C语言内存分配问题和C语言中的内存

<p style="margin-bottom: 10px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-size: 14px; line-height: 24px; font-family: ����; box-sizing: border-box !important; word-wrap: break-word !important;">C语言内存分配问题</p><p style="margin-bottom: 10px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-size: 14px; line-height: 24px; font-family: ����; box-sizing: border-box !important; word-wrap: break-word !important;">1、C中内存分为四个区<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />栈：用来存放函数的形参和函数内的局部变量。由编译器分配空间，在函数执行完后由编译器自动释放。 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />堆：用来存放由动态分配函数（如malloc）分配的空间。是由程序员自己手动分配的，并且必须由程序员使用free释放。如果忘记用free释放，会导致所分配的空间一直占着不放，导致内存泄露。 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />全局局：用来存放全局变量和静态变量。存在于程序的整个运行期间，是由编译器分配和释放的。 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />文字常量区：例如char *c = “123456”；则”123456”为文字常量，存放于文字常量区。也由编译器控制分配和释放。 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />程序代码区：用来存放程序的二进制代码。<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />例子(一) <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />int a = 0; //全局区 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />void main() <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{ <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />int b; //栈 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char s[] = abc; //s在栈,abc在文字常量区 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char *p1,*p2; //栈 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char *p3 = 123456; //123456在常量区，p3在栈上 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />static int c =0; //全局区 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />p1 = (char *)malloc(10); //p1在栈，分配的10字节在堆 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />p2 = (char *)malloc(20); //p2在栈，分配的20字节在堆 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />strcpy(p1, 123456); //123456放在常量区 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />} <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />例子(二) <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />//返回char型指针 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char *f() <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{ <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />//s数组存放于栈上 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char s[4] = {'1','2','3','0'}; <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />return s; //返回s数组的地址，但程序运行完s数组就被释放了 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />} <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />void main() <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{ <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char *s; <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />s = f(); <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />printf (%s, s); //打印出来乱码。因为s所指向地址已经没有数据 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />} <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />2、动态分配释放内存<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />用malloc动态分配内存后一定要判断一下分配是否成功，判断指针的值是否为NULL。 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />内存分配成功后要对内存单元进行初始化。 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />内存分配成功且初始化后使用时别越界了。 <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />内存使用完后要用free（p）释放，注意，释放后，p的值是不会变的，仍然是一个地址值，仍然指向那块内存区，只是这块内存区的值变成垃圾了。为了防止后面继续使用这块内存，应在free(p)后，立即p=NULL，这样后面如果要使用，判断p是否为NULL时就会判断出来。<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" /><br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" /><br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />NO.1<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />void GetMemory(char *p)<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />p = (char *)malloc(100);<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />}<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />void Test(void)<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char *str = NULL;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />GetMemory(str);<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />strcpy(str,hello world);<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />printf(str);<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />}<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />请问运行Test函数后会是什么样的结果？<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />NO.2<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char *GetMemory(void)<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char p[] = hello world;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />retrun p;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />}<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />void Test(void)<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char *str = NULL;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />str = GetMemory();<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />printf(str);<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />}<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />问题同NO.1<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />NO.3<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />void GetMemory2(char **p, int num)<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />*p = (char *)malloc(num);<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />}<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />void Test(void)<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char *str = NULL;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />GetMemory(&str,100);<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />strcpy(str,hello);<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />printf(str);<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />}<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />问题同NO.1<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />NO.4<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />void Test(void)<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char *str = (char *)malloc(100);<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />strcpy(str,hello);<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />free(str);<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />if(str != NULL)<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{ <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />strcpy(str,world);<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />printf(str);<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />}<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />}<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />问题同NO.1<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />我对以上问题的分析：<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />NO.1：程序首先申请一个char类型的指针str,并把str指向NULL（即str里存的是NULL的地址，*str为NULL中的值为０），调用函数的过程中做了如下动作：１申请一个char 类型的指针p，２把str的内容copy到了p里（这是参数传递过程中系统所做的）,3为p指针申请了100个空间，４返回Test函数．最后程序把字符串hello world拷贝到str指向的内存空间里．到这里错误出现了！str的空间始终为NULL而并没有实际的空间．深刻理解函数调用的第2步，将不难发现问题所在！（建议：画图理解）<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />NO.2：程序首先申请一个char类型的指针str,并把str指向NULL．调用函数的过程中做了如下动作：１申请一数组p[]并将其赋值为hello world(数组的空间大小为12)，２返回数组名p付给str指针（即返回了数组的首地址）．