搜集的一些面试题

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1。char *p = "ab";
printf("%d,%d",p,p+5);
地址问题，类型为char的地址下移5个为原地址值+5；今天做错了：（

2。int i=10,j=20,k=30;
k *= i+j;
printf("%d",k);

输出为900，考优先级，做对了：）

3。enum string{x1,x2,x3=10,x4,x5};
int x=x5;
printf("%d,%d",x,x1);

输出为12,0，做错了：（

4。int data[1024];

问实际使用的数组范围，应该是0~1023，呜呜，我还以为数组不能太大呢！原来这么大也可以！

5。typedef union data{
        long l;
        double d;
    }Data;
sizeof(data),sizeof(Data)的值都是8，取sizeof(double)的值，而不是sizeof(double)和sizeof(int)的和。

本文来自CSDN博客，转载请标明出处：http://blog.csdn.net/fengsanshao/archive/2007/03/16/1531748.aspx

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1. Given a rectangular (cuboidal for the puritans) cake with a rectangular piece removed (any size or orientation), how would you cut the remainder of the cake into two equal halves with one straight cut of a knife ?

2. You're given an array containing both positive and negative integers and required to find the sub-array with the largest sum (O(N) a la KBL). Write a routine in C for the above.

3. Given an array of size N in which every number is between 1 and N, determine if there are any duplicates in it. You are allowed to destroy the array if you like. [ I ended up giving about 4 or 5 different solutions for this, each supposedly better than the others ].

4. Write a routine to draw a circle (x ** 2 + y ** 2 = r ** 2) without making use of any floating point computations at all. [ This one had me stuck for quite some time and I first gave a solution that did have floating point computations ].

5. Given only putchar (no sprintf, itoa, etc.) write a routine putlong that prints out an unsigned long in decimal. [ I gave the obvious solution of taking % 10 and / 10, which gives us the decimal value in reverse order. This requires an array since we need to print it out in the correct order. The interviewer wasn't too pleased and asked me to give a solution which didn't need the array ].

6. Give a one-line C expression to test whether a number is a power of 2. [No loops allowed - it's a simple test.]

7. Given an array of characters which form a sentence of words, give an efficient algorithm to reverse the order of the words (not characters) in it.

8. How many points are there on the globe where by walking one mile south, one mile east and one mile north you reach the place where you started.

9. Give a very good method to count the number of ones in a "n" (e.g. 32) bit number.

ANS. Given below are simple solutions, find a solution that does it in log (n) steps.

Iterative
function iterativecount (unsigned int n)
begin
int count=0;
while (n)
begin
count += n & 0x1 ;
n >>= 1;
end
return count;
end
Sparse Count
function sparsecount (unsigned int n)
begin
int count=0;
while (n)
begin
count++;
n &= (n-1);
end
return count ;
end
10. What are the different ways to implement a condition where the value of x can be either a 0 or a 1. Apparently the if then else solution has a jump when written out in assembly. if (x == 0) y=a else y=b There is a logical, arithmetic and a data structure solution to the above problem.

11. Reverse a linked list.

12. Insert in a sorted list

13. In a X's and 0's game (i.e. TIC TAC TOE) if you write a program for this give a fast way to generate the moves by the computer. I mean this should be the fastest way possible.

The answer is that you need to store all possible configurations of the board and the move that is associated with that. Then it boils down to just accessing the right element and getting the corresponding move for it. Do some analysis and do some more optimization in storage since otherwise it becomes infeasible to get the required storage in a DOS machine.

14. I was given two lines of assembly code which found the absolute value of a number stored in two's complement form. I had to recognize what the code was doing. Pretty simple if you know some assembly and some fundaes on number representation.

15. Give a fast way to multiply a number by 7.

16. How would go about finding out where to find a book in a library. (You don't know how exactly the books are organized beforehand).

17. Linked list manipulation.

18. Tradeoff between time spent in testing a product and getting into the market first.

19. What to test for given that there isn't enough time to test everything you want to.

20. First some definitions for this problem: a) An ASCII character is one byte long and the most significant bit in the byte is always '0'. b) A Kanji character is two bytes long. The only characteristic of a Kanji character is that in its first byte the most significant bit is '1'.

