看到了這個網址http://www.gowrikumar.com/c/,作者收集了一系列網上和自己所遇到的C語言有趣的東西,我感覺挺像<Java解惑>的風格,我下面就嘗試解決一下作者提出的問題.
第一題
The expected output of the following C program is to print the elements in the array. But when actually run, it doesn't do so.
#include<stdio.h>
#define TOTAL_ELEMENTS (sizeof(array) / sizeof(array[0]))
int array[] = {23,34,12,17,204,99,16};
int main()
{
int d;
for(d=-1;d <= (TOTAL_ELEMENTS-2);d++)
printf("%d\n",array[d+1]);
return 0;
}
Find out what's going wrong.第二題:
I thought the following program was a perfect C program. But on compiling, I found a silly mistake. Can you find it out (without compiling the program :-) ?
#include<stdio.h>
void OS_Solaris_print()
{
printf("Solaris - Sun Microsystems\n");
}
void OS_Windows_print()
{
printf("Windows - Microsoft\n");
}
void OS_HP-UX_print()
{
printf("HP-UX - Hewlett Packard\n");
}
int main()
{
int num;
printf("Enter the number (1-3):\n");
scanf("%d",&num);
switch(num)
{
case 1:
OS_Solaris_print();
break;
case 2:
OS_Windows_print();
break;
case 3:
OS_HP-UX_print();
break;
default:
printf("Hmm! only 1-3 :-)\n");
break;
}
return 0;
}
第三道題:
What's the expected output for the following program and why?
enum {false,true};
int main()
{
int i=1;
do
{
printf("%d\n",i);
i++;
if(i < 15)
continue;
}while(false);
return 0;
}
第四道題:
The following program doesn't "seem" to print "hello-out". (Try executing it)
#include <stdio.h>
#include <unistd.h>
int main()
{
while(1)
{
fprintf(stdout,"hello-out");
fprintf(stderr,"hello-err");
sleep(1);
}
return 0;
}
What could be the reason?第五道題:
#include <stdio.h>
#define f(a,b) a##b
#define g(a) #a
#define h(a) g(a)
int main()
{
printf("%s\n",h(f(1,2)));
printf("%s\n",g(f(1,2)));
return 0;
}
Just by looking at the program one "might" expect the output to be, the same for both the printf statements. But on running the program you get it as:
bash$ ./a.out
12
f(1,2)
bash$
Why is it so?第六道題:
#include<stdio.h>
int main()
{
int a=10;
switch(a)
{
case '1':
printf("ONE\n");
break;
case '2':
printf("TWO\n");
break;
defa1ut:
printf("NONE\n");
}
return 0;
}
If you expect the output of the above program to be NONE, I would request you to check it out!!第七道題:
The following C program segfaults of IA-64, but works fine on IA-32.
int main()
{
int* p;
p = (int*)malloc(sizeof(int));
*p = 10;
return 0;
}
Why does it happen so?第八道題:
Here is a small piece of program(again just 14 lines of program) which counts the number of bits set in a number.
Input Output
0 0(0000000)
5 2(0000101)
7 3(0000111)
int CountBits (unsigned int x )
{
static unsigned int mask[] = { 0x55555555,
0x33333333,
0x0F0F0F0F,
0x00FF00FF,
0x0000FFFF
} ;
int i ;
int shift ; /* Number of positions to shift to right*/
for ( i =0, shift =1; i < 5; i ++, shift *= 2)
x = (x & mask[i ])+ ( ( x >> shift) & mask[i]);
return x;
}
Find out the logic used in the above program. 第九道題:
What do you think would be the output of the following program and why? (If you are about to say "f is 1.0", I would say check it out again)
#include <stdio.h>
int main()
{
float f=0.0f;
int i;
for(i=0;i<10;i++)
f = f + 0.1f;
if(f == 1.0f)
printf("f is 1.0 \n");
else
printf("f is NOT 1.0\n");
return 0;
}
第十道題:
I thought the following C program is perfectly valid (after reading about the comma operator in C). But there is a mistake in the following program, can you identify it?
#include <stdio.h>
int main()
{
int a = 1,2;
printf("a : %d\n",a);
return 0;
}
第十一道題:
What would be the output of the following C program? (Is it a valid C program?)
