C language preprocessing instruction

0. List

Instructions	explain
#include	Contains a file
#define	Defining macro
#undef	Cancel defined macro
#ifdef	If the macro has been defined , Then compile the following code
#ifndef	If the macro is not defined , Then compile the following code
#endif	End one #if……#else Conditional compilation block
#if	If the given condition is true , Then compile the following code
#elif	If the front #if Given conditions are not true , The current condition is true , Then compile the following code
#else	coordination #if Use
#	Stringize operator, Is to string macro parameters
##	Token pasting
#pragma	The most complicated , There is no way to elaborate here
#error	Report an error and stop precompiling
#warning	Warn and continue precompiling

1. brief introduction

C The language preprocessing instruction is a declaration （ Text ） Substitution expression , It uses # start , And it must be the first character ,# Next is the instruction keyword , In keywords and # Any number of white space characters are allowed between , The entire line of statements constitutes a processing instruction , This instruction performs some conversion of the source code before the compiler compiles .
C Preprocessing instructions are not C Part of the compiler , It's different from the standard C There are some grammatical differences , for example ：

• It uses # start .
• It is based on the behavior unit , Each statement has a separate line , But standard C It's not compulsory .
• It ends with a new behavior , But the standard C To use ; ending .

2. Instructions

2.1 #include

Used to import another file , Mainly header files . There are two ways

• #include <file.h>： from C Find the header file in the standard library of
• #include "path_to_header_file"： Look in the current directory , Then go again. C Find in the standard library , Mainly customized header files

2.2 #define

Macro definition , Given value / Constant / Expression defines a name , Format
#define MACRO_NAME macro_body
among MACRO_NAME yes C identifier , and macro_body Is string / value / expression .
give an example ：

#include <stdio.h>
// MACRO definition
#define COUNTRY "CHINA" //  String constant 
#define TRUE 1 //  integer constants 
#define FALSE 0 //  integer constants 
#define SUM(x,y) ((x)+(y)) //  Macro definition 
int main(void)
{
    
	printf("COUNTRY: %s\n", COUNTRY);
	printf("TRUE: %d\n", TRUE);
	printf("FALSE: %d\n", FALSE);
	printf("SUM(10+20): %d\n", SUM(10, 20));
	return 0;
}

result ：

COUNTRY: CHINA
TRUE: 1
FALSE: 0
SUM(10+20): 30

2.3 #undef

and #define The positive sign is opposite , Used to remove macro definitions , If it's already defined , The remove , If there is no definition , It doesn 't matter . Format ：#undef MACRO_NAME
give an example ：

#include <stdio.h>
// MARCO definition
#define TRUE 1
#define FALSE 0
int main(void)
{
    
	printf("TRUE: %d\n", TRUE);
    printf("FALSE: %d\n", FALSE);

    // Undefine a previously defined macro
    #undef TRUE
    #undef FALSE

    // Re-define macro values
    #define TRUE 0
    #define FALSE 1

    printf("\nMacro values are redefinition\n");
    printf("TRUE: %d\n", TRUE);
    printf("FALSE: %d\n", FALSE);

    return 0;
}

result

TRUE: 1
FALSE: 0

Macro values are redefinition
TRUE: 0
FALSE: 1

If in #undef After use, an error will be reported :

ex1.c: In function 'main':
ex1.c:14:26: error: 'TRUE' undeclared (first use in this function)
     printf("TRUE: %d\n", TRUE);
                          ^~~~
ex1.c:14:26: note: each undeclared identifier is reported only once for each function it appears in
ex1.c:15:27: error: 'FALSE' undeclared (first use in this function); did you mean 'FILE'?
     printf("FALSE: %d\n", FALSE);
                           ^~~~~
                           FILE

2.4 #ifdef, #ifndef, #endif

This is a preprocessing instruction with conditions , Used to check whether macros are defined . among #ifdef and #ifndef It's complementary .

