#include<stdio.h>         //输入字符时，程序死循环
 main()
 {
     int score,a,x,total=0,errtotal=0;
     int acount=0,bcount=0,ccount=0,dcount=0,ecount=0;
     printf("Please enter scores:\n");
     while (scanf("%d",&score)!=EOF)
     {
      a=score/10;
      if (score>=0&&score<=100)
      {
            switch(a)
           {
             case 10:
              case 9:
                 {
                     ++acount;
                     ++total;
                     break;
                 }
             case 8:
                 {
                     ++bcount;
                     ++total;
                     break;
                 }
              case 7:
                 {
                     ++ccount;
                     ++total;
                     break;
                 }
             case 6:
                 {
                     ++dcount;
                     ++total;
                     break;
                 }
             default:
                 {
                     ++ecount;
                     ++total;
                     break;
                 }
           }
           
     }
         else
             {
                 printf("Sorry.You have input a wrong score.\n");
                 ++errtotal;
             }
    }
     printf("\n有效输入次数为:%d,无效输入次数为:%d\n",total,errtotal);
     printf("总人数为:%d,成绩分布如下:\n",total);
     printf("A级\tB级\tC级\tD级\tE级\n");
     printf("%d人\t%d人\t%d人\t%d人\t%d人\n",acount,bcount,ccount,dcount,ecount);
 }

不能跳出循环是原因不在eof，在函数里！！

由于这个函数的while永远不会结束，导致了永远不可能break，也就永远退不出eof。

修改为\
    while(scanf("%d",&score)!='\0')  //这里修改了。
      你试下

eof是ctrl+z时发生的情况，当输入字符时候还是满足while的条件的。eof： end of file

scanf("%d",&score)!=EOF
后清空下缓冲区 scanf函数不清除错误流
后接fflush(stdin)