将整数，结构体，结构体数组，链表写到文件

在之前的学习中，忘文件中写的内容都是字符串或字符，本节学习如何写入其他各种类型的数据。

回看write和read函数的形式：

ssize_t write(int fd, const void *buf, size_t count);

ssize_t read(int fd, void *buf, size_t count);

其中，第二个参数都是一个无类型的指针，只不过之前一直将这里定义为一个字符串，其实，这个指针可以指向各种形式数据的地址。 

写入一个整数

demo7.c:

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>int main()
{int fd; // file descriptionint data = 100;int data2 = 0;fd = open("./file1",O_RDWR|O_CREAT|O_TRUNC, 0600);printf("file description = %d, open successfully!\n",fd);write(fd, &data, sizeof(int));lseek(fd,0,SEEK_SET);read(fd, &data2, sizeof(int));printf("context:%d\n",data2);close(fd); //close after writing return 0;
}

运行代码：

注意，如果此时打开file1:（此时需要使用vi打开）

发现是乱码，但是这并不影响程序运行时的读取和写入。

写入一个结构体

demo7.c:

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>struct Test
{int i;char c;
};int main()
{int fd; // file descriptionstruct Test data = {30,'k'};struct Test data2;fd = open("./file1",O_RDWR|O_CREAT|O_TRUNC, 0600);printf("file description = %d, open successfully!\n",fd);write(fd, &data, sizeof(struct Test));lseek(fd,0,SEEK_SET);read(fd, &data2, sizeof(struct Test));printf("data.i:%d,data.c=%c\n",data2.i,data2.c);close(fd); //close after writing return 0;
}

运行代码：

 

 

 写入一个结构体数组

demo7.c:

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>struct Test
{int i;char c;
};int main()
{int fd; // file descriptionstruct Test data[2] = {{30,'k'},{100,'p'}};struct Test data2[2];fd = open("./file1",O_RDWR|O_CREAT|O_TRUNC, 0600);printf("file description = %d, open successfully!\n",fd);write(fd, &data, sizeof(struct Test)*2);lseek(fd,0,SEEK_SET);read(fd, &data2, sizeof(struct Test)*2);printf("data[0].i:%d,data[0].c=%c\n",data2[0].i,data2[0].c);printf("data[1].i:%d,data[1].c=%c\n",data2[1].i,data2[1].c);close(fd); //close after writing return 0;
}

 运行代码：

写入一个链表 

demo7.c：

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>struct Test
{int data;struct Test *next;
};int main()
{int fd; // file descriptionstruct Test head = {20,NULL};struct Test node1 = {30,NULL};struct Test node2 = {40,NULL};head.next = &node1;node1.next = &node2;fd = open("./file1",O_RDWR|O_CREAT|O_TRUNC, 0600);printf("file description = %d, open successfully!\n",fd);write(fd, &head, sizeof(struct Test));write(fd, &node1, sizeof(struct Test));write(fd, &node2, sizeof(struct Test));lseek(fd,0,SEEK_SET);struct Test node_read_1 = {0,NULL};struct Test node_read_2 = {0,NULL};struct Test node_read_3 = {0,NULL};read(fd, &node_read_1, sizeof(struct Test)); read(fd, &node_read_2, sizeof(struct Test));read(fd, &node_read_3, sizeof(struct Test));printf("no.1=%d\n",node_read_1.data);printf("no.2=%d\n",node_read_2.data);printf("no.3=%d\n",node_read_3.data);close(fd);return 0;
}

运行代码：

 

但是以上的代码有点笨，且容错性太低，首先读取和写入应该封装成函数，并且我认为通常在读取链表的时候，不一定知道链表有多少元素，所以应该一边用尾插法动态创建链表一边读取。

改进代码demo8.c:

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>struct Test
{int data;struct Test *next;
};struct Test *insertBehind(struct Test *head, struct Test *new)
{struct Test *p = head;if(p == NULL){head = new;return head;}while(p->next != NULL){p = p->next;} //将p先移动到链表的尾部p->next = new;return head;
}void writeLink2File(int fd,struct Test *p){while(p != NULL){write(fd, p, sizeof(struct Test));p = p->next;}
}void readLinkFromFile(int fd,struct Test *head){struct Test *new;int size = lseek(fd,0,SEEK_END); //先计算文件大小lseek(fd,0,SEEK_SET); //然后不要忘记把光标移到文件头while(1){new = (struct Test *)malloc(sizeof(struct Test));read(fd, new, sizeof(struct Test));printf("link data:%d\n",new->data);if(lseek(fd,0,SEEK_CUR) == size){ //每次读取一个数据之后，动态创建下一个链表节点之前，都要判断“当前光标是否已经在文件尾”，如果是，说明读取已经完成printf("read finish\n");break;}head = insertBehind(head,new);}}int main()
{int fd; // file descriptionstruct Test head = {20,NULL};struct Test node1 = {30,NULL};struct Test node2 = {40,NULL};head.next = &node1;node1.next = &node2;fd = open("./file1",O_RDWR|O_CREAT|O_TRUNC, 0600);printf("file description = %d, open successfully!\n",fd);writeLink2File(fd,&head);struct Test *head_recv = NULL; //准备创建一个新的链表用于接收readLinkFromFile(fd,head_recv);close(fd);return 0;
}

改进代码运行：