刚学文件，程序的语法没错，语义有问题，无法输出结果。

#include<stdio.h>
main()
{
FILE *open; /*说明文件指针变量open */
int x,sum,aver,n;
if((open=fopen("30.cpp","r"))==NULL)
printf("打开文件失败.\n");
return;
}
sum=0;
n=0;
while(fscanf(open,"%d",&x)!=-1)
{
sum=sum+x;
n++;
}
fclose(open);
aver=sum/n;
printf("aver=%d\n",aver);
}

修改如下，供参考：

#include<stdio.h>
int main()
{
    FILE *open; /*说明文件指针变量open */
    int x,sum,aver=0,n;
    if((open=fopen("30.txt","r"))==NULL){ //if((open=fopen("30.cpp","r"))==NULL)
        printf("打开文件失败.\n");
        return -1;
    }
    sum=0;
    n=0;
    while(fscanf(open,"%d",&x)== 1) //while(fscanf(open,"%d",&x)!=-1)
    {
        sum=sum+x;
        n++;
    }
    fclose(open);
    if(n != 0) aver=sum/n;    //修改
    printf("aver=%d\n",aver);
    return 0;
}

while(fscanf(open,"%d",&x)&&x!=-1)

#include<stdio.h>
main()
{
   FILE *open;                                          /*说明文件指针变量open */
   int x,sum,aver=0,n;
  if((open=fopen("30.txt","r"))==NULL)                                /*打开文件失败*/
{
  printf("打开文件失败.\n");
  return -1;                                  /*终止程序*/
}
  sum=0;
  n=0;
while(fscanf(open,"%d",&x)==1)
{
    sum=sum+x;
    n++;
}
fclose(open);
if(n!=0)
aver=sum/n;
printf("aver=%d\n",aver);
return 0;
}