那么这样就可以打印出字符串＂hello world＂了么？当然是不能的！因为在函数调用的时候漏掉了最后一步．也就是在第２步return数组名后，函数调用还要进行一步操作，也就是释放内存空间．当一个函数被调用结束后它会释放掉它里面所有的变量所占用的空间．所以数组空间被释放掉了，也就是说str所指向的内容将不确定是什么东西．<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />NO.3：正确答案为可以打印出hello．但内存泄漏了！ <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />NO.4：申请空间，拷贝字符串，释放空间．前三步操作都没有任何问题．到if语句里的判断条件开始出错了，因为一个指针被释放之后其内容并不是NULL，而是一个不确定的值．所以if语句永远都不能被执行．这也是著名的＂野＂指针问题．所以我们在编写程序释放一个指针之后一定要人为的将指针付成NULL．这样就会避免出现＂野＂指针的出现．有人说＂野＂指针很可怕，会带来意想不到的错误．<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" /></p><p style="margin-bottom: 10px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 24px; box-sizing: border-box !important; word-wrap: break-word !important;">C语言内存对齐</p><p style="margin-bottom: 10px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-size: 14px; line-height: 24px; font-family: ����; box-sizing: border-box !important; word-wrap: break-word !important;"><br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" /></p><p style="margin-bottom: 10px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-size: 14px; line-height: 24px; font-family: ����; box-sizing: border-box !important; word-wrap: break-word !important;"><br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" /></p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">C99规定int、unsigned int和bool可以作为位域类型，但编译器几乎都对此作了扩展，允许其它类型类型的存在。</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">使用位域的主要目的是压缩存储，其大致规则为：<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />1) 如果相邻位域字段的类型相同，且其位宽之和小于类型的sizeof大小，则后面的字段将紧邻前一个字段存储，直到不能容纳为止；<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />2) 如果相邻位域字段的类型相同，但其位宽之和大于类型的sizeof大小，则后面的字段将从新的存储单元开始，其偏移量为其类型大小的整数倍；<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />3) 如果相邻的位域字段的类型不同，则各编译器的具体实现有差异，VC6采取不压缩方式，Dev-C++采取压缩方式；<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />4) 如果位域字段之间穿插着非位域字段，则不进行压缩；<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />5) 整个结构体的总大小为最宽基本类型成员大小的整数倍。</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">还是让我们来看看例子。<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />示例1：<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />struct BF1<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char f1 : 3;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char f2 : 4;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char f3 : 5;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />};<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />其内存布局为：<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />|_f1__|__f2__|_|____f3___|____|<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />0 3 7 8 1316<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />位域类型为char，第1个字节仅能容纳下f1和f2，所以f2被压缩到第1个字节中，而f3只能从下一个字节开始。因此sizeof(BF1)的结果为2。<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />示例2：<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />struct BF2<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char f1 : 3;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />short f2 : 4;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char f3 : 5;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />};<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />由于相邻位域类型不同，在VC6中其sizeof为6，在Dev-C++中为2。<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />示例3：<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />struct BF3<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char f1 : 3;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char f2;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />char f3 : 5;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />};</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;"> </p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">什么是内存对齐</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">    考虑下面的结构：</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">         struct foo<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />         {<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />           char c1;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />           short s;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />           char c2;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />           int i;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />          };<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    假设这个结构的成员在内存中是紧凑排列的，假设c1的地址是0，那么s的地址就应该是1，c2的地址就是3，i的地址就是4。也就是<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    c1 00000000, s 00000001, c2 00000003, i 00000004。</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">    可是，我们在Visual c/c++ 6中写一个简单的程序：</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">         struct foo a;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    printf("c1 %p, s %p, c2 %p, i %p/n",<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&a.c1 - (unsigned int)(void*)&a,<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&a.s - (unsigned int)(void*)&a,<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&a.c2 - (unsigned int)(void*)&a,<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&a.i - (unsigned int)(void*)&a);<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    运行，输出：<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />         c1 00000000, s 00000002, c2 00000004, i 00000008。</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">    为什么会这样？这就是内存对齐而导致的问题。</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">为什么会有内存对齐</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">    以下内容节选自《Intel Architecture 32 Manual》。<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    字，双字，和四字在自然边界上不需要在内存中对齐。（对字，双字，和四字来说，自然边界分别是偶数地址，可以被4整除的地址，和可以被8整除的地址。）<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    无论如何，为了提高程序的性能，数据结构（尤其是栈）应该尽可能地在自然边界上对齐。原因在于，为了访问未对齐的内存，处理器需要作两次内存访问；然而，对齐的内存访问仅需要一次访问。<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    一个字或双字操作数跨越了4字节边界，或者一个四字操作数跨越了8字节边界，被认为是未对齐的，从而需要两次总线周期来访问内存。一个字起始地址是奇数但却没有跨越字边界被认为是对齐的，能够在一个总线周期中被访问。<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    某些操作双四字的指令需要内存操作数在自然边界上对齐。如果操作数没有对齐，这些指令将会产生一个通用保护异常（#GP）。双四字的自然边界是能够被16整除的地址。其他的操作双四字的指令允许未对齐的访问（不会产生通用保护异常），然而，需要额外的内存总线周期来访问内存中未对齐的数据。</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">编译器对内存对齐的处理</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">    缺省情况下，c/c++编译器默认将结构、栈中的成员数据进行内存对齐。因此，上面的程序输出就变成了：<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />c1 00000000, s 00000002, c2 00000004, i 00000008。<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />编译器将未对齐的成员向后移，将每一个都成员对齐到自然边界上，从而也导致了整个结构的尺寸变大。尽管会牺牲一点空间（成员之间有空洞），但提高了性能。<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />也正是这个原因，我们不可以断言sizeof(foo) == 8。在这个例子中，sizeof(foo) == 12。</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">如何避免内存对齐的影响</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">    那么，能不能既达到提高性能的目的，又能节约一点空间呢？有一点小技巧可以使用。比如我们可以将上面的结构改成：</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">struct bar<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    char c1; <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    char c2;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    short s;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    int i;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />};<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    这样一来，每个成员都对齐在其自然边界上，从而避免了编译器自动对齐。