trivia. Question 12 is even worse, as the processing order is implementation defined. On my compiler, it prints out 60..40..60. C questions should be designed to show whether or not the candidate can write robust, professional quality code, not to test esoteric knowledge that even the interviewer can't get right. I was asked questions like these in an interview once. It was such a massive red flag to me that I went with another company (the final straw was arguing over the size of int, which he insisted was always 32 bits).

genbattle 14y ago
I think alot of these examples use esoteric or just plain "bad" C code just in order to create pits for programmers to fall into. In real life you would avoid writing code like this precisely because of these problems. The author even mentions for at least one of the examples that he had to disable or ignore compiler errors/warnings to run the code. Sure, it can be important to know what these problems are and how to identify them if you should have to read through some terrible code written by someone else. Still, I think the author goes a bit far; it should be more important that you hire a programmer who doesn't produce this sort of code in the first place. In the end I think there is no "best way" to technically test programmers; you're better off covering as many bases as possible with a bit of programming, a bit of documentation/explanation, some debugging, etc.
eridius 14y ago
Wow, question 12 is so bad that they completely ignore the C standard and make up rules (arguments evaluated left-to-right). There's no sequence point between function arguments.
tedunangst 14y ago
Further to point one, it's under no obligation to provide you with writable memory of the size you'd like. strncpy(argv[0], "NewName", 7); WTF does 7 come from, the source or the destination?

joezydeco 14y ago

Don't give me hints in the title of the question! That kind of wrecked the challenge.

jdoliner 14y ago

Well, Though the above code is not freeing up the memory allocated to ‘ptr’ but still this would not cause a memory leak as after the processing is done the program exits. Well this is always true, so I guess that means there's no such thing as memory leaks in C. Checkmate garbage collectors.

omra 14y ago
Generally modern operating systems free any memory left that the program is not using, but is still accessible. However, if you overwrite an old pointer, there is no way that the operating system could catch that. To illustrate the difference, I wrote two quick programs. This one has memory that the OS can recover: #include <stdlib.h> int main(void) { char *a = malloc(10); return 0; } This one has memory that the OS cannot recover: #include <stdlib.h> int main(void) { char *a = malloc(10); a = malloc(10); return 0; }
- mikeash 14y ago
  So if you run your second example a billion times or so, your OS will run out of RAM? No, that's not how it works....
- Jare 14y ago
  To get rid of the hard memory leak in the second example, just call free(a) twice before returning. They don't call it pointer magic for nothing! /sarcasm
- humbledrone 14y ago
  The OS always recovers all of the memory from a process that has terminated (barring horrible kernel bugs), and it doesn't need to look through the process image for pointers that need to be freed. When a process allocates memory, the kernel doesn't give it a physical memory address. It gives it a virtual address, which, when accessed, will be mapped to some physical address. The kernel has to keep track of which virtual addresses map to which physical addresses for each process, and it stores these mappings in a data structure called a "page table". At all times, the kernel knows what memory segments are in use by a given process. So, when a process exits, the kernel can just look at its page table to see what memory had been allocated to it. Provided that no other processes shared the same memory, it can free it.

mikeash 14y ago

I wonder why there are so many articles about C getting passed around where the author just doesn't know the language that well, or at the very least ventures well outside the boundaries of his knowledge. Where are the good articles on C, and why don't they ever show up on HN?

diggan 14y ago
The question is why it's on 12th place on the index. Don't people read articles before they press the little arrow?

romac 14y ago

Here is another interesting C/Objective-C Q&A: http://www.eosgarden.com/en/articles/objc-quizz/ http://www.eosgarden.com/en/articles/objc-quizz/

eswangren 14y ago

The author apparently doesn't know that casting the return value of malloc in C is redundant and can actually hide an error. I stopped reading when I saw that, this was not written by someone who knows C very well.

JBionics 14y ago

For those of you interested, the first stage in the SpaceX interview process, at least for the flight software group, is a series of questions like these, with A-G multiple choice answers. You had to identify the class of bug. The G choice was always "No, looks good". It's timed.

lurker14 14y ago

Is there a reason for any modern compiler to support linking to functions like gets() without setting an "--use-unsafe-libraries" flag?