Now you are given an array of a characters (both ASCII and Kanji) and, an index into the array. The index points to the start of some character. Now you need to write a function to do a backspace (i.e. delete the character before the given index).

21. Delete an element from a doubly linked list.

22. Write a function to find the depth of a binary tree.

23. Given two strings S1 and S2. Delete from S2 all those characters which occur in S1 also and finally create a clean S2 with the relevant characters deleted.

24. Assuming that locks are the only reason due to which deadlocks can occur in a system. What would be a foolproof method of avoiding deadlocks in the system.

25. Reverse a linked list.

Ans: Possible answers -

iterative loop
curr->next = prev;
prev = curr;
curr = next;
next = curr->next
endloop

recursive reverse(ptr)
if (ptr->next == NULL)
return ptr;
temp = reverse(ptr->next);
temp->next = ptr;
return ptr;
end

26. Write a small lexical analyzer - interviewer gave tokens. expressions like "a*b" etc.

27. Besides communication cost, what is the other source of inefficiency in RPC? (answer : context switches, excessive buffer copying). How can you optimize the communication? (ans : communicate through shared memory on same machine, bypassing the kernel _ A Univ. of Wash. thesis)

28. Write a routine that prints out a 2-D array in spiral order!

29. How is the readers-writers problem solved? - using semaphores/ada .. etc.

30. Ways of optimizing symbol table storage in compilers.

31. A walk-through through the symbol table functions, lookup() implementation etc. - The interviewer was on the Microsoft C team.

32. A version of the "There are three persons X Y Z, one of which always lies".. etc..

33. There are 3 ants at 3 corners of a triangle, they randomly start moving towards another corner.. what is the probability that they don't collide.

34. Write an efficient algorithm and C code to shuffle a pack of cards.. this one was a feedback process until we came up with one with no extra storage.

35. The if (x == 0) y = 0 etc..

36. Some more bitwise optimization at assembly level

37. Some general questions on Lex, Yacc etc.

38. Given an array t[100] which contains numbers between 1..99. Return the duplicated value. Try both O(n) and O(n-square).

39. Given an array of characters. How would you reverse it. ? How would you reverse it without using indexing in the array.

40. Given a sequence of characters. How will you convert the lower case characters to upper case characters. ( Try using bit vector - solutions given in the C lib -typec.h)

41. Fundamentals of RPC.

42. Given a linked list which is sorted. How will u insert in sorted way.

43. Given a linked list How will you reverse it.

44. Give a good data structure for having n queues ( n not fixed) in a finite memory segment. You can have some data-structure separate for each queue. Try to use at least 90% of the memory space.

45. Do a breadth first traversal of a tree.

46. Write code for reversing a linked list.

47. Write, efficient code for extracting unique elements from a sorted list of array. e.g. (1, 1, 3, 3, 3, 5, 5, 5, 9, 9, 9, 9) -> (1, 3, 5, 9).

48. Given an array of integers, find the contiguous sub-array with the largest sum.

ANS. Can be done in O(n) time and O(1) extra space. Scan array from 1 to n. Remember the best sub-array seen so far and the best sub-array ending in i.

49. Given an array of length N containing integers between 1 and N, determine if it contains any duplicates.

ANS. [Is there an O(n) time solution that uses only O(1) extra space and does not destroy the original array?]

50. Sort an array of size n containing integers between 1 and K, given a temporary scratch integer array of size K.

ANS. Compute cumulative counts of integers in the auxiliary array. Now scan the original array, rotating cycles! [Can someone word this more nicely?]

* 51. An array of size k contains integers between 1 and n. You are given an additional scratch array of size n. Compress the original array by removing duplicates in it. What if k << n?

ANS. Can be done in O(k) time i.e. without initializing the auxiliary array!

52. An array of integers. The sum of the array is known not to overflow an integer. Compute the sum. What if we know that integers are in 2's complement form?

ANS. If numbers are in 2's complement, an ordinary looking loop like for(i=total=0;i< n;total+=array[i++]); will do. No need to check for overflows!

53. An array of characters. Reverse the order of words in it.

ANS. Write a routine to reverse a character array. Now call it for the given array and for each word in it.

