C语言指针，求大侠帮忙解释解释。

2013-07-05 01:04:01 577 阅读 AI 写同款 · GEO 优化AI 选题数据监测一键分发 ›

C语言之内存对齐数相信大家都看过金庸的武侠小说，小说里那些大侠凭借一门绝世武功就可以打遍天下无敌手，而我们今天要讲的内存对齐术就相当于武侠小说里的绝世武功，理解它就可以了解结构体是如何在内存中存储的啦！本文就以结构体中的内存对齐来理解什仫是内存对齐术？为什仫要进行内存对齐？这两个问题，首先我们先来看一个简单地例子： #include #include typedef struct A { 阅读详情

大概是这样的：

typedef struct a s_a;

typedef struct b s_b;



struct a {

	char *s;

};



struct b {

	s_a r;

	s_a *p;

}



struct s_b *var;

我想通过ar访问a结构体中的s;
如果 var->p->s 我能够理解，因为都是指针嘛；

如果要访问 r 中的 s，该怎么访问？
var->r.s 还是 var.r->s 或者是 &var->r->s ?

谢谢大家！

...全文

欢聚时代2017校招C基础笔试题解析：指针、内存与字符串考点全攻克 C语言作为服务器开发的核心语言，其底层原理（如指针运算、内存布局）直接影响高并发系统的稳定性。指针与数组的步长计算、字符串操作的越界风险、结构体字节对齐的规则，往往是笔试中区分基础是否扎实的关键。本文以欢聚时代2017校招C基础类A卷为例，系统梳理了指针、内存管理、字符串、链表等高频考点的命题思路与解题技巧，并结合典型真题还原拆解过程。同时从备考方法论出发，给出三轮复习法与考场答题节奏建议，帮助读者将一套经典笔试题转化为完整的知识体系，从容应对C/C++相关岗位的校招笔试与技术面试。阅读详情

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NowDoIT 2013-08-12

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谢谢楼上各位，尤其是 @cfjtaishan ，亲自写代码验证。结贴散分，再次感谢！

一根烂笔头 2013-08-03

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结构指针访问成员用-> 结构实例访问成员用.

fldmxp 2013-08-03

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s是r的成员，s虽然是指针(s是不是指针没有关系)但是r并不是指针，所以是.而不是->

monkeydonkey 2013-07-06

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仔细看看书本上->和.各自使用场景

monkeydonkey 2013-07-06

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var->r.s

一根烂笔头 2013-07-06

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LZ的变量名s_b好奇葩阿

tianfyt 2013-07-06

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清_晨 2013-07-05

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->是指针指向其成员的运算符 .是结构体的成员运算符如：var->r.s="aaaaaa"; var是指针后面就接-> r是结构体（或者联合体）后面就接 . 如：var->p->s var是指针后面就接-> p还是指针后面就接->

X_Richard 2013-07-05

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引用 11 楼 cfjtaishan 的回复:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct a s_a;
typedef struct b s_b;
 
struct a {
    char *s;
};
 
struct b {
    s_a r;
    s_a *p;
};
int main(int argc, char *argv[])
{
	//struct s_b *var;    //这种写法会让编译器报未定义指针类型的变量var
	s_b *var = (s_b*)malloc(sizeof(s_b));
	if (NULL == var)
	{
		fprintf(stderr, "malloc failed!\n");
		exit(EXIT_FAILURE);
	}
	var->r.s = (char *)malloc(16);
	if (NULL == var->r.s)
	{
		fprintf(stderr, "malloc failed!\n");
		exit(EXIT_FAILURE);
	}
	var->p = &var->r;    //指针用于指向结构体变量
	strcpy(var->r.s, "helloworld");
	fprintf(stdout, "%s\n", var->p->s);
	//puts(var->p->s);
	free(var->r.s);
	free(var);
	
	return 0;
}

代码中有不足之处，大家可以发表自己意见，直接指出来，我不胜感激。但讨论应该有一个前提，那就是拿出证据，能够说出一个123，让大家信服。

哈哈，你说那个不堪负载，没那么严重，我只是开玩笑的说一下编码风格，项目组不可能就40行代码，别那么认真。 BTW, 看来你释放内存后也没有好习惯把pointer set NULL啊，如果悬垂指针引起问题就不那么好找咯~~（真正项目遇到后你就会注意了）。个人建议，误见怪~~

majia2011 2013-07-05

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可能是每个人的看法都不同吧，在我眼里，有括号反而更乱，呵呵呵