在这个例子中，sizeof(bar) == 8。</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">    这个技巧有一个重要的作用，尤其是这个结构作为API的一部分提供给第三方开发使用的时候。第三方开发者可能将编译器的默认对齐选项改变，从而造成这个结构在你的发行的DLL中使用某种对齐方式，而在第三方开发者哪里却使用另外一种对齐方式。这将会导致重大问题。<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    比如，foo结构，我们的DLL使用默认对齐选项，对齐为<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />c1 00000000, s 00000002, c2 00000004, i 00000008，同时sizeof(foo) == 12。<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />而第三方将对齐选项关闭，导致<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    c1 00000000, s 00000001, c2 00000003, i 00000004，同时sizeof(foo) == 8。</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">如何使用c/c++中的对齐选项</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">    vc6中的编译选项有 /Zp[1|2|4|8|16] ，/Zp1表示以1字节边界对齐，相应的，/Zpn表示以n字节边界对齐。n字节边界对齐的意思是说，一个成员的地址必须安排在成员的尺寸的整数倍地址上或者是n的整数倍地址上，取它们中的最小值。也就是：<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    min ( sizeof ( member ),  n)<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    实际上，1字节边界对齐也就表示了结构成员之间没有空洞。<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    /Zpn选项是应用于整个工程的，影响所有的参与编译的结构。<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    要使用这个选项，可以在vc6中打开工程属性页，c/c++页，选择Code Generation分类，在Struct member alignment可以选择。</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">    要专门针对某些结构定义使用对齐选项，可以使用#pragma pack编译指令。指令语法如下：<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />#pragma pack( [ show ] | [ push | pop ] [, identifier ] , n  )<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    意义和/Zpn选项相同。比如：</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">#pragma pack(1)<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />struct foo_pack<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    char c1;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    short s;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    char c2;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    int i;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />};<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />#pragma pack()</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">栈内存对齐</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">    我们可以观察到，在vc6中栈的对齐方式不受结构成员对齐选项的影响。（本来就是两码事）。它总是保持对齐，而且对齐在4字节边界上。</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">验证代码</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">#include <stdio.h></p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">struct foo<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    char c1;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    short s;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    char c2;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    int i;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />};</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">struct bar<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    char c1; <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    char c2;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    short s;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    int i;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />};</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">#pragma pack(1)<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />struct foo_pack<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    char c1;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    short s;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    char c2;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    int i;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />};<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />#pragma pack()</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;"><br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />int main(int argc, char* argv[])<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />{<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    char c1;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    short s;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    char c2;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    int i;</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">    struct foo a;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    struct bar b;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    struct foo_pack p;</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">    printf("stack c1 %p, s %p, c2 %p, i %p/n",<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&c1 - (unsigned int)(void*)&i,<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&s - (unsigned int)(void*)&i,<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&c2 - (unsigned int)(void*)&i,<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&i - (unsigned int)(void*)&i);</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">    printf("struct foo c1 %p, s %p, c2 %p, i %p/n",<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&a.c1 - (unsigned int)(void*)&a,<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&a.s - (unsigned int)(void*)&a,<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&a.c2 - (unsigned int)(void*)&a,<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&a.i - (unsigned int)(void*)&a);</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">    printf("struct bar c1 %p, c2 %p, s %p, i %p/n",<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&b.c1 - (unsigned int)(void*)&b,<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&b.c2 - (unsigned int)(void*)&b,<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&b.s - (unsigned int)(void*)&b,<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&b.i - (unsigned int)(void*)&b);</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">    printf("struct foo_pack c1 %p, s %p, c2 %p, i %p/n",<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&p.c1 - (unsigned int)(void*)&p,<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&p.s - (unsigned int)(void*)&p,<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&p.c2 - (unsigned int)(void*)&p,<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />        (unsigned int)(void*)&p.i - (unsigned int)(void*)&p);</p><p style="margin-top: 0px; margin-bottom: 0px; padding-top: 0px; padding-bottom: 0px; max-width: 100%; clear: both; min-height: 1em; widows: auto; color: rgb(51, 51, 51); font-family: Arial; font-size: 14px; line-height: 26px; box-sizing: border-box !important; word-wrap: break-word !important;">    printf("sizeof foo is %d/n", sizeof(foo));<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    printf("sizeof bar is %d/n", sizeof(bar));<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    printf("sizeof foo_pack is %d/n", sizeof(foo_pack));<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    <br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />    return 0;<br style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; word-wrap: break-word !important;" />}</p><div></div>