* 54. An array of integers of size n. Generate a random permutation of the array, given a function rand_n() that returns an integer between 1 and n, both inclusive, with equal probability. What is the expected time of your algorithm?

ANS. "Expected time" should ring a bell. To compute a random permutation, use the standard algorithm of scanning array from n downto 1, swapping i-th element with a uniformly random element <= i-th. To compute a uniformly random integer between 1 and k (k < n), call rand_n() repeatedly until it returns a value in the desired range.

55. An array of pointers to (very long) strings. Find pointers to the (lexicographically) smallest and largest strings.

ANS. Scan array in pairs. Remember largest-so-far and smallest-so-far. Compare the larger of the two strings in the current pair with largest-so-far to update it. And the smaller of the current pair with the smallest-so-far to update it. For a total of <= 3n/2 strcmp() calls. That's also the lower bound.

56. Write a program to remove duplicates from a sorted array.

ANS. int remove_duplicates(int * p, int size)
{
int current, insert = 1;
for (current=1; current < size; current++)
if (p[current] != p[insert-1])
{
p[insert] = p[current];
current++;
insert++;
} else
current++;

return insert;

}

57. C++ ( what is virtual function ? what happens if an error occurs in constructor or destructor. Discussion on error handling, templates, unique features of C++. What is different in C++, ( compare with unix).

58. Given a list of numbers ( fixed list) Now given any other list, how can you efficiently find out if there is any element in the second list that is an element of the first list (fixed list).

59. Given 3 lines of assembly code : find it is doing. IT was to find absolute value.

60. If you are on a boat and you throw out a suitcase, Will the level of water increase.

61. Print an integer using only putchar. Try doing it without using extra storage.

62. Write C code for (a) deleting an element from a linked list (b) traversing a linked list

63. What are various problems unique to distributed databases

64. Declare a void pointer ANS. void *ptr;

65. Make the pointer aligned to a 4 byte boundary in a efficient manner ANS. Assign the pointer to a long number and the number with 11...1100 add 4 to the number

66. What is a far pointer (in DOS)

67. What is a balanced tree

68. Given a linked list with the following property node2 is left child of node1, if node2 < node1 else, it is the right child.

O P
|
|
O A
|
|
O B
|
|
O C
How do you convert the above linked list to the form without disturbing the property. Write C code for that.

   O P
|
|
O B
/ /
/   /
/     /
O ?     O ?
determine where do A and C go

69. Describe the file system layout in the UNIX OS

ANS. describe boot block, super block, inodes and data layout

70. In UNIX, are the files allocated contiguous blocks of data

ANS. no, they might be fragmented

How is the fragmented data kept track of

ANS. Describe the direct blocks and indirect blocks in UNIX file system

71. Write an efficient C code for 'tr' program. 'tr' has two command line arguments. They both are strings of same length. tr reads an input file, replaces each character in the first string with the corresponding character in the second string. eg. 'tr abc xyz' replaces all 'a's by 'x's, 'b's by 'y's and so on. ANS.
a) have an array of length 26.
put 'x' in array element corr to 'a'
put 'y' in array element corr to 'b'
put 'z' in array element corr to 'c'
put 'd' in array element corr to 'd'
put 'e' in array element corr to 'e'
and so on.

the code
while (!eof)
{
c = getc();
putc(array[c - 'a']);
}

72. what is disk interleaving

73. why is disk interleaving adopted

74. given a new disk, how do you determine which interleaving is the best a) give 1000 read operations with each kind of interleaving determine the best interleaving from the statistics

75. draw the graph with performance on one axis and 'n' on another, where 'n' in the 'n' in n-way disk interleaving. (a tricky question, should be answered carefully)

76. I was a c++ code and was asked to find out the bug in that. The bug was that he declared an object locally in a function and tried to return the pointer to that object. Since the object is local to the function, it no more exists after returning from the function. The pointer, therefore, is invalid outside.

77. A real life problem - A square picture is cut into 16 squares and they are shuffled. Write a program to rearrange the 16 squares to get the original big square.

