報錯：warning: implicit declaration of function 'maxNum' is invalid in C99 [-Wimplicit-function-declaration]

代碼如下：

是gcc版本問題么？是不是hello.c中要引入max.o?怎么引入？

wubiao@xMac0?cStudy$cat?hello.c?
#include?<stdio.h>
int?main()
{
	int?a?=?33;
	int?b?=?20;
	int?m;
	m?=?maxNum(a,b);
	printf("the?max?value?is?%d\n",m);
	return?0;
}
wubiao@xMac0?cStudy$cat?max.c?
int?maxNum(int?a,int?b)
{
	return?a>b?a:b;
}
wubiao@xMac0?cStudy$gcc?-c?max.c?-o?max.o
wubiao@xMac0?cStudy$ls
hello.c????max.c??????max.o??????test.c?????whichDay.c
wubiao@xMac0?cStudy$gcc?max.o?hello.c?
hello.c:7:6:?warning:?implicit?declaration?of?function?'maxNum'?is?invalid?in?C99?[-Wimplicit-function-declaration]
????????m?=?maxNum(a,b);
????????????^
1?warning?generated.