fseek、fread和fwrite

#include<stdio.h>
#include<math.h>
struct record
{
	int x,y;
};
int main()
{
	FILE *fp=fopen("a.txt","w+");
	struct record a={1,2};
	struct record b={3,4};
	struct record c={5,6};
	fwrite((char*)&a,sizeof(struct record),1,fp);
	fwrite((char*)&b,sizeof(struct record),1,fp);
	fwrite((char*)&c,sizeof(struct record),1,fp);
	fseek(fp,0,0);
	struct record rec;
	while(fread((char*)&rec,sizeof(rec),1,fp)==1)
	{
		printf("%d %d\n",rec.x,rec.y);
		fseek(fp,-(long)sizeof(rec),1);
		rec.x*=2;
		rec.y*=2;
		fwrite((char*)&rec,sizeof(rec),1,fp);
		fseek(fp,0,1);//没有这句就死循环了
	}
	printf("\n");
	fseek(fp,0,0);
	while(fread((char*)&rec,sizeof(rec),1,fp)==1)
	{
		printf("%d %d\n",rec.x,rec.y);
	}
	fclose(fp);
	return 0;
}