78.
int *a;
char *c;
*(a) = 20;
*c = *a;
printf("%c",*c);

what is the output?

79. Write a program to find whether a given m/c is big-endian or little-endian!

80. What is a volatile variable?

81. What is the scope of a static function in C ?

82. What is the difference between "malloc" and "calloc"?

83. struct n { int data; struct n* next}node;
node *c,*t;
c->data = 10;
t->next = null;
*c = *t;
what is the effect of the last statement?

84. If you're familiar with the ? operator x ? y : z
you want to implement that in a function: int cond(int x, int y, int z); using only ~, !, ^, &, +, |, <<, >> no if statements, or loops or anything else, just those operators, and the function should correctly return y or z based on the value of x. You may use constants, but only 8 bit constants. You can cast all you want. You're not supposed to use extra variables, but in the end, it won't really matter, using vars just makes things cleaner. You should be able to reduce your solution to a single line in the end though that requires no extra vars.

85. You have an abstract computer, so just forget everything you know about computers, this one only does what I'm about to tell you it does. You can use as many variables as you need, there are no negative numbers, all numbers are integers. You do not know the size of the integers, they could be infinitely large, so you can't count on truncating at any point. There are NO comparisons allowed, no if statements or anything like that. There are only four operations you can do on a variable.
1) You can set a variable to 0.
2) You can set a variable = another variable.
3) You can increment a variable (only by 1), and it's a post increment.
4) You can loop. So, if you were to say loop(v1) and v1 = 10, your loop would execute 10 times, but the value in v1 wouldn't change so the first line in the loop can change value of v1 without changing the number of times you loop.
You need to do 3 things.
1) Write a function that decrements by 1.
2) Write a function that subtracts one variable from another.
3) Write a function that divides one variable by another.
4) See if you can implement all 3 using at most 4 variables. Meaning, you're not making function calls now, you're making macros. And at most you can have 4 variables. The restriction really only applies to divide, the other 2 are easy to do with 4 vars or less. Division on the other hand is dependent on the other 2 functions, so, if subtract requires 3 variables, then divide only has 1 variable left unchanged after a call to subtract. Basically, just make your function calls to decrement and subtract so you pass your vars in by reference, and you can't declare any new variables in a function, what you pass in is all it gets.

* 86. Under what circumstances can one delete an element from a singly linked list in constant time?

ANS. If the list is circular and there are no references to the nodes in the list from anywhere else! Just copy the contents of the next node and delete the next node. If the list is not circular, we can delete any but the last node using this idea. In that case, mark the last node as dummy!

* 87. Given a singly linked list, determine whether it contains a loop or not.

ANS. (a) Start reversing the list. If you reach the head, gotcha! there is a loop!
But this changes the list. So, reverse the list again.
(b) Maintain two pointers, initially pointing to the head. Advance one of them one node at a time. And the other one, two nodes at a time. If the latter overtakes the former at any time, there is a loop!

p1 = p2 = head;
do {
p1 = p1->next;
p2 = p2->next->next;
} while (p1 != p2);
88. Given a singly linked list, print out its contents in reverse order. Can you do it without using any extra space?

ANS. Start reversing the list. Do this again, printing the contents.

89. Given a binary tree with nodes, print out the values in pre-order/in-order/post-order without using any extra space.

90. Reverse a singly linked list recursively. The function prototype is node * reverse (node *) ;

ANS.

node * reverse (node * n)
{
node * m ;
if (! (n && n -> next))
return n ;
m = reverse (n -> next) ;
n -> next -> next = n ;
n -> next = NULL ;
return m ;
}
91. Given a singly linked list, find the middle of the list.

HINT. Use the single and double pointer jumping. Maintain two pointers, initially pointing to the head. Advance one of them one node at a time. And the other one, two nodes at a time. When the double reaches the end, the single is in the middle. This is not asymptotically faster but seems to take less steps than going through the list twice.

92. Reverse the bits of an unsigned integer.

ANS.

