LinuxC语言中调用函数之前没有函数声明会怎么样？

1.代码如下：

#include <stdio.h>
int main(void)
{
  printf("Three lines:\n");
  threeline();
  printf("Another three lines.\n");
  threeline();
  return 0;
} 

void newline(void)
{
  printf("\n");
} 
void threeline(void)
{
  newline();
  newline();
  newline();
}

2.执行结果及解释如下：

$ gcc main.c
main.c:17: warning: conflicting types for ‘threeline’
main.c:6: warning: previous implicit declaration of ‘threeline’ was here

参考：<Linux C 一站式编程>

3.其它eg