自信男孩 2013-07-05

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#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct a s_a;
typedef struct b s_b;
 
struct a {
    char *s;
};
 
struct b {
    s_a r;
    s_a *p;
};
int main(int argc, char *argv[])
{
	//struct s_b *var;    //这种写法会让编译器报未定义指针类型的变量var
	s_b *var = (s_b*)malloc(sizeof(s_b));
	if (NULL == var)
	{
		fprintf(stderr, "malloc failed!\n");
		exit(EXIT_FAILURE);
	}
	var->r.s = (char *)malloc(16);
	if (NULL == var->r.s)
	{
		fprintf(stderr, "malloc failed!\n");
		exit(EXIT_FAILURE);
	}
	var->p = &var->r;    //指针用于指向结构体变量
	strcpy(var->r.s, "helloworld");
	fprintf(stdout, "%s\n", var->p->s);
	//puts(var->p->s);
	free(var->r.s);
	free(var);
	
	return 0;
}

代码中有不足之处，大家可以发表自己意见，直接指出来，我不胜感激。但讨论应该有一个前提，那就是拿出证据，能够说出一个123，让大家信服。

自信男孩 2013-07-05

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引用 9 楼 rent19870817 的回复:

[quote=引用 6 楼 cfjtaishan 的回复:]

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct a s_a;
typedef struct b s_b;
 
struct a {
    char *s;
};
 
struct b {
    s_a r;
    s_a *p;
};
int main(int argc, char *argv[])
{
	//struct s_b *var;    //这种写法会让编译器报未定义指针类型的变量var
	s_b *var = (s_b*)malloc(sizeof(s_b));
	if (NULL == var)
	{
		fprintf(stderr, "malloc failed!\n");
		exit(EXIT_FAILURE);
	}
	var->r.s = (char *)malloc(16);
	if (NULL == var->r.s)
	{
		fprintf(stderr, "malloc failed!\n");
		exit(EXIT_FAILURE);
	}
	var->p = &var->r;    //指针用于指向结构体变量
	strcpy(var->r.s, "helloworld");
	fprintf(stdout, "%s\n", var->p->s);
	//puts(var->p->s);
	
	return 0;
}

我通过测试加不加括号是一样的，不过，加上括号能够让表达式更好理解。

都像你这么写代码，内存有多少都不够用的

[/quote] 嗯，我承认没有free我申请的空间，这只是一个demo程序，程序退出后会自动释放我申请的空间，所以没有手动free掉。除了这方面，我看不出我申请了多少空间，而让内存“不堪负载”。

X_Richard 2013-07-05

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引用 6 楼 cfjtaishan 的回复:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct a s_a;
typedef struct b s_b;
 
struct a {
    char *s;
};
 
struct b {
    s_a r;
    s_a *p;
};
int main(int argc, char *argv[])
{
	//struct s_b *var;    //这种写法会让编译器报未定义指针类型的变量var
	s_b *var = (s_b*)malloc(sizeof(s_b));
	if (NULL == var)
	{
		fprintf(stderr, "malloc failed!\n");
		exit(EXIT_FAILURE);
	}
	var->r.s = (char *)malloc(16);
	if (NULL == var->r.s)
	{
		fprintf(stderr, "malloc failed!\n");
		exit(EXIT_FAILURE);
	}
	var->p = &var->r;    //指针用于指向结构体变量
	strcpy(var->r.s, "helloworld");
	fprintf(stdout, "%s\n", var->p->s);
	//puts(var->p->s);
	
	return 0;
}

我通过测试加不加括号是一样的，不过，加上括号能够让表达式更好理解。

都像你这么写代码，内存有多少都不够用的

_胖胖 2013-07-05

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是这样的：结构体变量名.成员名；指针变量名->成员名。另外，指针变量名->成员名在计算机内部会被转化成(*指针变量名).成员名的方式来执行。所以你这个问题的结果是 var->r.s

自信男孩 2013-07-05

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var->p = &var->r;    //指针用于指向结构体变量

这一句中，最好能加上括号。如下：

var->p = &(var->r);    //指针用于指向结构体变量

自信男孩 2013-07-05

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#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct a s_a;
typedef struct b s_b;
 
struct a {
    char *s;
};
 
struct b {
    s_a r;
    s_a *p;
};
int main(int argc, char *argv[])
{
	//struct s_b *var;    //这种写法会让编译器报未定义指针类型的变量var
	s_b *var = (s_b*)malloc(sizeof(s_b));
	if (NULL == var)
	{
		fprintf(stderr, "malloc failed!\n");
		exit(EXIT_FAILURE);
	}
	var->r.s = (char *)malloc(16);
	if (NULL == var->r.s)
	{
		fprintf(stderr, "malloc failed!\n");
		exit(EXIT_FAILURE);
	}
	var->p = &var->r;    //指针用于指向结构体变量
	strcpy(var->r.s, "helloworld");
	fprintf(stdout, "%s\n", var->p->s);
	//puts(var->p->s);
	
	return 0;
}

我通过测试加不加括号是一样的，不过，加上括号能够让表达式更好理解。

Regan-lin 2013-07-05

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(var->r).s

水平不流 2013-07-05

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楼上都解决了....

