Linux C language: Pretreatment

1. What is pretreated

Compiling divided into four steps:
① .c file generation .i file, pretreatment;
② .s .i file generation file, the compiler;
③ .s .o file file generation, compilation;
④ .o file an executable file, link;

File: $ directly to the bottom of the file;

gcc -o main.i main.c -E

Generate a preprocessed file, -E representation only pretreatment;,
see the preprocessed file onlyvi main.i

The first thing to do pre-treatment: Expand the header file, the file can be displayed in .i, no longer appears in the form of include; then macro replacement.

2. Pretreatment of macros

#define R  10 

The second step is the macro substitution pretreatment such int a = R;a statement such as replacingint a = 10;

Do not think this is 10 int, 10 and think this is a string, the replacement process simple string to replace it;

Nature macro: string replacement occurs in pretreatment stage;

Example:

#include<stdio.h>
#define R 10
#define M int main()

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

Programs can also run out.

In the pretreatment stage, does not consider the macro syntax compiler;

3 Pretreatment of macro function

#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);
	printf("b=%d\n",b);
	return 0;
}

operation result:

linux@ubuntu:~/workspace2/les1$ ./a.out
a=20
b=200

.i file int b=N(a);has been replacedint b=a*10；

#define ADD(a,b) a+b
int main()
{
	int a=20;
	int b=200;
	int d=ADD(a,b);
	int e=ADD(a,b) * ADD(a,b);
}

The results of d may fulfill the functions of ADD function;
desired result e is 48400, the actual result is 4220;
Reason: .i file macro substitution result int e= a+b*a+b；, computation time after the compilation will be executed, in the preprocessing stage It does not perform an arithmetic operation.
.C files need to be changed int e= (ADD(a,b))*(ADD(a,b));
(YY replication, p is pasted, u is withdrawn, ctrl + r revoked canceled);

? Macro function with the normal function of any difference
macro function does not return type and parameter types, the type can not be considered;

In the preprocessing stage, in addition to the macro, also it provides a conditional compilation functions may be different according to different conditions to compile part of the program, resulting in different code files, to the transplant procedure and for debugging.

4 . typedef

typedef is a keyword, you can give individual names from the variable type;

typedef int tni;

typedef int* p;
p q=NULL;// 写法等同于int *q = NULL;

Int * to type the name of the individual from p;

typedef and macro difference:
results of a semicolon 1.typedef;
2.typedef the pretreatment is not to be replaced, the file is not replaced .i;
below 3 can be used after the macro definition, have the effect typedef ? domain, the general body of the function in this
typedef usually give their own custom data types from the alias:

typedef unsigned long size_t;

Unused structure typedef:

struct stu{
};
struct stu a;

Structure using typedef:

typedef struct stu{
}stu_t;
stu_t a;