C PUZZLE

• Declare an array of N pointers to functions returning pointers to functions returning pointers to characters?

Answer:

            (char*(*)( )) (*ptr[N])( );

• main(){

struct student {

char name[30];

struct date dob;

}stud;

struct date{     

int day,month,year;

};

scanf("%s%d%d%d", stud.rollno, &student.dob.day, &student.dob.month,      &student.dob.year);

}

Answer:

Compiler Error: Undefined structure date

Explanation:

Inside the struct definition of ‘student’ the member of type struct date is given. The compiler doesn’t have the definition of date structure (forward  reference is not allowed in C in this case) so it issues an error.

• main(){

struct date;

struct student{

char name[30];

struct date dob;

}stud;

struct date{

         int day,month,year;

 };

scanf("%s%d%d%d", stud.rollno, &student.dob.day, &student.dob.month, &student.dob.year);

}

Answer:

Compiler Error: Undefined structure date

Explanation:

Only declaration of struct date is available inside the structure definition of ‘student’ but to have a variable of type struct date the definition of the structure is required.

• There were 10 records stored in “somefile.dat” but the following program printed 11 names. What went wrong?

void main(){

struct student{  

char name[30], rollno[6];

}stud;     

FILE *fp = fopen(“somefile.dat”,”r”);

while(!feof(fp)) {

                  fread(&stud, sizeof(stud), 1 , fp);

puts(stud.name);

}

}

Explanation:

fread reads 10 records and prints the names successfully. It will return EOF only when fread tries to read another record and fails reading EOF (and returning EOF). So it prints the last record again. After this only the condition feof(fp) becomes false, hence comes out of the while loop.

• Is there any difference between the two declarations,

int foo(int *arr[]) and

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.

• What is the subtle error in the following code segment?

void fun(int n, int arr[]){

int *p=0;

int i=0;

while(i++<n)

            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).    

• 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.

• 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. 

• 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'.