while (TRUE)
{
char cmdbuf[4];
fgets(cmdbuf,4,stdin);
               //这里输入了许多数据，大于4，然后回车。
               //我想判断标准输入缓冲区是否有数据？如果有我就循环调用getchar()清空缓冲区数据。
               // int c;
               //while((c = getchar()) != '\n' && c != EOF);

              //但是如果没有数据的话，getchar这里会等待输入。
               //问题就是怎么判断标准输入缓冲区是否有数据？
printf("%s\n",cmdbuf);
}

Portably, you can get the next character in the input stream with getchar() and then push it back with ungetc(), which results in a state as if the character wasn't removed from the stream.

or you could use setvbuf on stdin with your own buffer, so you can peek whenever you want