gcc 编译出错了

gcc max.o min.o hello.c

hello.c: In function ‘main’:
hello.c:18:16: warning: implicit declaration of function ‘max’; did you mean ‘main’? [-Wimplicit-function-declaration]
     int maxNum=max(a1,a2);
                ^~~
                main
hello.c:19:16: warning: implicit declaration of function ‘min’; did you mean ‘main’? [-Wimplicit-function-declaration]
     int minNum=min(a1,a2);
                ^~~
                main