#include <stdio.h>
int main()
{
int i=43;
printf("%d\n",printf("%d",printf("%d",i)));
return 0;
}
第十二道題:
void duff(register char *to, register char *from, register int count)
{
register int n=(count+7)/8;
switch(count%8){
case 0: do{ *to++ = *from++;
case 7: *to++ = *from++;
case 6: *to++ = *from++;
case 5: *to++ = *from++;
case 4: *to++ = *from++;
case 3: *to++ = *from++;
case 2: *to++ = *from++;
case 1: *to++ = *from++;
}while( --n >0);
}
}
Is the above valid C code? If so, what is it trying to acheive and why would anyone do something like the above? 第十三道題:
Here is yet another implementation of CountBits. Verify whether it is correct (how do you that???). If so, find out the logic used.
int CountBits(unsigned int x)
{
int count=0;
while(x)
{
count++;
x = x&(x-1);
}
return count;
}第十四道題:
Are the following two function prototypes same?
int foobar(void);
int foobar();
The following programs should be of some help in finding the answer: (Compile and run both the programs and see what happens)
Program 1:
#include <stdio.h>
void foobar1(void)
{
printf("In foobar1\n");
}
void foobar2()
{
printf("In foobar2\n");
}
int main()
{
char ch = 'a';
foobar1();
foobar2(33, ch);
return 0;
}
Program 2:
#include <stdio.h>
void foobar1(void)
{
printf("In foobar1\n");
}
void foobar2()
{
printf("In foobar2\n");
}
int main()
{
char ch = 'a';
foobar1(33, ch);
foobar2();
return 0;
}第十五道題:
What's the output of the following program and why?
#include <stdio.h>
int main()
{
float a = 12.5;
printf("%d\n", a);
printf("%d\n", *(int *)&a);
return 0;
}第十六道題:
The following is a small C program split across files. What do you expect the output to be, when both of them compiled together and run?
File1.c
int arr[80];
File2.c
extern int *arr;
int main()
{
arr[1] = 100;
return 0;
}第十七道:
Explain the output of the following C program (No, the output is not 20).
#include<stdio.h>
int main()
{
int a=1;
switch(a)
{ int b=20;
case 1: printf("b is %d\n",b);
break;
default:printf("b is %d\n",b);
break;
}
return 0;
}第十八道:
What is the output of the following program? (Again, it is not 40, (if the size of integer is 4)).
#define SIZE 10
void size(int arr[SIZE])
{
printf("size of array is:%d\n",sizeof(arr));
}
int main()
{
int arr[SIZE];
size(arr);
return 0;
}第十九道:
The following is a simple c program, in which there is a function called Error to display errors. Can you see a potential problem with the way Error is defined?
#include <stdlib.h>
#include <stdio.h>
void Error(char* s)
{
printf(s);
return;
}
int main()
{
int *p;
p = malloc(sizeof(int));
if(p == NULL)
{
Error("Could not allocate the memory\n");
Error("Quitting....\n");
exit(1);
}
else
{
/*some stuff to use p*/
}
return 0;
}第二十道題:
What is the differnce between the following function calls to scanf?(Please notice the space carefully in the second call. Try removing it and observe the behaviour of the program)
#include <stdio.h>
int main()
{
char c;
scanf("%c",&c);
printf("%c\n",c);
scanf(" %c",&c);
printf("%c\n",c);
return 0;
}第二十一道:
What is the potential problem with the following C program?
#include <stdio.h>
int main()
{
char str[80];
printf("Enter the string:");
scanf("%s",str);
printf("You entered:%s\n",str);
return 0;
}字符串結尾問題,如果輸入的過長,超過了80個字符,則str字符數組最後一個元素就不是\0,printf函數在打印時則會造成程序的越界訪問甚至會崩潰.
第二十二道:
What is the output of the following program?
#include <stdio.h>
int main()
{
int i;
i = 10;
printf("i : %d\n",i);
printf("sizeof(i++) is: %d\n",sizeof(i++));
printf("i : %d\n",i);
return 0;
}第二十三道:
Why does the following program give a warning? (Please remember that sending a normal pointer to a function requiring const pointer does not give any warning)
#include <stdio.h>
void foo(const char **p) { }
int main(int argc, char **argv)
{
foo(argv);
return 0;
}第二十四道:
What is the output of the following program?
#include <stdio.h>
int main()
{
int i;
i = 1,2,3;
printf("i:%d\n",i);
return 0;
}第二十五道:
The following is a piece of code which implements the reverse Polish Calculator. There is a(are) serious(s) bug in the code. Find it(them) out!!! Assume that the function getop returns the appropriate return values for operands, opcodes, EOF etc..