• #ifdef： If macros have been defined , Then compile the following code , Format ：#ifdef MACRO_NAME
• #ifndef： If no macro is defined , Then compile the following code , Format ：#ifndef MACRO_NAME
• #endif： End the above code , Matching use of , One for one , There must be
  give an example ：

#include <stdio.h>
// MACRO definition
#define COUNTRY "CHINA"

int main(void)
{
    
	#ifdef COUNTRY
		printf("Country is defined\n");
	#endif
	
	#ifndef PROVINCE
		printf("PROVINCE is not defined. Defining PROVINCE... \n");
		#define PROVINCE "HN"
	#endif
	printf("Province is: %s\n", PROVINCE);
		
	return 0;
}

result

Country is defined
PROVINCE is not defined. Defining PROVINCE...
Province is: HN

The main purpose of these instructions is to prevent a header file from being loaded repeatedly many times . For example, there is a custom header file myfile.h, In multiple files *.c Use... In the document , In order to prevent multiple loading , This file can be defined as follows

#ifndef _MY_FILE_H
	#define _MY_FILE_H
	//  Declare other macros 
	//  Declare variables and methods 
#endif

At the first load ,C The preprocessor will check that there is no definition _MY_FILE_H, Will execute the definition , And declare other macros 、 Variables and methods , When loading and importing for the second time , It's already defined _MY_FILE_H, There will be no restatement .

2.5 #if, #elif, #else and #endif

These instructions are similar C In language if...else... sentence , For conditional compilation . Format ：

#if expression
	// If condition is true
#elif expression
	// If else if condition is true
#else
	// If no condition is true
#endif

give an example ：

#include <stdio.h>

#define IND 1
#define USA 2
#define UK 3

#define COUNTRY IND

int main()
{
    
	#if COUNTRY == IND
		printf("Selected country code is: %d\n", COUNTRY);
        // Do some task if country is India
	#elif COUNTRY == USA
		printf("Selected country code is: %d\n", COUNTRY);
        // Do some task if country is USA
	#else
		printf("Selected country code is: %d\n", COUNTRY);
        // Do some task if country is UK
	#endif
	
	return 0;
}

2.6 #,##

These are two special C Preprocessing instruction , Is used to process strings .

• #, Used to convert macro parameters into strings , Make it unnecessary to use double quotation marks to enclose the display , Format ：#define MACRO_NAME(param) #param, give an example ：

#include <stdio.h>

#define PRINT(msg) #msg
int main(void)
{
    
	printf(PRINT(I love programming));
	return 0;
}

result ：

I love programming

• ##, Used to connect two data , Put them together as strings . Format ：```#define MACTRO_NAME(param1, param2) param1##param2, give an example ：

#include <stdio.h>
#define CONCAT(a,b) a##b

int main(void)
{
    
	printf("CONCAT(10, 20) = %d\n", CONCAT(10, 20));
	return 0;
}

result ：

CONCAT(10, 20) = 1020

2.7 #pragma

In all preprocessing instructions ,#pragma Instructions are probably the most complex , Its function is to set the state of the compiler or instruct the compiler to complete some specific actions .#pragma Instructions give a method to each compiler , In keeping with C and C++ When the language is fully compatible , Give host or operating system specific features . By definition , Compilation instructions are machine or operating system specific , And it's different for each compiler .
Format ：#pragma Para

2.7.1 message

message Parameters can output corresponding information in the compilation information output window , This is very important for the control of source code information . Format ：#pragma message("Text"). When the compiler encounters this instruction, it prints out the message text in the compile output window . give an example ：

#ifdef _X86
#pragma message("_X86 macro activated!")
#endif

2.7.2 code_seg

Format ：#pragma code_seg(["section-name"[, "section-class"]])
It can set the code segment stored in the function code in the program , It will be used when developing drivers .

2.7.3 once

This is more commonly used , It is mainly used to add this instruction at the beginning of the header file to ensure that the header file is imported and compiled only once . and #ifndef That's similar to , But this has compatibility problems , Some compilers do not support , Some already supported are also considering whether to continue to support .
The rest , When I use , Add more specific explanations . You can also refer to Baidu Encyclopedia 了 .

2.7.4 hdrstop

2.7.5 resource

2.7.6 warning

2.7.7 comment

2.7.8 disable

2.7.9 data_seg

2.7.10 region

3. quote

1. C preprocessor directives – #include, #define, #undef and conditional directives
2. C Summary of language preprocessing instructions
3. Stringize (#) and token pasting (##) operator in C language
4. C Language: Preprocessor Directives
5. #pragma