C Aptitude Questions with Answers Part 31

1)

#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 < 100);  
  else 
    printf("This statement becomes else for if in assert macro"); 
} 

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()) \ 
} 
 

2) 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.

3) 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.

4) Is the following code legal?

typedef struct a { 
   int x; 
   aType *b; 
 }aType; 

Answer:
No
Explanation:
The typename aType is not known at the point of declaring the structure (forward references are not made for typedefs).

5) Is the following code legal?

typedef struct a aType; 
struct a { 
   int x; 
   aType *b; 
}; 

Answer:
Yes
Explanation:
The typename aType is known at the point of declaring the structure, because it is already typedefined.

6) Is the following code legal?

void main() {
   typedef struct a aType; 
   aType someVariable; 
  struct a { 
      int x; 
      aType *b; 
   }; 
} 

Answer:
No
Explanation:
  When the declaration, typedef struct a aType; is encountered body of struct a is not known. This is known as ‘incomplete types’.

7)

void main() {
    printf(“sizeof (void *) = %d \n“, sizeof( void *));
    printf(“sizeof (int *)    = %d \n”, sizeof(int *)); 
    printf(“sizeof (double *)  = %d \n”, sizeof(double *)); 
    printf(“sizeof(struct unknown *) = %d \n”, sizeof(struct unknown *)); 
 } 

Answer :
sizeof (void *) = 2
sizeof (int *) = 2
sizeof (double *) = 2
sizeof(struct unknown *) = 2
Explanation:
The pointer to any type is of same size.

8)

char inputString[100] = {0};

To get string input from the keyboard which one of the following is better?
1) gets(inputString)
2) fgets(inputString, sizeof(inputString), fp)

Answer & Explanation:
The second one is better because gets(inputString) doesn't know the size of the string passed and so, if a very big input (here, more than 100 chars) the charactes will be written past the input string. When fgets is used with stdin performs the same operation as gets but is safe.

9) Which version do you prefer of the following two,
1) printf(“%s”,str); // or the more curt one
2) printf(str);

Answer & Explanation:
Prefer the first one. If the str contains any format characters like %d then it will result in a subtle bug.

10)

void main() {
   int i=10, j=2; 
   int *ip= &i, *jp = &j; 
   int k = *ip/*jp; 
   printf(“%d”,k); 
}   
 

Answer:
Compiler Error: “Unexpected end of file in comment started in line 5”.
Explanation:
The programmer intended to divide two integers, but by the “maximum munch” rule, the compiler treats the operator sequence / and * as /* which happens to be the starting of comment. To force what is intended by the programmer,

int k = *ip/ *jp;   
// give space explicity separating / and *  
//or 
int k = *ip/(*jp); 
// put braces to force the intention 

will solve the problem.