这是hello.c文件源码：

#include <stdio.h>
//#include "max.c"
//#include "min.c"

int main()
{
    int a1=33;
    int a2=21;
    int maxNum=max(a1,a2);
    int minNum=min(a1,a2);
    printf("the max value is %d\nthe min value is %d\n",maxNum,minNum);
    return 0;

}

这种情况下输入命令：gcc hello.c max.o min.o 然后回车为什么会：

hello.c: In function ‘main’:
hello.c:9:16: warning: implicit declaration of function ‘max’ [-Wimplicit-function-declaration]
     int maxNum=max(a1,a2);
                ^
hello.c:10:16: warning: implicit declaration of function ‘min’ [-Wimplicit-function-declaration]
     int minNum=min(a1,a2);