gcc demo.c max.c 报错 识别不到max方法 不是说gcc可以自动识别么？

gcc demo.c  max.c 

demo.c: In function ‘main’:

demo.c:7:18: warning: implicit declaration of function ‘max’; did you mean ‘main’? [-Wimplicit-function-declaration]

     int maxNum = max(a1,a2);

                  ^~~

                  main

代码如下：

#include <stdio.h>

// #include "max.c"

int main()

{

    int a1 = 30;

    int a2 = 32;

    int maxNum = max(a1,a2);

    printf("the max value is %d\n",maxNum);

    return 0;

}