Member Avatar for Dwija

Hello
Members/Administrator

I urgently needed help for the following questions as
i have very little time in my hand for preparing the interview.

->Why doesn't C have nested functions?
->What is the most efficient way to count the number of bits which are set in a value?
->How can I call a function, given its name as a string?
->How can I return multiple values from a function?
->How can I invoke another program from within a C program?
->How can I access memory located at a certain address?
->How can I allocate arrays or structures bigger than 64K?
->How can I find out how much memory is available?
->How can I read a directory in a C program?
->How can I increase the allowable number of simultaneously open files?
->What's wrong with the call "fopen("c:\newdir\file.dat", "r")"?


its
"dwija"

  • 6 Contributors
  • 10 Replies
  • 264 Views
  • 2 Months Discussion Span
  • Latest Post Latest Post by neoderf

Dani AI

Generated

Short, interview‑ready clarifications and corrections to Dwija’s list — synthesizes points raised by and others and adds compact examples and caveats.

Standard C and nested functions

  • ISO C has no syntax for defining a function inside another because nested definitions require capturing an outer function’s stack frame (closures) and complicate calling conventions and lifetime rules. Some compilers (GCC) offer nested functions as a nonstandard extension; mention non‑portability in an interview and prefer file‑scope static helpers, function pointers, or move to a language with closures when closures are required.

Calling by name at runtime

  • Turning an identifier into a string with the preprocessor does not call the function. Two practical approaches: build a dispatch table that maps names to function pointers, or perform dynamic symbol lookup for shared libraries (dlsym/GetProcAddress). Example dispatch table:
typedef void (*fn_t)(void);
struct { const char *name; fn_t fn; } tbl[] = {
  { "hello", hello },
  { NULL, NULL }
};
/* lookup: strcmp(tbl[i].name, query) and call tbl[i].fn(); */

Counting bits efficiently

  • Best: use compiler intrinsics that map to CPU POPCNT (e.g. __builtin_popcount on GCC/Clang). Portable fallback: Kernighan’s algorithm — repeatedly clear the lowset bit until zero:
unsigned int popcount(unsigned int v){
  unsigned int c = 0;
  while (v) { v &= v - 1; c++; }
  return c;
}

Quick fire corrections & caveats

  • Multiple returns: use out‑parameters (pointers) or return a struct.
  • Invoke programs: prefer fork+exec (Unix) or CreateProcess (Windows); system() is simple but has security/shell issues.
  • Read directory: POSIX opendir/readdir or Windows FindFirstFile family.
  • 64K limit: a 64K cap is a legacy 16‑bit issue; on modern 32/64‑bit systems use heap allocation (malloc) or OS APIs.
  • Accessing an arbitrary address: cast and dereference only when the address is valid for the process (use volatile for MMIO; mmap or kernel code for physical memory).
  • fopen("c:\newdir\file.dat", "r") fails because backslashes are escapes; use "c:\\newdir\\file.dat" or "c:/newdir/file.dat".

Where pointed in the thread is directionally useful but watch portability and safety when using OS‑specific APIs.

Recommended Answers

All 10 Replies

Member Avatar for infamous

1) b/c C is a minimal, spartan like language
2) shift each bit out and test it:

unsigned int num = 0xdeadbeef;
 for(int x = 0; x < (sizeof(int) * 8); x++)
 	if(num & (1 << x) ) printf("bit %d is on\n", x);

3)using the `stringification' operator in a preprocessor macro:
#define(func2call) #func2call()
4)by passing some arguments by reference, or by returning a structure packed full of stuff
5)by using execve() or CreateProcess()
6)by dereferencing a p ointer to that address
7)the only limit on dynamically allocated memory is the size of ur RAM i believe
8)see above
9)by using the readdir() function
9)by suing the setrlimit() function in linux
10)looks fine to me

Member Avatar for BountyX

I disagree C does have nested functions...recursion is pretty nested to me.

Member Avatar for infamous

that's not what 'nested' functions means. nested functions means being able to define a function inside another. and some compilers allow and others dont, so it's not something u can rely on.

Member Avatar for alc6379

ummm... are we sure this is an interview? This sounds more like a homework assignment to me... Who's doing the interviewing?

Member Avatar for BountyX

that's not what 'nested' functions means. nested functions means being able to define a function inside another. and some compilers allow and others dont, so it's not something u can rely on.

oh, i see. Sounds like java, main is inside another class method, is that a correct analogy?

maybe the interview is a homework assignment :lol:

Member Avatar for infamous

exactly.

Member Avatar for samaru

Infamous, you have better things to do than waste your time doing someone else's homework.

Member Avatar for infamous

what am i, psychic? i didnt know if it was homework or not, i was just trying to clear up some of the old posts; worry about yourself. that post was almost 3 months old anyhow, dont think it helped him much if it was homework.

Member Avatar for BountyX

well it helped me at least, now I know what a nested function is :D

Member Avatar for neoderf

well dicounting the other questions
lets drop the how and the way you solve the can i part is

yes!!!!!

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