#include <stdio.h>
#include <stdlib.h>
#define MAX 80
#define NUMBER '0'
int getop(char[]);
void push(double);
double pop(void);
int main()
{
int type;
char s[MAX];
while((type = getop(s)) != EOF)
{
switch(type)
{
case NUMBER:
push(atof(s));
break;
case '+':
push(pop() + pop());
break;
case '*':
push(pop() * pop());
break;
case '-':
push(pop() - pop());
break;
case '/':
push(pop() / pop());
break;
/* ...
* ...
* ...
*/
}
}
}第二十六道:
The following is a simple program which implements a minimal version of banner command available on most *nix systems. Find out the logic used in the program.
#include<stdio.h>
#include<ctype.h>
char t[]={
0,0,0,0,0,0,12,18,33,63,
33,33,62,32,62,33,33,62,30,33,
32,32,33,30,62,33,33,33,33,62,
63,32,62,32,32,63,63,32,62,32,
32,32,30,33,32,39,33,30,33,33,
63,33,33,33,4,4,4,4,4,4,
1,1,1,1,33,30,33,34,60,36,
34,33,32,32,32,32,32,63,33,51,
45,33,33,33,33,49,41,37,35,33,
30,33,33,33,33,30,62,33,33,62,
32,32,30,33,33,37,34,29,62,33,
33,62,34,33,30,32,30,1,33,30,
31,4,4,4,4,4,33,33,33,33,
33,30,33,33,33,33,18,12,33,33,
33,45,51,33,33,18,12,12,18,33,
17,10,4,4,4,4,63,2,4,8,
16,63
};
int main(int argc,char** argv)
{
int r,pr;
for(r=0;r<6;++r)
{
char *p=argv[1];
while(pr&&*p)
{
int o=(toupper(*p++)-'A')*6+6+r;
o=(o<0||o>=sizeof(t))?0:o;
for(pr=5;pr>=-1;--pr)
{
printf("%c",( ( (pr>=0) && (t[o]&(1<<pr)))?'#':' '));
}
}
printf("\n");
}
return 0;
}
第二十七道:
What is the output of the following program?
#include <stdio.h>
#include <stdlib.h>
#define SIZEOF(arr) (sizeof(arr)/sizeof(arr[0]))
#define PrintInt(expr) printf("%s:%d\n",#expr,(expr))
int main()
{
/* The powers of 10 */
int pot[] = {
0001,
0010,
0100,
1000
};
int i;
for(i=0;i<SIZEOF(pot);i++)
PrintInt(pot[i]);
return 0;
}第二十八道:
The following is the implementation of the Euclid's algorithm for finding the G.C.D(Greatest Common divisor) of two integers. Explain the logic for the below implementation and think of any possible improvements on the current implementation.
BTW, what does scanf function return?
#include <stdio.h>
int gcd(int u,int v)
{
int t;
while(v > 0)
{
if(u > v)
{
t = u;
u = v;
v = t;
}
v = v-u;
}
return u;
}
int main()
{
int x,y;
printf("Enter x y to find their gcd:");
while(scanf("%d%d",&x, &y) != EOF)
{
if(x >0 && y>0)
printf("%d %d %d\n",x,y,gcd(x,y));
printf("Enter x y to find their gcd:");
}
printf("\n");
return 0;
}
Also implement a C function similar to the above to find the GCD of 4 integers.第二十九道:
What's the output of the following program. (No, it's not 10!!!)
#include <stdio.h>
#define PrintInt(expr) printf("%s : %d\n",#expr,(expr))
int main()
{
int y = 100;
int *p;
p = malloc(sizeof(int));
*p = 10;
y = y/*p; /*dividing y by *p */;
PrintInt(y);
return 0;
}第三十道:
The following is a simple C program to read a date and print the date. Run it and explain the behaviour
#include <stdio.h>
int main()
{
int day,month,year;
printf("Enter the date (dd-mm-yyyy) format including -'s:");
scanf("%d-%d-%d",&day,&month,&year);
printf("The date you have entered is %d-%d-%d\n",day,month,year);
return 0;
}第三十一道:
The following is a simple C program to read and print an integer. But it is not working properly. What is(are) the mistake(s)?
#include <stdio.h>
int main()
{
int n;
printf("Enter a number:\n");
scanf("%d\n",n);
printf("You entered %d \n",n);
return 0;
}首先scanf中n是不對的,應該傳入&n,此外scanf("%d\n",n)中的\n也是問題,scanf中儘量不要使用非輸入控制符,像上面的程序就需要輸入兩個數字程序才能運行完畢.此外,這個程序估計還有別的我不知道的問題.
