C language: 14. Preprocessing

1、c Program execution

c Language programming executable files are divided into 4 Step ：

Preprocessing ： from .c The file to .i The process of file processing is called preprocessing
compile ：.i File generation .s The process of file compilation
assembly ：.s File generation .o The process of documentation is called assembly
link ：.o The process of generating executable files from files is called linking

2、 Preprocessing

The parameters below -E Express gcc Just do pretreatment .
The following command will helloworld.c Pretreatment into a helloworld.i Preprocessing files .
gcc -o helloworld.i helloworld.c -E
The following figure shows the header file stdio.h And hellworld.c The code in the file is carried out and merged .

The first thing preprocessing does is to expand the header file , At the same time, macro replacement should also be carried out during preprocessing .

2、 Macro replace

#include <stdio.h>
#define R 10

int main()
{
    
    int a = R;
    printf("a=%d\n", a);
    printf("hello world");
    return 0;
}

Preprocess the above code , Check the preprocessing file again , Here's the picture

It can be seen from the above figure , Pretreatment stage , The macro will be replaced with the corresponding value by the compiler .

Be careful ： Macro replacement is just a simple string replacement , Will not consider c Grammatical , As code below .

#include <stdio.h>
#define M int main(
M){
    
	printf(" At the top of the M Will be replaced by int main(\n");
	return 0;
}

The above code has been preprocessed gcc -o helloworld.i helloworld.c -E after ,helloworld.i The content in is as follows

2.1、 Macro functions

#include <stdio.h>
#define R 20
#define M int main(
#define N(n) n*10

M){
    
    int a = R;
    printf("a=%d\n", a);

    int b = N(a);//N(a) replaced a*10
    printf("b=%d\n",b);
    return 0;
}

The result of code preprocessing above , Here's the picture ：

2.2、 Macro function considerations （ Operator priority ）

Because macro functions directly replace characters , So you need to pay attention to the errors in the figure below

The macro function definition above must be bracketed

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

int main(){
    
    int b = ADD(2,3) * ADD(2,3);
    printf("b=%d\n",b);
    return 0;
}

2.3、 Advantages of macro functions

2.4 Conditional compilation

@todo