The macro definition in the header file in C language,

The macro definition in the header file in C language,

I don't know what it means when I see Conditional compilation in the header file, but the master says, "I have to write it like that." I did it, but I don't know why.

Okay, I admit that I am just a code Porter. Oh, no, maybe not.

The following is your understanding after learning ~~~

Code:

1 # ifndef DSP_ADC_BSP_H _ // prevent header files from being repeatedly contained 2 # define DSP_ADC_BSP_H _ 3 4 # ifdef _ DSP_ADC_BSP_C _ // determine whether the file is defined 5 # define ADCBSPEXT // 6 # else 7 # define ADCBSPEXT extern // when undefined, execute 8 # endif // _ DSP_ADC_BSP_C _ 9 10 ADCBSPEXT void InitileAdc (void ); 11 ADCBSPEXT void CaptureAdcInt (); 12 ADCBSPEXT unsigned char CaptureAdcInt_data (); 13 14 # endif/* DSP_ADC_BSP_H _*/

Maybe I am a zombie. In short, I did not.

Explanation:

Rows 1st, 2, and 14th: avoid repeated definitions.

# Ifndef DSP_ADC_BSP_H _ // If DSP_ADC_BSP_H _ is not defined, run the following command:
# Define DSP_ADC_BSP_H _ // defines DSP_ADC_BSP_H _
...
# Endif/* DSP_ADC_BSP_H _ * // The annotation indicates the corresponding # ifndef command to the programmer.

When this file is contained for the first time, the macro DSP_ADC_BSP_H _ is not defined, and the Preprocessor is allowed to retain multiple lines between # ifndef and # endif.

If the file is contained again, the Preprocessor deletes the content between # ifndef and # endif.

This means that if two. c files contain the. h file at the same time, it will be defined only once.

3rd ~ Row 12: external functions and variables can be used in other modules only once in the. h file.

First, describe extern:

Extern int I;

The declared information provided by the compiler is that I is an int variable, but the compiler does not allocate storage units for I. The above Declaration is not a definition of variables, but a prompt that the compiler needs to access the variables defined elsewhere.

Variables can be declared multiple times in the program, but can only be defined once.

# Ifdef _ DSP_ADC_BSP_C _ // judge whether _ DSP_ADC_BSP_C _ is defined
# Define ADCBSPEXT // if the execution is defined
# Else
# Define ADCBSPEXT extern // if no execution is defined
# Endif

ADCBSPEXT void InitileAdc (void );
ADCBSPEXT void CaptureAdcInt ();
ADCBSPEXT unsigned char CaptureAdcInt_data ();

The. c file of the corresponding module contains the following sentence:

# Define _ DSP_ADC_BSP_C _ // defines _ DSP_ADC_BSP_C _

When this module. c file is used, the ADCBSPEXT is replaced with a blank one.

The function declaration is:

void InitileAdc(void);void CaptureAdcInt();unsigned char CaptureAdcInt_data();

When these functions are used for other. c files, ADCBSPEXT is replaced with extern for use.

extern void InitileAdc(void);extren void CaptureAdcInt();extern unsigned char CaptureAdcInt_data();

  

One cainiao for your reference only.