Pengge C language sixth class

One . Original code , Inverse code , Complement code

Integer in C Language is stored in binary code . Unsigned integer binary code unsigned bit , Only numeric digits ; The highest bit of binary code with signed integer is the sign bit , The following is numeric digit .0 For integers ,1 It's a negative number . Positive integer complement （ The original code, inverse code and complement of positive integer numbers are the same ） stored , Negative integer numbers are stored in complement .

From the literal value of signed integer to the original code ：

         The symbol is +, Symbol bit 0;

        The symbol is -, Symbol bit 1.

The original code, inverse code and complement of positive integer numbers are the same . The original code of negative integer numbers is in addition to the sign bit , According to the not , Get the inverse ; Counter code plus 1, Get the complement .

Negative integer numbers are stored in memory in the form of complements , This is for the convenience of addition （ Subtraction ） operation . But when translated into literal quantity , The original code is used （ Will translate the complement into the original code ）.

Consider the following code ：

#include <stdio.h>
int main() {
	int a = 0;//00000000 00000000 00000000 00000000
	int b = ~a;   // This assignment statement means “11111111 11111111 11111111 11111111” yes b In memory form （ Complement code ）
	printf("%d\n",b);// The result is -1
	return 0;
}

Two .auto,register,static keyword ,typedef Preliminary usage of keywords

typedef： type （ heavy ） Define the operator .

One of the simplest functions is to redefine types

typedef unsigned int u_int;
unsigned int a = 10;//unsigned int It may be too long 
// Equivalent to  u_int a = 10;

// Use typedef A common mistake of is to associate it with #define confusion , I didn't add it later ;

The problem of the redefined scope of use will not be discussed for the time being .

auto： All local variables default to automatic （automatic） Variable . meaning ： Enter the scope and automatically create , Out of scope auto destroy . Such as ：auto int a=10; however auto Generally omit . So it seems ,auto Only applicable to local variables , It reflects the relationship between the life cycle of local variables and their scope .

static： There are three situations .

        1. Modify local variables

The difference in auto, Changing the life cycle of local variables . Once the local variable is created , Can survive until the end of the program . 

        2. Modify global variable

Changed the scope of local variables . Is a global variable that can only be used in the source file that defines it . This is very useful for multi file programming when setting variable access permissions between files

        3. Modify function

Similar to global variables . Changed the link properties of the function . Can be called by a source file other than the one that defines it , External link properties called functions ; Can only be called by the source file that defines it , Internal link attribute called function .

Functions should also be declared before use , And the function is externally linkable by default . Add... Before it static Cancel its external link attribute , Make it an internal link function .

（ From the study here, we can find , The previous understanding of the life cycle of local variables is incorrect ： Previously, it was thought that the life cycle of local variables starts from entering its scope , To leave its scope . This is not comprehensive , Not thinking about static The condition of embellishment ）

// Source 1.cpp
int sum(int a, int b) {
	return a + b;

}
int a = 10;

// Source 2.cpp
#include <stdio.h>
int sum(int, int);// Functions and variables must be declared before use 
extern int a;
int main() {
	
	printf("%d\n",sum(1,2));
	printf("%d\n",a);
	return 0;
}

// correct , Can run 

// if sum and a add static keyword , Running source 2.cpp An unresolved error will occur when 
// Source 1.cpp
static int sum(int a, int b) {
	return a + b;

}
static int a = 10;

3、 ... and .#define Defining macro

#define Previously used to define identifier constants . Now used to define macros . Macros can be used to replace some simple functions .

#include <stdio.h>
#define MAX(X,Y) X>Y?X:Y
int main(){
    int max=MAX(1,2);
    printf("%d\n",max);
    return 0;
}

Put it further later

Four . Origin of pointer and address

First, introduce the origin of the address . The operating system is divided into 32 Bit and 64 position . With 32 For example, bit operating system ,32 Bit means 32 Root address line .32 The root address line means there is 2^32 Different addresses . These addresses correspond to the unique memory unit . Each such address unit is 1 byte, thus ,2^32 A memory unit means 4G Of memory .

This is the origin of the address .

Here is the pointer . Pointers are variables used to store addresses . The data type of pointer variable can be ：int*, char*, double* wait . The memory occupied by pointer variables depends on the type of operating system ,64 The pointer of the bit system occupies 8bytes,32 The pointer of the bit system occupies 4bytes. Related operators are ：& Fetch address ;* Indirect access operators （ Dereference operator ）