#define reverse(x) /
(x=x>>16|(0x0000ffff&x)<<16, /
x=(0xff00ff00&x)>>8|(0x00ff00ff&x)<<8, /
x=(0xf0f0f0f0&x)>>4|(0x0f0f0f0f&x)<<4, /
x=(0xcccccccc&x)>>2|(0x33333333&x)<<2, /
x=(0xaaaaaaaa&x)>>1|(0x55555555&x)<<1)
* 93. Compute the number of ones in an unsigned integer.

ANS.

#define count_ones(x) /
(x=(0xaaaaaaaa&x)>>1+(0x55555555&x), /
x=(0xcccccccc&x)>>2+(0x33333333&x), /
x=(0xf0f0f0f0&x)>>4+(0x0f0f0f0f&x), /
x=(0xff00ff00&x)>>8+(0x00ff00ff&x), /
x=x>>16+(0x0000ffff&x))
94. Compute the discrete log of an unsigned integer.

ANS.

#define discrete_log(h) /
(h=(h>>1)|(h>>2), /
h|=(h>>2), /
h|=(h>>4), /
h|=(h>>8), /
h|=(h>>16), /
h=(0xaaaaaaaa&h)>>1+(0x55555555&h), /
h=(0xcccccccc&h)>>2+(0x33333333&h), /
h=(0xf0f0f0f0&h)>>4+(0x0f0f0f0f&h), /
h=(0xff00ff00&h)>>8+(0x00ff00ff&h), /
h=(h>>16)+(0x0000ffff&h))
If I understand it right, log2(2) =1, log2(3)=1, log2(4)=2..... But this macro does not work out log2(0) which does not exist! How do you think it should be handled?
* 95. How do we test most simply if an unsigned integer is a power of two?

ANS. #define power_of_two(x) / ((x)&&(~(x&(x-1))))

96. Set the highest significant bit of an unsigned integer to zero.

ANS. (from Denis Zabavchik) Set the highest significant bit of an unsigned integer to zero
#define zero_most_significant(h) /
(h&=(h>>1)|(h>>2), /
h|=(h>>2), /
h|=(h>>4), /
h|=(h>>8), /
h|=(h>>16))

97. Let f(k) = y where k is the y-th number in the increasing sequence of non-negative integers with the same number of ones in its binary representation as y, e.g. f(0) = 1, f(1) = 1, f(2) = 2, f(3) = 1, f(4) = 3, f(5) = 2, f(6) = 3 and so on. Given k >= 0, compute f(k).

98. A character set has 1 and 2 byte characters. One byte characters have 0 as the first bit. You just keep accumulating the characters in a buffer. Suppose at some point the user types a backspace, how can you remove the character efficiently. (Note: You cant store the last character typed because the user can type in arbitrarily many backspaces)

99. What is the simples way to check if the sum of two unsigned integers has resulted in an overflow.

100. How do you represent an n-ary tree? Write a program to print the nodes of such a tree in breadth first order.

101. Write the 'tr' program of UNIX. Invoked as

tr -str1 -str2. It reads stdin and prints it out to stdout, replacing every occurance of str1[i] with str2[i].

e.g. tr -abc -xyz
to be and not to be <- input
to ye xnd not to ye <- output

本文来自CSDN博客，转载请标明出处：http://blog.csdn.net/fengsanshao/archive/2007/03/16/1531759.aspx

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#include <stdio.h>
struct ss
{
    char c;
    int n;
    union menbers
    {
        char letter;
        int number;
        struct aa
        {
            char s[4];
        }aa1;
    }x;
};

void sizetest(char a[100],char *str)
{
    printf("%d %d/n",sizeof(a),sizeof(*a));
    printf("%d %d/n",sizeof(str),sizeof(*str));
}

int main()
{
    char a[100];
    char *str = "hello!";
    struct ss ss1;
    struct ss *ss2;

    printf("%d %d/n",sizeof(a),sizeof(*a));
    printf("%d %d/n",sizeof(str),sizeof(*str));
    printf("%d %d/n",sizeof(ss1),sizeof(&ss1)); /* 能不能解释一下这里的&ss1 */
    printf("%d %d/n",sizeof(*ss2),sizeof(ss2)); /* 还有这里的ss2 */
    sizetest(a,str);

return 0;
}

只写对了几个，高手是不是可以不用编译器写出正确的答案呢？

1、 sizeof应用在结构上的情况

  请看下面的结构：

  struct MyStruct

  {

  double dda1;

  char dda;

  int type

  };