第三十二道:
The following is a simple C program which tries to multiply an integer by 5 using the bitwise operations. But it doesn't do so. Explain the reason for the wrong behaviour of the program.
#include <stdio.h>
#define PrintInt(expr) printf("%s : %d\n",#expr,(expr))
int FiveTimes(int a)
{
int t;
t = a<<2 + a;
return t;
}
int main()
{
int a = 1, b = 2,c = 3;
PrintInt(FiveTimes(a));
PrintInt(FiveTimes(b));
PrintInt(FiveTimes(c));
return 0;
}第三十三道:
Is the following a valid C program?
#include <stdio.h>
#define PrintInt(expr) printf("%s : %d\n",#expr,(expr))
int max(int x, int y)
{
(x > y) ? return x : return y;
}
int main()
{
int a = 10, b = 20;
PrintInt(a);
PrintInt(b);
PrintInt(max(a,b));
}第三十四道:
The following is a piece of C code, whose intention was to print a minus sign 20 times. But you can notice that, it doesn't work.
#include <stdio.h>
int main()
{
int i;
int n = 20;
for( i = 0; i < n; i-- )
printf("-");
return 0;
}
Well fixing the above code is straight-forward. To make the problem interesting, you have to fix the above code, by changing exactly one character. There are three known solutions. See if you can get all those three.第三十五道:
What's the mistake in the following code?
#include <stdio.h>
int main()
{
int* ptr1,ptr2;
ptr1 = malloc(sizeof(int));
ptr2 = ptr1;
*ptr2 = 10;
return 0;
}第三十六道:
What is the output of the following program?
#include <stdio.h>
int main()
{
int cnt = 5, a;
do {
a /= cnt;
} while (cnt --);
printf ("%d\n", a);
return 0;
}當cnt爲1時while(cnt--)爲真,所以繼續進行循環,而此時cnt經過自減操作cnt爲0,所以a /= cnt成了a /= 0,會發生除零錯誤
第三十七道題:
What is the output of the following program?
#include <stdio.h>
int main()
{
int i = 6;
if( ((++i < 7) && ( i++/6)) || (++i <= 9))
;
printf("%d\n",i);
return 0;
}第三十八道題:
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;
}第三十九道題:
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;
}第四十道題:
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;
}第四十一道題:
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.c
bash$ ./a.out
a = 10
Hmm!! no compilation/linker error!!! Why is it so?? 第四十二道題:
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)))
這是stddef.h裏定義的一個宏,其返回值爲size_t,它適用於結構和union,用於檢索結構成員相對於父結構的偏移量,第一個參數爲結構名稱,第二個參數爲結構中的字段名,其主要實現如下:將地址0強制轉換爲對應結構指針,此時結構的地址爲0,然後使用->獲得b的內容,再使用&操作符獲得b的地址,再將b的地址轉換爲int類型,使用b的地址減去結構地址0(因爲是0強轉的),即爲結構字段b相對於結構的偏移量,這種在嵌入式裏面用的比較多,這裏有一篇詳解它的文章.它的使用例子如下:
#include <stdio.h>
#include <stddef.h>
void main(void)
{
struct test
{
int a;
short b;
char c;
int d;
};
struct test a;
size_t offset = offsetof(struct test,c);
printf("the offset of c is %d\n",offset); /* 輸出6,因爲32位機器上int佔4字節,short爲2字節 */
}第四十三道題:
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? 第四十四道題:
What is the use of the following macro?
#define DPRINTF(x) printf("%s:%d\n",#x,x)
第四十五道題:
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?) 第四十六道題:
What does the following macro do?
#define ROUNDUP(x,n) ((x+n-1)&(~(n-1)))
第四十七道題:
Most of the C programming books, give the following example for the definition of macros.
#define isupper(c) (((c) >= 'A') && ((c) <= 'Z'))
But there would be a serious problem with the above definition of macro, if it is used as follows (what is the problem??)
char c;
/* ... */
if(isupper(c++))
{
/* ... */
}
But most of the libraries implement the isupper (declared in ctypes.h) as a macro (without any side effects). Find out how isupper() is implemented on your system. 如果調用issuper(c++),那麼實際上c++在宏中調用了兩次,比如c = 'Y',調用issuper(c++)完後Y就變成了字符[,與作者本意不同,因爲作者本意是c++運算後,此時運算爲字符'Z',一般機器上的都是用位運算來做的,因爲大小寫字母剛好差32,換算成十六進制就是0x20,即大小寫字母只是在第六位(從右到左數)比特位上不相同,其他比特字節相同,所以用位運算又快又準確.
