Part II - C language improvement_ 11. Pretreatment

C The four steps of language processing for source program ： Preprocessing 、 compile 、 assembly 、 link .

Preprocessing is before the program source code is compiled , By preprocessor （Preprocessor） Processing of program source code . This process does not parse the source code syntax of the program , But it will split or process the source code into specific symbols to prepare for the next compilation .

11.1 The file contains instructions (#include)

11.1.1 The file contains processing

“ The file contains processing ” It means that one source file can include all the contents of another file .Ｃ Language provides #include The command is used to implement “ File contains ” The operation of .

11.1.2 #incude<> and #include"" difference

1. "" The presentation system First stay file1.c Current directory found file1.h, If you can't find it , Again Search according to the directory specified by the system .
2. < > The presentation system direct Search according to the directory specified by the system .

Be careful ：

1. #include <> It is often used in header files containing library functions ;

2. #include "" It is often used to include customized header files ;

3. Theoretically #include Can contain files of any format (.c .h etc. ) , But it's usually used for The header file The inclusion of ;

11.2 Macro definition

11.2.1 Macro definition without parameters ( Macro constants )

If it is widely used in the program 100 This value , So for the convenience of management , We can define it as ：

const int num = 100; But if we use num Define an array , No support for c99 Standard compilers do not support , because num Not a compiler constant , If you want to get a compiler constant , Then you can use ：

#define num 100

During compilation preprocessing , Put all the num Use both 100 Instead of . This method allows the user to replace a long string with a simple name , The process of replacing the macro name with a string at precompile is called “ Macro expansion ”. Macro definition , It only works in the file of macro definition .

#define PI 3.1415
void test()
{
	double r = 10.0;
	double s = PI * r * r;
	printf("s = %lf\n", s);
}

Output results

s = 314.150000

explain ：

1. Macro names are usually capitalized , To distinguish it from variables ;
2. Macro definitions can be constants 、 Expression etc. ;
3. Macro definitions are not checked for syntax , Only after the compilation is expanded, the source program will report an error ;
4. The valid range of macro name is from definition to the end of this source file ;
5. It can be used #undef Command to terminate the scope of the macro definition ;
6. In the macro definition , You can refer to the defined macro name ;

11.2.2 Macro definition with parameters ( Macro functions )

In the project , Often write some short and frequently used functions as macro functions , This is because macro functions do not have ordinary function parameters on the stack 、 Jump 、 The cost of returning and so on , It can improve the efficiency of the program .

Macros use parameters , You can create function like macros with shapes and functions similar to functions . Macro parameters are also enclosed in parentheses .

#define SUM(x,y) (( x )+( y ))
void test()
{
	// Just do text substitution   The following example is replaced by  int ret = ((10)+(20));
	int ret = SUM(10, 20);
	printf("ret:%d\n",ret);
}

Output results

ret:30

Be careful :

1. The name of macro No spaces in , But there can be spaces in the replaced string .ANSI C Allow spaces in parameter lists ;
2. Enclose each parameter in parentheses , And the overall definition of the macro .
3. Use capital letters to represent the function name of the macro .
4. If you plan to use macros instead of functions to speed up the program . If you only use macros once in the program, the running time of the program will not be greatly improved .

11.3 Conditional compilation

11.3.1 Basic concepts

In general , All the lines in the source program are compiled . But sometimes you want to compile some source program lines only when certain conditions are met , That is, specify the compilation conditions for this part of the source program line .

11.3.2 Conditional compilation

Prevent header files from being repeatedly referenced ;

#ifndef _SOMEFILE_H
#define _SOMEFILE_H

// Variables that need to be declared 、 function 
// Macro definition 
// Structure 

#endif

11.4 Some special booking macros

C compiler , Several special forms of predefined macros are provided , It can be directly used in practical programming , Very convenient .

//	__FILE__			 The source file name of the macro file  
//	__LINE__			 The line number of the macro's line 
//	__DATE__			 The date the code was compiled 
//	__TIME__			 The time the code was compiled 

void test()
{
	printf("%s\n", __FILE__);
	printf("%d\n", __LINE__);
	printf("%s\n", __DATE__);
	printf("%s\n", __TIME__);
}

Output results

E:\TortoiseGit-2.8.0.0-64bit\cs_-c\C_2022_07_26\C_2022_07_26\test.c
40
Jul 26 2022
11:03:04