  对结构MyStruct采用sizeof会出现什么结果呢？sizeof(MyStruct)为多少呢？也许你会这样求：

  sizeof(MyStruct)=sizeof(double)+sizeof(char)+sizeof(int)=13

  但是当在VC中测试上面结构的大小时，你会发现sizeof(MyStruct)为16。你知道为什么在VC中会得出这样一个结果吗？

  其实，这是VC对变量存储的一个特殊处理。为了提高CPU的存储速度，VC对一些变量的起始地址做了“对齐”处理。在默认情况下，VC规定各成员变量存放的起始地址相对于结构的起始地址的偏移量必须为该变量的类型所占用的字节数的倍数。下面列出常用类型的

对齐方式(vc6.0,32位系统)。

  类型
  对齐方式（变量存放的起始地址相对于结构的起始地址的偏移量）

  Char
  偏移量必须为sizeof(char)即1的倍数

  int
  偏移量必须为sizeof(int)即4的倍数

  float
  偏移量必须为sizeof(float)即4的倍数

  double
  偏移量必须为sizeof(double)即8的倍数

  Short
  偏移量必须为sizeof(short)即2的倍数

  各成员变量在存放的时候根据在结构中出现的顺序依次申请空间，同时按照上面的对齐方式调整位置，空缺的字节VC会自动填充。同时VC为了确保结构的大小为结构的字节边界数（即该结构中占用最大空间的类型所占用的字节数）的倍数，所以在为最后一个成员变量申请空间后，还会根据需要自动填充空缺的字节。

  下面用前

面的例子来说明VC到底怎么样来存放结构的。

  struct MyStruct

  {

  double dda1;

  char dda;

  int type

  }；

  为上面的结构分配空间的时候，VC根据成员变量出现的顺序和对齐方式，先为第一个成员dda1分配空间，其起始地址跟结构的起始地址相同（刚好偏移量0刚好为sizeof(double)的倍数），该成员变量占用sizeof(double)=8个字节；接下来为第二个成员dda分配空间，这时下一个可以分配的地址对于结构的起始地址的偏移量为8，是sizeof(char)的倍数，所以把dda存放在偏移量为8的地方满足对齐方式，该成员变量占用sizeof(char)=1个字节；接下来为第三个成员type分配空间，这时下一个可以分配的地址对于结构的起始地址的偏移量为9，不是sizeof(int)=4的倍数，为了满足对齐方式对偏
移量的约束问题，VC自动填充3个字节（这三个字节没有放什么东西），这时下一个可以分配的地址对于结构的起始地址的偏移量为12，刚好是sizeof(int)=4的倍数，所以把type存放在偏移量为12的地方，该成员变量占用sizeof(int)=4个字节；这时整个结构的成员变量已经都分配了空间，总的占用的空间大小为：8+1+3+4=16，刚好为结构的字节边界数（即结构中占用最大空间的类型所占用的字节数sizeof(double)=8）的倍数，所以没有空缺的字节需要填充。所以整个结构的大小为：sizeof(MyStruct)=8+1+3+4=16，其中有3个字节是VC自动填充的，没有放任何有意义的东西。

  下面再举个例子，交换一下上面的MyStruct的成员变量的位置，使它变成下面的情况：

  struct MyStruct

  {

  char dda;

  double dda1;



int type

  }；

  这个结构占用的空间为多大呢？在VC6.0环境下，可以得到sizeof(MyStruc)为24。结合上面提到的分配空间的一些原则，分析下VC怎么样为上面的结构分配空间的。（简单说明）

