這是hello.c文件源碼：

#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;

}

這種情況下輸入命令：gcc hello.c max.o min.o 然后回車為什么會：

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);