是啊，忘了把文件里的内容重新读出来了，改过后的
# include <stdio.h>
# include <stdlib.h>
# include <time.h>
# include <string.h>

time_t result;
char * data ;
FILE*fp;
FILE*lfp;
int count;
char buf [100];
char lbuf [10];
int main (void)
{
   
    fp = fopen ("./a.txt","a");
    if ( fp == NULL)
    {
        perror ("fopen failed");
        return -1;
    }
   
    lfp = fopen ("./lin","a+");
    if (lfp == NULL)
    {
        perror ("fopen failed");
        return -1;
    }

    fread (lbuf,5,2,lfp);//将lfp文件的内容读到buf
    count = atoi(lbuf);//把buf中的字符转换成整数
    lfp = freopen("./lin", "w", lfp);
    while (1)
    {  
        //bzero (buf,100);//清空buf
        count++;
        result = time (NULL);
        data = asctime( localtime(&result));
        sprintf(buf,"%d.%s",count,data);
        fwrite (buf,strlen(buf),1,fp);//将buf里的写入fp的文件里
        fprintf(lfp, "%d\n", count);
        printf ("%s",buf);
        fseek(lfp, 0 , SEEK_SET);
        fflush (lfp);
        fflush (fp);//强退还能将buf里的存到fp的文件里
        sleep (1);
    }
   
    exit(0);
}

嗯，能达成目标就行。
看你在一楼的代码使用了freopen函数，以为你打算用重定向的方式读取lin文件的序号，所以我用“lfp = freopen("./lin", "r",stdin);if(lfp!=NULL)scanf("%d",&count);”读取序号到count，无需整形数转换，一样达成目的。