第三十八题

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=; i<SIZE; i++)
*(a + i) = i * i;
for (i=; i<SIZE; i++)
printf("%d\n", *a++);
free(a);
return ;
}
题目讲解:
printf打印栈中值时,a的值被改变,所以最后free(a)中a的值并不是之前分配的栈的起始地址。

第三十九题

Is the following a valid C program? If so, what is the output of it?
#include <stdio.h>
int main()
{
int a=, b = ; printf(&a["Ya!Hello! how is this? %s\n"], &b["junk/super"]);
printf(&a["WHAT%c%c%c %c%c %c !\n"], ["this"],
["beauty"],["tool"],["is"],["sensitive"],["CCCCCC"]);
return ;
}
知识点讲解:
对于数组中的元素,最常见的表示方法是:地址[偏移],如a[0],a[1],a[2]。
还有一种不常见的表示方法是:偏移[地址],如0[a],1[a],2[a]。
举例:
#include <stdio.h>
int main()
{
int a[] = {, , };
printf(“%d, %d, %d\n”, [a], [a], [a]);
return ;
}
运行结果为:0, 1, 2
printf(“%d, %d, %d\n”, 0[a], 1[a], 2[a]);
等效于
printf(“%d, %d, %d\n”, a[0], a[1], a[2]); 题目讲解:
printf(&a["Ya!Hello! how is this? %s\n"], &b["junk/super"]);
等效于
printf("Hello! how is this? %s\n", "super"); printf(&a["WHAT%c%c%c  %c%c  %c !\n"], 1["this"],
2["beauty"],0["tool"],0["is"],3["sensitive"],4["CCCCCC"]);
等效于
printf("T%c%c%c  %c%c  %c !\n", ‘h’,‘a’,’t’,’i’,’s’,’C’);

第四十题

What is the output of the following, if the input provided is:
Life is beautiful
#include <stdio.h>
int main()
{
char dummy[];
printf("Enter a string:\n");
scanf("%[^a]",dummy);
printf("%s\n",dummy);
return ;
}
知识点讲解:
scanf格式控制符:
%[...]:读取字符串,直到遇到不是[]中的字符;
%[^...]:读取字符串,直到遇到[]中的字符。
如:
scanf("%[a-z]",dummy);输入为”abc123”时,dummy为”abc”;
scanf("%[^a-z]",dummy);输入为”123abc”时,dummy为”123”;
题目讲解:
“%[^a]”表示读取字符串,直到遇到字符’a’。所以当输入为”Life is beautiful”时,dummy为”Life is be”。
 

第四十一题

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 = ;
main.c
------
extern int a;
int main()
{
printf("a = %d\n",a);
return ;
}
Let's see what happens, when the files are compiled together:
bash$ gcc a.c b.c main.c
bash$ ./a.out
a =
Hmm!! no compilation/linker error!!! Why is it so??
知识点讲解:
参考《C陷阱与缺陷》第67页 4.2 申明与定义。
若一个文件中定义int a,另一个文件中定义int a,编译不会有问题,编译器将a初始化为0;
若一个文件中定义int a,另一个文件中定义int a=10,a的值为10;
若一个文件中定义int a=1,另一个文件中定义int a=10,编译会有重复定义的错误。

第四十二题

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)))
知识点讲解:
计算结构体成员变量在结构体中的偏移。a为结构体类型,b为成员变量。

第四十三题

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?
知识点讲解:
参考博文:http://www.cnblogs.com/tanghuimin0713/p/3220665.html的评论。
此方法有如下局限:
1)a和b不能是同一个变量,即如果执行SWAP(a, a)那么不管原来a值是多少,执行后a值被置为0; 
2)a和b不能是浮点数,异或操作对浮点数没有意义; 
3)a和b不能是结体体等复合数据类型,原因同上; 
4)a或b不能是表达式;

第四十四题

What is the use of the following macro?
#define DPRINTF(x) printf("%s:%d\n",#x,x)
题目讲解:
打印x的值。
如a=10,DPRINTF(a)的结果为“a:10“。

第四十五题

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?)
题目讲解:
检测溢出的方法参考:http://www.fefe.de/intof.html
04-23 07:01