Program environment and pretreatment

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List of articles

1. The translation environment of the program

2. The execution environment of the program

3. Detailed explanation ：

   c Compilation of language programs + link

4. Explanation of predefined symbols

5. Preprocessing instruction #define

6. The comparison between macro and function

7. Preprocessing operators # and ## Introduction to

8. Command definition

9. Preprocessing instruction #include

10 Preprocessing instruction #undef

11. Conditional compilation

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1. Compilation and linking of programs

3. Pretreatment details

3.1 Predefined symbols

__FILE __   // Source files to compile

__LINE_ _   // The current line number of the file

__DATE_ _ // The date the file was compiled

__TIME_ _   // When the file was compiled

__STDC_ _// If the compiler follows ANSI C, Its value is 1, Otherwise, it is not defined

#include<stdio.h>
int main()
{
	printf("line:%d,date:%s,time:%s", __LINE__, __DATE__, __TIME__);
	return 0;

}

3.2

 grammar ：
    #define name stuff
 for instance ：
#define MAX 1000
#define reg  register
#define do_forever for(;;)
#define  CASE break;case
#define  bug   printf("line:%d\t \
                       date:%s\ttime:%s\n",\
                       __FILE__,__LINE__,     \
                       __DATE__,__TIME__)

 3.2#define Defining macro

 Macro declaration form 
#define name( Parameters ）stuff
 Be careful ：
 The left parenthesis of the argument list must be the same as name Next door neighbor 
 If there is any gap between the two , The parameter list will be interpreted as stuff Part of 

#define double(x)    x+x
#include<stdio.h>
int main()
{
	printf("%d", double(2));
	return 0;
}

Again ：

#define  SQUARE(x) x*x

If after the above statement , You put

SOUARE(5);

Put in the program , The preprocessor will replace the above expression with the following expression

5*5

There are some problems with this macro ：
Observe the following code snippet

int a=5;
printf("%d\n",SQUARE(a+1))

At first glance , You may think this paragraph is printed 36 This value , In fact, he can print 11.

Actually, it's because of the parameters x Replaced with a+1, It actually became   printf（"%d\n",a+1*a+1),

So you need to add two parentheses to the macro definition , In this way, the problem can be solved easily

#define SQUARE(x) (x)*(x)

printf("%d\n",(a+1)*(a+1));

There is also a macro definition

#define DQUBLE(x)   (x)+(x)
 We use parentheses in the definition , However, new errors may appear in this macro ：
int a=5;
printf("%d\n",10*DOUBLE(a));

This question looks like printing 100, But what is actually printed is 55.

We found that after the replacement ：

printf（"%d\n",10*(5)+(5));

The multiplication operation first adds with the macro definition , So it's here 55. So we need to add brackets

#define DOUBLE(x) ((x)+(x)), When writing macros in the future, you should put parentheses in this way , Avoid the unexpected consequences of operator priority .

3.2.4 ##

## Its function is to combine the symbols on both sides of it into one symbol

#define hu(class,num) class##num
#include<stdio.h>
int main()
{
   int class106=100;
    printf("%d",hu(class,106));
   return 0;
}

3.2.5 Macro parameters with side effects

#define MAX(x,y) ((x)>(y)?(x):(y))
#include<stdio.h>
int main()
{
	int a = 2;
	int b = 3;
	int c = MAX(a++, b++);
	printf("%d", c);
	printf("%d%d", a, b);
	return 0;
}

 

  So why is it such a result ？ We can get   (a++)>(b++)?(a++):(b++) First ( a++)>(b++)?a<b, then a++,b++,a b The values of are 3,4, Then return b++,c Is equal to b be equal to 4, then b++,b=5.

3.2.6  Macro and function comparison

Macros are often used to perform simple calculations , For example, find the larger of the two numbers .

#define MAX(a,b) ((a)>(b)?(a):(b))

Then why not use functions to perform this task

1. Macros are faster than functions

2. The parameters of a function must be declared as specific parameters , Macros are good for shaping 、 Long integer 、 floating-point

The disadvantages of macro ：

1. Macros cannot be debugged

2. Macros are type independent , It's not rigorous enough

3. Macros can cause operator priority problems

4. Every time you use a macro , A copy of the code defined by the macro is inserted into the program , Unless the macro is short , Otherwise, the length of the program may be greatly increased .

3.3#undef

This instruction is used to remove a macro definition

#define MAX 100
#include<stdio.h>
int main()
{
#undef MAX
	printf("%d", MAX);
	return 0;
}

 

 3.5 Conditional compilation ：

1.#if  Constant expression

     //

  #endif

Such as ：

#define__debug__1

#if__debug__

//

#endif

2. Conditional compilation of multiple branches

#if Constant expression

//

#elif    Constant expression

//else

//

#endif

3. Judge whether it is defined

#if defined（symbol） 

#ifdef symbol

#if ！defined（symbol）

#ifndef symbol

#define MAX 100

int main()
{
#ifndef MAX
	printf("max\n");
#endif

	return 0;
}

int main()
{
#if !defined(MAX)
	printf("max\n");
#endif

	return 0;
}
#define max 100
#include<stdio.h>
int main()
{
#ifdef max
	printf("%d", max);
#endif
	return 0;
}
#define max 100
#include<stdio.h>
int main()
{
#if defined ( max)
	printf("%d", max);
#endif
	return 0;
}