The following code is a single-chip program, 51 single-chip, compiler HT-IDE3000,

Simply speaking

The header file can only state,

When variables are declared in the header file, the keyword extern should be added to tell the compiler that variables are defined in other files,

enum is not a variable, but a data type defined by ourselves. Like int char, it's just defined by ourselves, so we don't need to add,

 1 extern u16    g_u16TimeBaseCount; 
 2 extern u8    g_8count_2s;
 3 extern u8    g_8count_8s;
 4 extern u8    g_8count_18s;
 5 extern u8    g_8count_60s;
 6 extern u8    g_8count_200s;
 7 extern u8    g_8count_6h;
 8 extern u8   g_8count_200sw;//6hour wash
 9 extern u8   g_8count_6hw;//6hour wash
10 extern u8   w_6hw_start;
11 extern u8   w_6hw_end;
12 extern u8   g_state;
13 extern u16    make_6h_count;
14 extern u8   g_once;
15 extern u16  s_c1;//Intermediate variable
16 extern u16  s_c2;
17 extern u16  s_c3;
18 extern u16  s_c4;
19 
20 extern u8 UARTReceive;
21 extern u8 AD[6];
22 extern u8 TDS[6];
23 extern u8 count_AD;  
24 extern u8 count_ADS;       
25 extern u16 sumTDS_H,sumTDS_L,sum;
26 extern u16 temer;
27 
28 typedef enum GPIOPin
29 {
30   GPIO_Pin_0    = ((u8)0x01),   /*!< Pin 0 selected */
31   GPIO_Pin_1    = ((u8)0x02),   /*!< Pin 1 selected */
32   GPIO_Pin_2    = ((u8)0x04),   /*!< Pin 2 selected */
33   GPIO_Pin_3    = ((u8)0x08),   /*!< Pin 3 selected */
34   GPIO_Pin_4    = ((u8)0x10),   /*!< Pin 4 selected */
35   GPIO_Pin_5    = ((u8)0x20),   /*!< Pin 5 selected */
36   GPIO_Pin_6    = ((u8)0x40),   /*!< Pin 6 selected */
37   GPIO_Pin_7    = ((u8)0x80),   /*!< Pin 7 selected */
38   GPIO_Pin_LNib = ((u8)0x0F),   /*!< Low nibble pins selected */
39   GPIO_Pin_HNib = ((u8)0xF0),   /*!< High nibble pins selected */
40   GPIO_Pin_All  = ((u8)0xFF)    /*!< All pins selected */
41 }GPIO_Pin_TypeDef;

The function is also in the header file. It can only be stated that the function body cannot be added

u8 UART_Receive();

The implementation of variables and functions is the function body, which can only be defined in the source file

 1 uFlg uF1,uF2,uF3,uF4;
 2 u16    g_u16TimeBaseCount; 
 3 u8    g_8count_2s;
 4 u8    g_8count_8s;
 5 u8    g_8count_18s;
 6 u8    g_8count_60s;
 7 u8    g_8count_200s;
 8 u8    g_8count_6h;
 9 u8   g_8count_200sw;//6hour wash
10 u8   g_8count_6hw;//6hour wash
11 u8   w_6hw_start;
12 u8   w_6hw_end;
13 u8   g_state;
14 u16    make_6h_count;
15 u8   g_once;
16 u16  s_c1;//Intermediate variable
17 u16  s_c2;
18 u16  s_c3;
19 u16  s_c4;
20 u8 UARTReceive=0;
21 u8 AD[6]={0,0,0,0,0,0};
22 u8 TDS[6]={0,0,0,0,0,0};
23 u8 count_AD=0; 
24 u8 count_ADS=0; 
25 u16 sumTDS_H=0,sumTDS_L=0,sum=0;
26 u16 temer;
27 
28 //===========================Data reception=========================================//
29 u8 UART_Receive()
30     {
31        u8 data;
32        if(_rxif==1)//If data is received
33        {
34              data=_txr_rxr;
35              return data;
36            }
37      }