  struct MyStruct

  {

  char dda;//偏移量为0，满足对齐方式，dda占用1个字节；

  double dda1;//下一个可用的地址的偏移量为1，不是sizeof(double)=8

  //的倍数，需要补足7个字节才能使偏移量变为8（满足对齐

  //方式），因此VC自动填充7个字节，dda1存放在偏移量为8

  //的地址上，它占用8个字节。

  int type；//下一个可用的地址的偏移量为16，是sizeof(int)=4的倍

  //数，满足int的对齐方式，所以不需要VC自动填充，type存

  //放在偏移量

为16的地址上，它占用4个字节。

  }；//所有成员变量都分配了空间，空间总的大小为1+7+8+4=20，不是结构

  //的节边界数（即结构中占用最大空间的类型所占用的字节数sizeof

  //(double)=8）的倍数，所以需要填充4个字节，以满足结构的大小为

  //sizeof(double)=8的倍数。

  所以该结构总的大小为：sizeof(MyStruc)为1+7+8+4+4=24。其中总的有7+4=11个字节是VC自动填充的，没有放任何有意义的东西。

  VC对结构的存储的特殊处理确实提高CPU存储变量的速度，但是有时候也带来了一些麻烦，我们也屏蔽掉变量默认的对齐方式，自己可以设定变量的对齐方式。

  VC中提供了#pragma pack(n)来设定变量以n字节对齐方式。n字节对齐就是说变量存放的起始地址的偏移量有两种情况：

第一、如果n大于等于该变量所占用的字节数，那么偏移量必须满足默认的对齐方式，第二、如果n小于该变量的类型所占用的字节数，那么偏移量为n的倍数，不用满足默认的对齐方式。结构的总大小也有个约束条件，分下面两种情况：如果n大于所有成员变量类型所占用的字节数，那么结构的总大小必须为占用空间最大的变量占用的空间数的倍数；

  否则必须为n的倍数。下面举例说明其用法。

  #pragma pack(push) //保存对齐状态

  #pragma pack(4)//设定为4字节对齐

  struct test

  {

  char m1;

  double m4;

  int m3;

  };

  #pragma pack(pop)//恢复对齐状态

  以上结构的大小为16，下面分析其存储情况，首先为m1分配空间，其偏移量为0，满足我们自

己设定的对齐方式（4字节对齐），m1占用1个字节。接着开始为m4分配空间，这时其偏移量为1，需要补足3个字节，这样使偏移量满足为n=4的倍数（因为sizeof(double)大于n）,m4占用8个字节。接着为m3分配空间，这时其偏移量为12，满足为4的倍数，m3占用4个字节。这时已经为所有成员变量分配了空间，共分配了16个字节，满足为n的倍数。如果把上面的#pragma pack(4)改为#pragma pack(16)，那么我们可以得到结构的大小为24。（请读者自己分析）

  2、 sizeof用法总结

  在VC中，sizeof有着许多的用法，而且很容易引起一些错误。下面根据sizeof后面的参数对sizeof的用法做个总结。

  A．参数为数据类型或者为一般变量。例如sizeof(int),sizeof(long)等等。这种情况要注意的是不同系统系统或者不同编译器得到的结果可能是不同的

。例如int类型在16位系统中占2个字节，在32位系统中占4个字节。

  B．参数为数组或指针。下面举例说明.

  int a[50]; //sizeof(a)=4*50=200; 求数组所占的空间大小

  int *a=new int[50];// sizeof(a)=4; a为一个指针，sizeof(a)是求指针

  //的大小,在32位系统中，当然是占4个字节。

  C．参数为结构或类。Sizeof应用在类和结构的处理情况是相同的。但有两点需要注意，第一、结构或者类中的静态成员不对结构或者类的大小产生影响，因为静态变量的存储位置与结构或者类的实例地址无关。

  第二、没有成员变量的结构或类的大小为1，因为必须保证结构或类的每一

  个实例在内存中都有唯一的地址。

  下面举例说明，

  Class Test{int a;static double

c};//sizeof(Test)=4.

  Test *s;//sizeof(s)=4,s为一个指针。

  Class test1{ };//sizeof(test1)=1;

  D．参数为其他。下面举例说明。

  int func(char s[5]);

  {

  cout<
  //数的参数在传递的时候系统处理为一个指针，所

  //以sizeof(s)实际上为求指针的大小。

  return 1;

  }

  sizeof(func(“1234”))=4//因为func的返回类型为int，所以相当于

  //求sizeof(int).

  以上为sizeof的基本用法，在实际的使用中要注意分析VC的分配变量的分配策略，这样的话可以避免一些错误。