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struct Person{ char name[64]; int age;};typedef struct _PERSON{ char name[64]; int age;}Person;
注意:定义结构体类型时不要直接给成员赋值,结构体只是一个类型,编译器还没有为其分配空间,只有根据其类型定义变量时,才分配空间,有空间后才能赋值。
struct Person{ char name[64]; int age;}p1; //定义类型同时定义变量struct{ char name[64]; int age;}p2; //定义类型同时定义变量struct Person p3; //通过类型直接定义
struct Person{ char name[64]; int age;}p1 = { "john",10}; //定义类型同时初始化变量struct{ char name[64]; int age;}p2 = { "Obama",30}; //定义类型同时初始化变量struct Person p3 = { "Edward",33}; //通过类型直接定义
struct Person{ char name[64]; int age;};void test(){ //在栈上分配空间 struct Person p1; strcpy(p1.name, "John"); p1.age = 30; //如果是普通变量,通过点运算符操作结构体成员 printf("Name:%s Age:%d\n", p1.name, p1.age); //在堆上分配空间 struct Person* p2 = (struct Person*)malloc(sizeof(struct Person)); strcpy(p2->name, "Obama"); p2->age = 33; //如果是指针变量,通过->操作结构体成员 printf("Name:%s Age:%d\n", p2->name, p2->age);}
相同的两个结构体变量可以相互赋值,把一个结构体变量的值拷贝给另一个结构体,这两个变量还是两个独立的变量。
struct Person{ char name[64]; int age;};void test(){ //在栈上分配空间 struct Person p1 = { "John" , 30}; struct Person p2 = { "Obama", 33 }; printf("Name:%s Age:%d\n", p1.name, p1.age); printf("Name:%s Age:%d\n", p2.name, p2.age); //将p2的值赋值给p1 p1 = p2; printf("Name:%s Age:%d\n", p1.name, p1.age); printf("Name:%s Age:%d\n", p2.name, p2.age);}
//一个老师有N个学生typedef struct _TEACHER{ char* name;}Teacher;void test(){ Teacher t1; t1.name = malloc(64); strcpy(t1.name , "John"); Teacher t2; t2 = t1; //对手动开辟的内存,需要手动拷贝 t2.name = malloc(64); strcpy(t2.name, t1.name); if (t1.name != NULL){ free(t1.name); t1.name = NULL; } if (t2.name != NULL){ free(t2.name); t1.name = NULL; }}
struct Person{ char name[64]; int age;};void test(){ //在栈上分配空间 struct Person p1[3] = { { "John", 30 }, { "Obama", 33 }, { "Edward", 25} }; struct Person p2[3] = { "John", 30, "Obama", 33, "Edward", 25 }; for (int i = 0; i < 3;i ++){ printf("Name:%s Age:%d\n",p1[i].name,p1[i].age); } printf("-----------------\n"); for (int i = 0; i < 3; i++){ printf("Name:%s Age:%d\n", p2[i].name, p2[i].age); } printf("-----------------\n"); //在堆上分配结构体数组 struct Person* p3 = (struct Person*)malloc(sizeof(struct Person) * 3); for (int i = 0; i < 3;i++){ sprintf(p3[i].name, "Name_%d", i + 1); p3[i].age = 20 + i; } for (int i = 0; i < 3; i++){ printf("Name:%s Age:%d\n", p3[i].name, p3[i].age); }}
struct Person{ char* name; int age;};void allocate_memory(struct Person** person){ if (person == NULL){ return; } struct Person* temp = (struct Person*)malloc(sizeof(struct Person)); if (temp == NULL){ return; } //给name指针分配内存 temp->name = (char*)malloc(sizeof(char)* 64); strcpy(temp->name, "John"); temp->age = 100; *person = temp;}void print_person(struct Person* person){ printf("Name:%s Age:%d\n",person->name,person->age);}void free_memory(struct Person** person){ if (person == NULL){ return; } struct Person* temp = *person; if (temp->name != NULL){ free(temp->name); temp->name = NULL; } free(temp);}void test(){ struct Person* p = NULL; allocate_memory(&p); print_person(p); free_memory(&p);}
