C Programming Puzzlers
These questions originally appeared as an article on programmersheaven.com, written by Ashok K. Pathak, a researcher at Bharat Electronics Limited (CRL), Ghaziabad. They are reproduced here with minor modifications.
The questions test advanced knowledge of the C language, including some rarely-used features. Effective C programming requires a strong understanding of concepts like undefined behavior, recursion, and pointer arithmetic, but the deliberately convoluted examples on this page are not representative of real-world code, and certainly won't win any prizes for clarity and maintainability.
Performance on these questions is not a good indicator of broader competence in software development. As such, they are unlikely to be useful in an interview setting.
Steve Kobes, 8/25/04 (amended 6/19/11)
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1. Consider the following program:
#include <stdio.h>
#include <setjmp.h>
static jmp_buf buf;
int main(void){
volatile int b = 3;
if (setjmp(buf) != 0)
{
printf("%d\n", b);
exit(0);
}
b = 5;
longjmp(buf, 1);
}
What is the output of this program?
(a)
3
(b) 5
(c)
0
(d)
none of the above
Show answer
2. Consider the following program:
#include <stdio.h>
int main(void){
struct node
{
int a;
int b;
int c;
};
struct node s = { 3, 5, 6 };
struct node *pt = &s;
printf("%d\n", *(int*)pt);
return 0;
}
What is the output of this program?
(a) 3
(b)
5
(c)
6
(d)
7
Show answer
3. Consider the following code segment:
int foo(int x, int n){
int val = 1;
if (n > 0)
{
if (n % 2 == 1)
val *= x;
val *= foo(x * x, n / 2);
}
return val;
}
What function of x and n is computed by foo?
(a) xn
(b)
x
×
n
(c)
nx
(d)
none of the above
Show answer
4. Consider the following program:
#include <stdio.h>
int main(void){
int a[5] = { 1, 2, 3, 4, 5 };
int *ptr = (int*)(&a + 1);
printf("%d %d\n", *(a + 1), *(ptr - 1));
return 0;
}
What is the output of this program?
(a)
2 2
(b)
2 1
(c) 2 5
(d)
none of the above
Show answer
5. Consider the following program:
#include <stdio.h>
void foo(int[][3]);
int main(void){
int a[3][3] = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} };
foo(a);
printf("%d\n", a[2][1]);
return 0;
}
void foo(int b[][3]){
++b;
b[1][1] = 9;
}
What is the output of this program?
(a)
8
(b) 9
(c)
7
(d)
none of the above
Show answer
6. Consider the following program:
#include <stdio.h>
int main(void){
int a, b, c, d;
a = 3;
b = 5;
c = a, b;
d = (a, b);
printf("c=%d ", c);
printf("d=%d\n", d);
return 0;
}
What is the output of this program?
(a)
c=3 d=3
(b)
c=5 d=3
(c) c=3 d=5
(d)
c=5 d=5
Show answer
7. Consider the following program:
#include <stdio.h>
int main(void){
int a[][3] = {1, 2, 3, 4, 5, 6};
int (*ptr)[3] = a;
printf("%d %d ", (*ptr)[1], (*ptr)[2]);
++ptr;
printf("%d %d\n", (*ptr)[1], (*ptr)[2]);
return 0;
}
What is the output of this program?
(a) 2 3 5 6
(b)
2 3 4 5
(c)
4 5 0 0
(d)
none of the above
Show answer
8. Consider the following code segment:
#include <stdlib.h>
int *f1(void){
int x = 10;
return &x;
}
int *f2(void){
int *ptr;
*ptr = 10;
return ptr;
}
int *f3(void){
int *ptr;
ptr = malloc(sizeof *ptr);
return ptr;
}
Which of these functions uses pointers incorrectly?
(a)
f3
only
(b)
f1
and
f3
(c) f1 and f2
(d)
f1
,
f2
, and
f3
Show answer
9. Consider the following program:
#include <stdio.h>
int main(void){
int i = 3;
int j;
j = sizeof(++i + ++i);
printf("i=%d j=%d\n", i, j);
return 0;
}
What is the output of this program on an implementation where int occupies 2 bytes?
(a)
i=4 j=2
(b) i=3 j=2
(c)
i=5 j=2
(d)
the behavior is undefined
Show answer
10. Consider the following program:
#include <stdio.h>
void f1(int*, int);
void f2(int*, int);
void (*p[2])(int*, int);
int main(void){
int a = 3;
int b = 5;
p[0] = f1;
p[1] = f2;
p[0](&a, b);
printf("%d %d ", a, b);
p[1](&a, b);
printf("%d %d\n", a, b);
return 0;
}
void f1(int *p, int q){
int tmp = *p;
*p = q;
q = tmp;
}
void f2(int *p, int q){
int tmp = *p;
*p = q;
q = tmp;
}
What is the output of this program?
(a) 5 5 5 5
(b)
3 5 3 5
(c)
5 3 3 5
(d)
none of the above
Show answer
11. Consider the following program:
#include <stdio.h>
void e(int);
int main(void){
int a = 3;
e(a);
putchar('\n');
return 0;
}
void e(int n){
if (n > 0)
{
e(--n);
printf("%d ", n);
e(--n);
}
}
What is the output of this program?
(a) 0 1 2 0
(b)
0 1 2 1
(c)
1 2 0 1
(d)
0 2 1 1
Show answer
12. Consider the following code segment:
typedef int (*test)(float*, float*);
test tmp;
What is the type of tmp?
(a)
function taking two pointer-to-
float
arguments and returning pointer to
int
(b)
pointer to
int
(c) pointer to function taking two pointer-to-float arguments and returning int(d)
none of the above
Show answer
13. Consider the following program:
#include <stdio.h>
int main(void){
char p;
char buf[10] = {1, 2, 3, 4, 5, 6, 9, 8};
p = (buf + 1)[5];
printf("%d\n", p);
return 0;
}
What is the output of this program?
(a)
5
(b)
6
(c) 9
(d)
none of the above
Show answer
14. Consider the following program:
#include <stdio.h>
void f(char**);
int main(void){
char *argv[] = { "ab", "cd", "ef", "gh", "ij", "kl" };
f(argv);
return 0;
}
void f(char **p){
char *t;
t = (p += sizeof(int))[-1];
printf("%s\n", t);
}
What is the output of this program on an implementation where int and all pointer types occupy 2 bytes?
(a)
ab
(b)
cd
(c)
ef
(d) gh
Show answer
15. Consider the following program:
#include <stdarg.h>
#include <stdio.h>
int ripple(int n, ...){
int i, j, k;
va_list p;
k = 0;
j = 1;
va_start(p, n);
for (; j < n; ++j)
{
i = va_arg(p, int);
for (; i; i &= i - 1)
++k;
}
va_end(p);
return k;
}
int main(void){
printf("%d\n", ripple(3, 5, 7));
return 0;
}
What is the output of this program?
(a)
7
(b)
6
(c) 5
(d)
3
Show answer
16. Consider the following program:
#include <stdio.h>
int counter(int i){
static int count = 0;
count = count + i;
return count;
}
int main(void){
int i, j;
for (i = 0; i <= 5; i++)
j = counter(i);
printf("%d\n", j);
return 0;
}
What is the output of this program?
(a)
10
(b) 15
(c)
6
(d)
7