C语言中读写交替时出现的问题分析

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

int main(void){
	FILE *fp;
	char b;
	//每次测试保证txt文件中只"有123456"这6个字符；
	fp=fopen("file.txt","a+");   //只仅仅修改这里的模式为a+和a,和保证txt开始文本只有123456
	rewind(fp);  //指针在文件开始
	b = fgetc(fp);
	printf("第一次读：%c\n",b);
	fseek(fp,2,0);
	printf("第二次读：%c\n",fgetc(fp));
	fseek(fp,2,0);
	fputs("sag",fp);
	printf("第三次读：%c\n",fgetc(fp));

	fclose(fp);
	system("type file.txt ");  //只是在cmd窗口显示文本内容，懒得每次打开txt查看
	printf("\n");
	system("pause");	
	return 0;
}

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

int main(void){
	FILE *fp;
	char a[4],b;

	fp=fopen("file.txt","r+");
	rewind(fp);
	b = fgetc(fp);
	printf("第一次读：%c\n",b);
	fseek(fp,2,0);
	printf("第二次读：%c\n",fgetc(fp));
	fseek(fp,1,0);
	fputs("sa",fp);
	//fseek(fp,2,0);
	printf("第三次读：%c\n",fgetc(fp));

	fclose(fp);
	system("type file.txt ");
	printf("\n");
	system("pause");	
	return 0;
}