Pointer ( key of c )
Memory is an operating system abstraction of physical memory - virtual memory.
The virtual memory can be viewed as a continuous small squares, each square is the size of one byte (byte) = 8 bit, can store a binary number of eight, each of the small box 1 will have a number , the virtual memory size is 2 2gb range 1024 ^ 3 byte number from 0 to 1024 ^ 3-1 
pointer about analysis: pointer is a variable, the value may become his, his address which is stored Suppose we define a pointer to int p; int a; if p = & a; this time we know, a is a variable, then the memory must give him assign an address to store him, p points to a meaning that is p kept inside is a address . So p is a deposit with variable addresses. Now that we know which kept a p address, so we are looking for a very convenient, * p value of p it means the address stored inside, which is the value of a. This sentence is how to understand it?
~ .p address is stored, compared to where you live, then p to represent you, because p would indicate that the address data inside. This time p and a is exactly the same, and if you want to change a value. a = a + 1 and P = P +. 1 is exactly the same, to achieve the purpose of operating a. However, p = p + 1 and p = (p + 1) is not the same, this time with special attention because p which is the address, then (p + 1) says that after this address plus 1, address plus 1 is not that a change of address? After a change of address on the inside is certainly not a deposit, just may be your neighbors. Because the address has changed, so p is changed, and therefore * has changed, this place is a little difficult to understand, the landlord more than pondering pondering. To sum it up: A pointer is a master key, can point to any address, you can change any address inside the data (except for read-only), using pointer pay attention to safety, in order to avoid an exception occurs.
1, the operator &
scanf ( "% d", & i); // here is an operator &
· Role: to obtain the address of a variable, its operand must be a variable.
· In the C language, sizeof () is a data type determines the length of the operator or expression
·int i；printf("%x",&i);
`int i; printf ("% p ", & i); // address output by the address and size of the int% p depends extended compilers:% X means is a hexadecimal number in the form of integer output
1.% c: A character is read
2.% d: decimal integer
3.% f,% F,% e,% E,% g,% G for inputting a real number, or can be entered as a decimal exponent
4.% o: octal
5.% s: read into a string, the case of spaces, tabs, or new end.
6.% u: unsigned decimal number
7 %%: percent sign% output
8.% i read in decimal, octal, hexadecimal integer
9.% p a read pointer
10.% n characters equivalent to this value has been read
11% [] set of characters scanned
12.% a,% A reads a floating point value (valid only C99)
& Can not be advanced after taking out the address of something without an address, top-down stack
a, a pointer is a variable holding address
int i;
* p = i & int; : p represents a pointer to int, to the address of i p p represents the value of i. p i at the address where the value of that variable, then said p points to i.
* P int, Q; int P, Q; // P is an int, q is not a pointer. Whether and int and p are close or relying on the past. int P, Q *; case p, q is a pointer to the.
p is an int numbers, p * p is a pointer.
· P is the name of a pointer variable indicates the memory address of the pointer variable points to, if you use% p to output, it will be a hexadecimal number
· * P represents the content of this memory address pointer stored in, and is generally consistent with a pointer type variables or constants
· Is & address operator, is to take the address pointer p & p,
b, value is the memory address of the variable.
· Common variable is the value of the actual value.
· Pointer variable value is the address of a variable with real values.
c、
void f (int * p); f need int pointer // get the address of a variable is called at the time.
int i = 0; f (& i); if this function is called when it should be left to an address with the address & made variable, this variable is then passed to the pointer. // this i can access the outside through this pointer in a function.
#include <stdio.h>
void f(int *p);
void g(int k);
int main(void)
{
int i = 6;
printf("&i=%p\n", &i); //&i：地址值&i被传进了函数f中
f(&i);
g(&i);
return 0;
}
void f(int *p) //p：通过这个地址
{
printf(" p=%p\n", p);
printf("*p=%d\n", *p);
*p = 26; //*p：用*p这种方式访问到i变量
} //指针就是保存了内存的一个地址（门牌号），通过这个地址就能够访问对于内存中的数据
void g(int k)
{
printf("k=%d\n", k) //***p就是i=6的值，而p=&i，所以p和&i都是地址。 \*地址=值 注：\*p可以是和指针类型相同的变量也可以是常量**
} //p是i的地址，*p就是i（的值）。
d、
* Is a unary operator, - when the definition is merely illustrative of pointer variables
· Access to this variable pointer stored address
· Pointer address used to access the value of the variable on the address indicated.
· Can be left value may also be the right value.
·int k=*p;
*p=k+1;
f, value left
An assignment on the left is not a variable, but the value is the result of the calculation expression.
a [0] = 2; * p = 3; these two variables are not left.
g, reaction
& yptr is -> (& yper) -> (yper address) -> both yper reaction is to represent the original that variable & yper -> & (* yper) -> & (y) -> get y address -> yper
h, incoming address
· I = 6; int i; scanf ( "% d", i); this way is input to the compiler not being given, then the operation will be an error in the 32-bit integer as large as the address and the compiler mistaken i address is passed, the actual upload into the value of i is 6.
Practical application i, the pointer
I, the exchange value of two variables
#include <stdio.h>
void swap(int *pa, int *pb);
int main()
{
int a = 5;
int b = 6;
swap(&a,&b); //将a b的地址传递过awap函数中
printf("a=%d b=%d\n", a, b);
return 0;
}
void swap(int *pa, int *pb) //参数是两个指针，如果不用指针，那么从main函数传来的就是值，而不是变量。
{
int t=*pa; //*pa取出pa所代表的变量，赋值给t
*pa=*pb;
*pb= t;
}B, two application function that returns a pointer operation state, the result returned by the pointer