//一个老师有N个学生typedef struct _TEACHER{ char name[64]; char** students;}Teacher;void create_teacher(Teacher** teacher,int n,int m){ if (teacher == NULL){ return; } //创建老师数组 Teacher* teachers = (Teacher*)malloc(sizeof(Teacher)* n); if (teachers == NULL){ return; } //给每一个老师分配学生 int num = 0; for (int i = 0; i < n; i ++){ sprintf(teachers[i].name, "老师_%d", i + 1); teachers[i].students = (char**)malloc(sizeof(char*) * m); for (int j = 0; j < m;j++){ teachers[i].students[j] = malloc(64); sprintf(teachers[i].students[j], "学生_%d", num + 1); num++; } } *teacher = teachers; }void print_teacher(Teacher* teacher,int n,int m){ for (int i = 0; i < n; i ++){ printf("%s:\n", teacher[i].name); for (int j = 0; j < m;j++){ printf(" %s",teacher[i].students[j]); } printf("\n"); }}void free_memory(Teacher** teacher,int n,int m){ if (teacher == NULL){ return; } Teacher* temp = *teacher; for (int i = 0; i < n; i ++){ for (int j = 0; j < m;j ++){ free(temp[i].students[j]); temp[i].students[j] = NULL; } free(temp[i].students); temp[i].students = NULL; } free(temp);}void test(){ Teacher* p = NULL; create_teacher(&p,2,3); print_teacher(p, 2, 3); free_memory(&p,2,3);}
//一旦结构体定义下来,则结构体中的成员内存布局就定下了typedef struct Teacher{ char a; int b; int c; } Teacher;void test(){ Teacher t1; Teacher*p = NULL; p = &t1; int offsize1 = (int)&(p->b) - (int)p; //age 相对于结构体 Teacher的偏移量 int offsize2 = (int)&(((Teacher *)0)->b);//绝对0地址 age的偏移量 int offsize3 = offsetof(Teacher, b); printf("offsize1:%d \n", offsize1); printf("offsize2:%d \n", offsize2); printf("offsize3:%d \n", offsize3);}
在用sizeof运算符求算某结构体所占空间时,并不是简单地将结构体中所有元素各自占的空间相加,这里涉及到内存字节对齐的问题。
从理论上讲,对于任何变量的访问都可以从任何地址开始访问,但是事实上不是如此,实际上访问特定类型的变量只能在特定的地址访问,这就需要各个变量在空间上按一定的规则排列, 而不是简单地顺序排列,这就是内存对齐。我们知道内存的最小单元是一个字节,当cpu从内存中读取数据的时候,是一个一个字节读取,所以内存对我们应该是入下图这样:
但是实际上cpu将内存当成多个块,每次从内存中读取一个块,这个块的大小可能是2、4、8、16等,
那么下面,我们来分析下非内存对齐和内存对齐的优缺点在哪? 内存对齐是操作系统为了提高访问内存的策略。操作系统在访问内存的时候,每次读取一定长度(这个长度是操作系统默认的对齐数,或者默认对齐数的整数倍)。如果没有对齐,为了访问一个变量可能产生二次访问。至此大家应该能够简单明白,为什么要简单内存对齐?
手动设置对齐模数:
#pragma pack(4)typedef struct _STUDENT{ int a; char b; double c; float d;}Student;typedef struct _STUDENT2{ char a; Student b; double c;}Student2;void test01(){ //Student //a从偏移量0位置开始存储 //b从4位置开始存储 //c从8位置开始存储 //d从12位置开存储 //所以Student内部对齐之后的大小为20 ,整体对齐,整体为最大类型的整数倍 也就是8的整数倍 为24 printf("sizeof Student:%d\n",sizeof(Student)); //Student2 //a从偏移量为0位置开始 8 //b从偏移量为Student内部最大成员整数倍开始,也就是8开始 24 //c从8的整数倍地方开始,也就是32开始 //所以结构体Sutdnet2内部对齐之后的大小为:40 , 由于结构体中最大成员为8,必须为8的整数倍 所以大小为40 printf("sizeof Student2:%d\n", sizeof(Student2));}
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