C programming Interview Questions and Answers

Predict the output or error(s) for the following:

81. Is there any difference between the two declarations, 
1.      int foo(int *arr[]) and
2.      int foo(int *arr[2])

Answer:
No 

Explanation:
Functions can only pass pointers and not arrays. The numbers that are allowed inside the [] is just for more readability. So there is no difference between the two declarations.

82.  What is the subtle error in the following code segment?
void fun(int n, int arr[])
{
int *p=0;
int i=0;
while(i++
                        p = &arr[i];
*p = 0;
}

Answer & Explanation:
If the body of the loop never executes p is assigned no address. So p remains NULL where *p =0 may result in problem (may rise to runtime error “NULL pointer assignment” and terminate the program).    

83.  What is wrong with the following code?  
int *foo()
{
int *s = malloc(sizeof(int)100);
assert(s != NULL);
return s;
}

Answer & Explanation:
assert macro should be used for debugging and finding out bugs. The check s != NULL is for error/exception handling and for that assert shouldn’t be used. A plain if and the corresponding remedy statement has to be given.

84.  What is the hidden bug with the following  statement?
assert(val++ != 0);

Answer & Explanation:
Assert macro is used for debugging and removed in release version. In assert, the experssion involves side-effects. So the behavior of the code becomes different in case of debug version and the release version thus leading to a subtle bug. 
Rule to Remember:
Don’t use expressions that have side-effects in assert statements. 

85.    void main()
{
int *i = 0x400;  // i points to the address 400
*i = 0;              // set the value of memory location pointed by i;
}

Answer:
Undefined behavior 

Explanation:
The second statement results in undefined behavior because it points to some location whose value may not be available for modification.  This type of pointer in which the non-availability of the implementation of the referenced location is known as 'incomplete type'.

86.     #define assert(cond) if(!(cond)) \
  (fprintf(stderr, "assertion failed: %s, file %s, line %d \n",#cond,\
 __FILE__,__LINE__), abort())
 
void main()
{
int i = 10;
if(i==0)            
    assert(i <>}

Answer:
No output

Explanation:
The else part in which the printf is there becomes the else for if in the assert macro. Hence nothing is printed. 
The solution is to use conditional operator instead of if statement,
#define assert(cond) ((cond)?(0): (fprintf (stderr, "assertion failed: \ %s, file %s, line %d \n",#cond, __FILE__,__LINE__), abort()))
 
Note:
However this problem of “matching with nearest else” cannot be solved by the usual method of placing the if statement inside a block like this,
#define assert(cond) { \
if(!(cond)) \
  (fprintf(stderr, "assertion failed: %s, file %s, line %d \n",#cond,\
 __FILE__,__LINE__), abort()) \
}

87.    Is the following code legal?
struct a
    { 
int x;
 struct a b;
    }

Answer:
No

Explanation:
Is it not legal for a structure to contain a member that is of the same
type as in this case. Because this will cause the structure declaration to be recursive without end.

88. Is the following code legal?
struct a
    { 
int x;
            struct a *b;
    }

Answer:
Yes.

Explanation:
*b is a pointer to type struct a and so is legal. The compiler knows, the size of the pointer to a structure even before the size of the structure
is determined(as you know the pointer to any type is of same size). This type of structures is known as ‘self-referencing’ structure.