问题
今天督促队友写代码，他反馈移植的OELD代码报错Symbol oled_pow multiply defined。我查看了一下，发现在main.c和SR04.c中都引用了OLED（#include "oled.h"）。如果将超声波代码里面的#include "oled.h"注释掉就没有报错了，我查看了他头文件，发现加上了#ifndef __OLED_H_#define __OLED_H_#endif。测试了几次还是一大堆的Symbol oled_pow multiply defined 报错。
报错
Robot_competition\Robot_competition.axf: Error: L6200E: Symbol F6x8 multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol F8X16 multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol Hzk multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol BMP multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol BMP1 multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol BMP2 multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol BMP3 multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol IIC_Start multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol IIC_Stop multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol IIC_Wait_Ack multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol OLED_Clear multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol OLED_Display_On multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol OLED_DrawBMP multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol OLED_GPIO multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol OLED_Init multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol OLED_Set_Pos multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol OLED_ShowCHinese multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol OLED_ShowChar multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol OLED_ShowNum multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol OLED_ShowString multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol OLED_WR_Byte multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol Write_IIC_Byte multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol Write_IIC_Command multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol Write_IIC_Data multiply defined (by main.o and sr04.o). Robot_competition\Robot_competition.axf: Error: L6200E: Symbol oled_pow multiply defined (by main.o and sr04.o).
尝试过程
（1）于是我开始加入extern关键字，发现依旧无用。
（2）然后我就开始查看他的OLED代码，发现OLED代码似乎也没有问题。
（3）然后我突然想到了去字模库看看。后面发现，他将所有的字模变量定义都放在了.h文件。
（4）然后我突然想起我也遇到过类似的问题，然后搜索了一下，在头文件定义变量有什么问题。结果是得到： 头文件中不可以放变量的定义 ！ 一般头文件中只是放变量的声明，因为头文件要被其他文件包含#include，如果把定义放在头文件的话，就不能避免多次 定义变量 。
结论
我们的OLED字模变量定义应该放在.C文件中，而不是.h文件。只需要把.h文件剪切到.C文件即可。
