Day 8 of learning C language

Binary

All files are ultimately binary

Text and binary

The advantage of text is that it is easy for people to read and write . And cross platform

The disadvantage of text is that the program input and output must be formatted , Spending big

The disadvantage of binary system is that it is difficult for human to read and write , And not cross platform

int The sizes of are inconsistent . The problem of big and small end ...

The advantage of binary is that the program reads and writes fast

Locate... In the file

long ftell(FILE*stream);

int fseek(FILE*stream,long offset,int whence);

SEEK_SET : Start from scratch

SEEK_CUR : From the current position

SEEK_END : Start at the end ( Come back )

Bitwise operation

C Bitwise operators of language :

&	Bitwise AND
|	Bitwise OR
~	According to the not
^	Bitwise XOR
<<	Move left
>>	Move right

Bitwise AND &

If (x)i ==1 also (y)i ==1, that ( x & y)i = 1

Otherwise (x & y) i= 0

Bitwise and are commonly used in two applications :
Let one or more bits be 0: x & 0xFE

Take a paragraph from a number :x & 0xFF

Press bit or |

If (x)i == 1 or (y)i == 1, that (x | y)i = 1

Otherwise , (x | y)i == 0

Bitwise OR commonly used in two applications :

Make one or more persons 1: x | 0x01

Put the two numbers together : 0x00FF | 0xFF00 

According to the not ~

(~x)i = 1 - (x)i

hold 1 A into 0,0 Displacement 1

Want to get all the bits as 1 Number of numbers :  ~0

7 The binary of is 0111, x|7 Make low 3 Position as 1

x& ~7, It makes it low 3 Position as 0

Bitwise XOR ^

If (x)i ==(y)i, that (x^y)i = 0

Otherwise , (x ^ y)i == 1

If the two bits are equal , So the result is 0; It's not equal , The result is 1

If x and y equal , that x^y As the result of the 0

XOR a variable twice with the same value ,  It's like doing nothing

x ^ y ^ y---> x

Move left

i<<j

i  All the bits in the move to the left  j  A place ,  And the right side is filled with 0

All less than  int  The type of , Shift to int The way to do it , The result is  int

x<<=1 Equivalent to x *=2

x<<=n Equivalent to x *=2^n

Move right

i >> j

i All the positions in the move to the right j position

All less than int The type of , Shift to int The way to do it , The result is int

about unsigned The type of ,  Fill in on the left 0

about signed The type of , Fill in the original highest position on the left ( Leave the sign unchanged )

x>>=1 Equivalent to x /= 2

x>>=n Equivalent to x/=2^n

Logical operation and bitwise operation

For logical operations , He only sees two values : 0 and 1

It can be considered that logical operation is equivalent to all non 0 It's worth it 1, Then do bitwise operations

5 & 4 --->4 and 5&&4---->1&1---->1

5 | 4--->5 and 5 || 4---->1 | 1--->1

~4--->3 and !4--->!1---->0

Bit segment

Can be accessed directly with the member name of the bit field

Specific shift . And Or it's convenient

The compiler arranges the bits , Not portable

When the required bits exceed int Multiple... Will be used int