buyong 2013-07-05

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(var->r).s

赵4老师 2013-07-05

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(var->r).s

//C++ Operators
//  Operators specify an evaluation to be performed on one of the following:
//    One operand (unary operator)
//    Two operands (binary operator)
//    Three operands (ternary operator)
//  The C++ language includes all C operators and adds several new operators.
//  Table 1.1 lists the operators available in Microsoft C++.
//  Operators follow a strict precedence which defines the evaluation order of
//expressions containing these operators.  Operators associate with either the
//expression on their left or the expression on their right;    this is called
//“associativity.” Operators in the same group have equal precedence and are
//evaluated left to right in an expression unless explicitly forced by a pair of
//parentheses, ( ).
//  Table 1.1 shows the precedence and associativity of C++ operators
//  (from highest to lowest precedence).
//
//Table 1.1   C++ Operator Precedence and Associativity
// The highest precedence level is at the top of the table.
//+------------------+-----------------------------------------+---------------+
//| Operator         | Name or Meaning                         | Associativity |
//+------------------+-----------------------------------------+---------------+
//| ::               | Scope resolution                        | None          |
//| ::               | Global                                  | None          |
//| [ ]              | Array subscript                         | Left to right |
//| ( )              | Function call                           | Left to right |
//| ( )              | Conversion                              | None          |
//| .                | Member selection (object)               | Left to right |
//| ->               | Member selection (pointer)              | Left to right |
//| ++               | Postfix increment                       | None          |
//| --               | Postfix decrement                       | None          |
//| new              | Allocate object                         | None          |
//| delete           | Deallocate object                       | None          |
//| delete[ ]        | Deallocate object                       | None          |
//| ++               | Prefix increment                        | None          |
//| --               | Prefix decrement                        | None          |
//| *                | Dereference                             | None          |
//| &                | Address-of                              | None          |
//| +                | Unary plus                              | None          |
//| -                | Arithmetic negation (unary)             | None          |
//| !                | Logical NOT                             | None          |
//| ~                | Bitwise complement                      | None          |
//| sizeof           | Size of object                          | None          |
//| sizeof ( )       | Size of type                            | None          |
//| typeid( )        | type name                               | None          |
//| (type)           | Type cast (conversion)                  | Right to left |
//| const_cast       | Type cast (conversion)                  | None          |
//| dynamic_cast     | Type cast (conversion)                  | None          |
//| reinterpret_cast | Type cast (conversion)                  | None          |
//| static_cast      | Type cast (conversion)                  | None          |
//| .*               | Apply pointer to class member (objects) | Left to right |
//| ->*              | Dereference pointer to class member     | Left to right |
//| *                | Multiplication                          | Left to right |
//| /                | Division                                | Left to right |
//| %                | Remainder (modulus)                     | Left to right |
//| +                | Addition                                | Left to right |
//| -                | Subtraction                             | Left to right |
//| <<               | Left shift                              | Left to right |
//| >>               | Right shift                             | Left to right |
//| <                | Less than                               | Left to right |
//| >                | Greater than                            | Left to right |
//| <=               | Less than or equal to                   | Left to right |
//| >=               | Greater than or equal to                | Left to right |
//| ==               | Equality                                | Left to right |
//| !=               | Inequality                              | Left to right |
//| &                | Bitwise AND                             | Left to right |
//| ^                | Bitwise exclusive OR                    | Left to right |
//| |                | Bitwise OR                              | Left to right |
//| &&               | Logical AND                             | Left to right |
//| ||               | Logical OR                              | Left to right |
//| e1?e2:e3         | Conditional                             | Right to left |
//| =                | Assignment                              | Right to left |
//| *=               | Multiplication assignment               | Right to left |
//| /=               | Division assignment                     | Right to left |
//| %=               | Modulus assignment                      | Right to left |
//| +=               | Addition assignment                     | Right to left |
//| -=               | Subtraction assignment                  | Right to left |
//| <<=              | Left-shift assignment                   | Right to left |
//| >>=              | Right-shift assignment                  | Right to left |
//| &=               | Bitwise AND assignment                  | Right to left |
//| |=               | Bitwise inclusive OR assignment         | Right to left |
//| ^=               | Bitwise exclusive OR assignment         | Right to left |
//| ,                | Comma                                   | Left to right |
//+------------------+-----------------------------------------+---------------+

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C语言指针，求大侠帮忙解释解释。

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