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C Interview Questions
Note : All the programs are tested under Turbo C/C++ compilers.
It is assumed that,

Programs run under DOS environment,

The underlying machine is an x86 system,

Program is compiled using Turbo C/C++ compiler.
The program output may depend on the information based on this
assumptions (for example sizeof(int) == 2 may be assumed).
Predict the output or error(s) for the following:
1. void main()
{
int const * p=5;
printf("%d",++(*p));
}
Answer:
Compiler error: Cannot modify a constant value.
Explanation:
p is a pointer to a "constant integer". But we tried to change the
value of the "constant integer".
2. main()
{
char s[ ]="man";
int i;
for(i=0;s[ i ];i++)
printf("\n%c%c%c%c",s[ i ],*(s+i),*(i+s),i[s]);
}
Answer:
mmmm
aaaa
nnnn
Explanation:
s[i], *(i+s), *(s+i), i[s] are all different ways of expressing the
same idea. Generally array name is the base address for that array. Here s is
the base address. i is the index number/displacement from the base address. So,
indirecting it with * is same as s[i]. i[s] may be surprising. But in the case of C
it is same as s[i].
3. main()
{
float me = 1.1;
double you = 1.1;
if(me==you)
printf("I love U");
else
printf("I hate U");
}
Answer:
I hate U
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Explanation:
For floating point numbers (float, double, long double) the values
cannot be predicted exactly. Depending on the number of bytes, the precession
with of the value represented varies. Float takes 4 bytes and long double takes
10 bytes. So float stores 0.9 with less precision than long double.
Rule of Thumb:
Never compare or at-least be cautious when using floating point
numbers with relational operators (== , >, <, <=, >=,!= ) .
4. main()
{
static int var = 5;
printf("%d ",var--);
if(var)
main();
}
Answer:
5 4 3 2 1
Explanation:
When static storage class is given, it is initialized once. The change
in the value of a static variable is retained even between the function calls. Main
is also treated like any other ordinary function, which can be called recursively.
5. main()
{
int c[ ]={2.8,3.4,4,6.7,5};
int j,*p=c,*q=c;
for(j=0;j<5;j++) {
printf(" %d ",*c);
++q; }
for(j=0;j<5;j++){
printf(" %d ",*p);
++p; }
}
Answer:
2 2 2 2 2 2 3 4 6 5
Explanation:
Initially pointer c is assigned to both p and q. In the first loop,
since only q is incremented and not c , the value 2 will be printed 5 times. In
second loop p itself is incremented. So the values 2 3 4 6 5 will be printed.
6. main()
{
extern int i;
i=20;
printf("%d",i);
}
Answer:
Linker Error : Undefined symbol '_i'
Explanation:
extern storage class in the following declaration,
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extern int i;
specifies to the compiler that the memory for i is allocated in some other
program and that address will be given to the current program at the time of
linking. But linker finds that no other variable of name i is available in any other
program with memory space allocated for it. Hence a linker error has occurred .
7. main()
{
int i=-1,j=-1,k=0,l=2,m;
m=i++&&j++&&k++||l++;
printf("%d %d %d %d %d",i,j,k,l,m);
}
Answer:
0 0 1 3 1
Explanation :
Logical operations always give a result of 1 or 0 . And also the
logical AND (&&) operator has higher priority over the logical OR (||) operator. So
the expression ?i++ && j++ && k++? is executed first. The result of this
expression is 0 (-1 && -1 && 0 = 0). Now the expression is 0 || 2 which
evaluates to 1 (because OR operator always gives 1 except for ?0 || 0?
combination- for which it gives 0). So the value of m is 1. The values of other
variables are also incremented by 1.
8. main()
{
char *p;
printf("%d %d ",sizeof(*p),sizeof(p));
}
Answer:
1 2
Explanation:
The sizeof() operator gives the number of bytes taken by its
operand. P is a character pointer, which needs one byte for storing its value (a
character). Hence sizeof(*p) gives a value of 1. Since it needs two bytes to store
the address of the character pointer sizeof(p) gives 2.
9. main()
{
int i=3;
switch(i)
{
default:printf("zero");
case 1: printf("one");
break;
case 2:printf("two");
break;
case 3: printf("three");
break;
}
}
Answer :
three
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Explanation :
The default case can be placed anywhere inside the loop. It is
executed only when all other cases doesn't match.