第四十八道題:
I hope you know that ellipsis (...) is used to specify variable number of arguments to a function. (What is the function prototype declaration for printf?) What is wrong with the following delcaration?
int VarArguments(...)
{
/*....*/
return 0;
}第四十九道題:
Write a C program to find the smallest of three integers, without using any of the comparision operators. 我的思路,使用位運算
#include <stdio.h>
/**
* 找到兩個數中的較小的數,如果相等返回同一個數
* 這種方法只適用於使用補碼錶示有符號數的計算機,且是8比特補碼系統
*/
int findLess(int a,int b)
{
int c = a - b;
int flag = c >> (sizeof(int) * 8 - 1); /* 獲取標誌位 */
if(flag) /* 標誌位爲1表示結果爲負數 */
{
return a;
}
else if(c) /* 此時標誌位爲0,表示c爲非負數,如果c爲真,表示c大於0,返回b */
{
return b;
}
else /* c爲0,表示兩個數相等 */
{
return a;
}
printf("compare error\n");
return -1;
}
void main(void)
{
int a = 1,b = 2,c = 2;
printf("the small integer is %d\n",findLess(a,findLess(b,c)));
}第五十道題:
What does the format specifier %n of printf function do?msdn描述如下,%n表示當前已經成功寫入流或者緩衝的字符數目;該值保存在一個整數中,其地址作爲參數傳給printf函數,且說明%n內在並不安全,所以默認情況下是禁用的,要使用%n,請查看_set_printf_count_output等.例子如下:
#include <stdio.h>
/**
* gcc編譯
*/
void main(void)
{
char* str = "hello world";
int n;
printf("%s\n%n",str,&n); /* 傳入n的地址 */
printf("%d\n",n); /* 打印n的值 */
}第五十一道題:
Write a C function which does the addition of two integers without using the '+' operator. You can use only the bitwise operators.(Remember the good old method of implementing the full-adder circuit using the or, and, xor gates....) 第五十二道題:
How do you print I can print % using the printf function? (Remember % is used as a format specifier!!!) #include <stdio.h>
void main(void)
{
printf("I can print %% use the printf function\n");
}第五十三道題:
What's the difference between the following two C statements?
const char *p;
char* const p;#include <stdio.h>
int main()
{
const char a = 'a';
const char b = 'b';
char c = 'c';
const char *p;
p = &a;
printf("%c\n",*p); /* 輸出a */
p = &b;
printf("%c\n",*p); /* 輸出b */
p = &c;
printf("%c\n",*p); /* 輸出c */
/*
*p = 'd'; 這句需要註釋掉,否則會報錯error: assignment of read-only location '*p'
*/
return 0;
}#include <stdio.h>
int main()
{
const char a = 'a';
char b = 'b';
char c = 'c';
/*
這句註釋掉,否則會報warning: initialization discards 'const' qualifier from pointer target type [enabled by default]
這也說明不要將常量的地址賦值給char* const 類型的指針
char* const p = &a;
*/
char* const p = &b; /* 定義時即賦值 */
printf("%c\n",*p);
*p = c; /* 可以使用p修改p所指向的數據 */
printf("%c\n",b);
printf("%c\n",c);
/*
p = &c; 這句需要註釋掉,否則報錯error: assignment of read-only location '*p'
*/
}
第五十四道題:
What is the difference between memcpy and memmove? 如果傳入的原地址與目標地址有重疊的話,memmove總能正確處理,memcpy則不一定可行.
第五十五道題:
What is the format specifiers for printf to print double and float values? 第五十六道題:
Write a small C program to determine whether a machine's type is little-endian or big-endian.方法一:
#include <stdio.h>
int main(void)
{
int a = 1;
char *p = (char*) &a;
if(*p == 1)
{
printf("the machine is little endian\n");
}
else
{
printf("the machine is big endian\n");
}
return 0;
}
方法二:
#include <stdio.h>
typedef union endian_
{
int a;
char b;
} Endian;
int main(void)
{
Endian a;
a.a = 1;
if(a.b == 1)
{
printf("the machine is little endian\n");
}
else
{
printf("the machine is big endian\n");
}
return 0;
}Write a C program which prints Hello World! without using a semicolon!!! stackOverflow給輸了多種答案,下面是其中的一種
#include <stdio.h>
void main(void)
{
if(printf("hello world\n")){}
}