C puzzles [38-45]

38th questions

What is the bug in the following program?   #include <stdlib.h>  #include <stdio.h>  #define SIZE 15   int main()  {      int *a, i;      a = malloc(SIZE*sizeof(int));      for (i=0; i<SIZE; i++)          *(a + i) = i * i;      for (i=0; i<SIZE; i++)          printf("%d\n", *a++);      free(a);      return 0;  }

Description:

When printf prints the value of the stack, the value of a is changed. Therefore, the value of a in free (a) is not the starting address of the previously allocated stack.

39th questions

Is the following a valid C program? If so, what is the output of it?   #include <stdio.h>  int main()  {    int a=3, b = 5;    printf(&a["Ya!Hello! how is this? %s\n"], &b["junk/super"]);    printf(&a["WHAT%c%c%c  %c%c  %c !\n"], 1["this"],       2["beauty"],0["tool"],0["is"],3["sensitive"],4["CCCCCC"]);    return 0;  }

Knowledge points:

The most common expression of elements in an array is address [offset], such as a [0], a [1], and a [2].
There is also an uncommon representation: Offset [address], such as 0 [a], 1 [A], 2 [A].

Example:

#include <stdio.h>int main(){int a[] = {0, 1, 2};printf(“%d, %d, %d\n”, 0[a], 1[a], 2[a]);return 0;}

Running result: 0, 1, 2
Printf ("% d, % d, % d \ n", 0 [a], 1 [A], 2 [a]);
Equivalent
Printf ("% d, % d, % d \ n", a [0], a [1], a [2]);

Description:
Printf (& A ["ya! Hello! How is this? % S \ n "], & B [" junk/super "]);
Equivalent
Printf ("Hello! How is this? % S \ n "," super ");

Printf (& A ["What % C! \ N "], 1 [" this "], 2 [" beauty "], 0 [" tool "], 0 [" is "], 3 [" sensitive "], 4 ["cccccc"]);
Equivalent
Printf ("T % C! \ N ", 'h', 'A', 't', 'I', 's', 'C ');

40th questions

What is the output of the following, if the input provided is:Life is beautiful   #include <stdio.h>  int main()  {      char dummy[80];      printf("Enter a string:\n");      scanf("%[^a]",dummy);      printf("%s\n",dummy);      return 0;  }

Knowledge points:

Scanf format controller:

% [...]: Read the string until it encounters a character that is not in;

% [^...]: Read the string until it encounters a character in.

For example:

Scanf ("% [A-Z]", dummy); when "ABC123" is input, dummy is "ABC ";

Scanf ("% [^ A-Z]", dummy); when "123abc" is input, dummy is "123 ";

Description:

"% [^ A]" means to read the string until the character 'a' is encountered '. So when the input is "Life is beauul ul", dummy is "Life is be ".

 

41st questions

Note : This question has more to do with Linker than C language We have three files a.c, b.c and main.c respectively as follows: a.c--- int a;b.c--- int a = 10;main.c------ extern int a;int main(){        printf("a = %d\n",a);        return 0;}Let‘s see what happens, when the files are compiled together: bash$ gcc a.c b.c main.cbash$ ./a.outa = 10Hmm!! no compilation/linker error!!! Why is it so??

Knowledge points:

Refer to page 67th declaration and definition of c traps and defects.

If int A is defined in one file and int A is defined in the other file, the compiler will initialize a to 0;

If int A is defined in one file, int A = 10 is defined in the other file, and A is 10;

If int A = 1 is defined in one file and int A = 10 is defined in the other file, the compilation will produce a duplicate definition error.

42nd questions

The following is the offset macros which is used many a times. Figure out what is it trying to do and what is the advantage of using it.   #define offsetof(a,b) ((int)(&(((a*)(0))->b)))

Knowledge points:

Calculates the offset of the struct member variable in the struct. A is a struct type, and B is a member variable.

43rd questions

The following is the macro implementation of the famous, Triple xor swap.   #define SWAP(a,b) ((a) ^= (b) ^= (a) ^= (b))What are the potential problems with the above macro?

Knowledge points:

For more information, see http://www.cnblogs.com/tanghuimin0713/p/3220665.html.

This method has the following limitations:

1) A and B cannot be the same variables. That is, if swap (A, A) is executed, the value is set to 0 no matter what the original a value is;

2) A and B cannot be floating-point numbers. The XOR operation has no significance for floating-point numbers;

3) A and B cannot be Composite data types such as, for the same reason as above;

4) A or B cannot be expressions;

44th questions

What is the use of the following macro?   #define DPRINTF(x) printf("%s:%d\n",#x,x)

Description:

Print the value of X.

For example, if a = 10, the result of dprintf (a) is "A: 10".

45th questions

Let‘s say you were asked to code a function IAddOverFlow which takes three parameters, pointer to an integer where the result is to be stored, and the two integers which needs to be added. It returns 0 if there is an overflow and 1 otherwise:   int IAddOverFlow(int* result,int a,int b)  {      /* ... */  }So, how do you code the above function? (To put in a nutshell, what is the logic you use for overflow detection?)

Description:

Reference for methods of detecting overflow: http://www.fefe.de/intof.html

C puzzles [38-45]