结构体的指针(续）

注意这几行

void printBook( struct Books *book ) {

   printf( "Book title : %s\n", book->title);
   printf( "Book author : %s\n", book->author);
   printf( "Book subject : %s\n", book->subject);
   printf( "Book book_id : %d\n", book->book_id);
}



struct Books Book1;  


printBook( &Book1 );

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

struct Books {
   char  title[50];
   char  author[50];
   char  subject[100];
   int   book_id;
};

/* function declaration */
void printBook( struct Books *book );
int main( ) {

   struct Books Book1;        /* Declare Book1 of type Book */
   struct Books Book2;        /* Declare Book2 of type Book */

   /* book 1 specification */
   strcpy( Book1.title, "C Programming");
   strcpy( Book1.author, "Nuha Ali");
   strcpy( Book1.subject, "C Programming Tutorial");
   Book1.book_id = 6495407;

   /* book 2 specification */
   strcpy( Book2.title, "Telecom Billing");
   strcpy( Book2.author, "Zara Ali");
   strcpy( Book2.subject, "Telecom Billing Tutorial");
   Book2.book_id = 6495700;

   /* print Book1 info by passing address of Book1 */
   printBook( &Book1 );

   /* print Book2 info by passing address of Book2 */
   printBook( &Book2 );

   return 0;
}

void printBook( struct Books *book ) {

   printf( "Book title : %s\n", book->title);
   printf( "Book author : %s\n", book->author);
   printf( "Book subject : %s\n", book->subject);
   printf( "Book book_id : %d\n", book->book_id);
}

 结构体数组(结构体的第二种实例化方法）

struct Books books[2];

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

struct Books {
    char  title[50];
    char  author[50];
    char  subject[100];
    int   book_id;
};

/* function declaration */
void printBook( struct Books *book );
int main( ) {

    struct Books books[2];

    /* book 1 specification */
    strcpy( books[0].title, "C Programming");
    strcpy( books[0].author, "Nuha Ali");
    strcpy( books[0].subject, "C Programming Tutorial");
    books[0].book_id = 6495407;

    /* book 2 specification */
    strcpy( books[1].title, "Telecom Billing");
    strcpy( books[1].author, "Zara Ali");
    strcpy( books[1].subject, "Telecom Billing Tutorial");
    books[1].book_id = 6495700;

    /* print Book1 info by passing address of Book1 */
    printBook( &books[0] );

    /* print Book2 info by passing address of Book2 */
    printBook( &books[1] );

    return 0;
}

void printBook( struct Books *book ) {

    printf( "Book title : %s\n", book->title);
    printf( "Book author : %s\n", book->author);
    printf( "Book subject : %s\n", book->subject);
    printf( "Book book_id : %d\n", book->book_id);
}

结构体占用内存计算 （函数实现）

sizeof(struct Books)

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

struct Books {
};

int main( ) {
    printf("%d\n", (int) sizeof(struct Books)); /*0*/
    return 0;
}
#include <stdio.h>
#include <string.h>

struct Books {
    char title[50];
    char author[50];
    char subject[100];
    int book_id;
};

int main() {
    printf("%d\n", (int) sizeof(struct Books)); /*204*/
    return 0;
}

 

位域   （没太搞懂）

有时候我们内存紧张的时候，我们可以使用位域定义结构体成员变量，比如当我们需要定义一个表示true或false的时候，如果想这样定义

int isOpen;

明显很浪费空间，因为一个真假值只需要一个字位表示，所以我们可以这样定义

unsigned int isOpen:1;

但是如果你直接写在函数中是会报错的，我们应该写在结构体中

int main() {
    unsigned int isOpen:1; /*编译无法通过*/
    return 0;
}

正确姿势

struct packed_struct {
   unsigned int f1:1;
   unsigned int f2:1;
   unsigned int f3:1;
   unsigned int f4:1;
   unsigned int type:4;
   unsigned int my_int:9;
} pack;

C尽可能紧凑地自动打包上述位字段，前提是字段的最大长度小于或等于计算机的整数字长。如果不是这种情况，那么一些编译器可能允许字段存储器重叠，而其他编译器会将下一个字段存储在下一个字中。

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

struct packed_struct {
    unsigned int f1:1;
    unsigned int f2:1;
    unsigned int f3:1;
    unsigned int f4:1;
    unsigned int type:4;
    unsigned int my_int:9;
} pack;
int main() {
    printf("%d\n", (int) sizeof(struct packed_struct));
    return 0;
}

输出结果 8