int main(void)

{
char s[70];
FILE *fp;
fp=fopen("123.txt","r");

if((fp=fopen("123.txt","w"))==NULL) //if 语句就是创建了一个空的.txt文件
{

printf("Open the file failure...\n");
exit(0);
}
while(1)
{
printf("Input a string...\ns=");
if(gets(s),strlen(s)<70)
break;
printf("Too long, redo: ");
}
fputs(s,fp);

fclose(fp);
printf("\n");
return 0;
}