Embedded-c Language-1

One 、 about C Description of language

You can understand it as ： A language that enables people to communicate with computers , You write down the functions you want to achieve through the keyboard , Then there will be corresponding translation officers （ Compiler ） To help you translate , Let the computer understand what you think , Then the computer will mobilize its own resources to execute the commands you write .

1. Next, let's try to write ！

On the way to writing, I will introduce some things involved in writing the project linux Common commands or shortcut keys .

ctrl+alt+t ： Bring up the terminal （ You can also click , But try to master the keyboard ）

mkdir -p /home/chenpi/csdn_c1 ：（ Note blank space ）

mkdir The meaning of creating a file .

-p If mkdir Parent directory not found , This parameter will help create the parent directory first .

/home/chenpi/csdn_c1 The folder path you want to create .

vim hello.c ： Use vim The editor writes the code , Save and exit after writing , This operation will create a .c The files are in this directory .

  Get into vim after , Press i Enter edit mode , After writing the code ,

According to the first esc Exit to visual mode , At this time, you can only see , No more code changes ,

Then press shift+： Enter command line mode , Input wq, It means save and exit .

  Save after exit , Input ：

gcc helloworld.c  ( compiler , It will generate a file that can be recognized by the computer )

gcc function : It is also a program , yes C The compiler of the program , Similar to the function of the translator , take C The source file of the program is translated into a file that the computer can recognize and understand .gcc Some grammars of are very common , Just study on Baidu by yourself .

ls  -lh（ You can use only ls, add -lh It can display information such as file size ）

./a.out （ Run the program a.out , Ahead ./ Represents... In the current directory )

You can see the printed information .

2. Let's talk about defining variables ：

int val;（ Although I didn't count , But there will be a random number in it ）

int var = 250;

int var1, var2, var3;

int var1 = 1, var2 = 2, var3 = 3;

All of the above , But pay attention to the following , Will report a mistake ：

 int 2var; //gcc Report errors , Because the first letter cannot be a number . It can only be letters or underscores , Case sensitive

Words with special meanings and functions , These words cannot be used casually

for example : int , return, void, main, include etc.

int int = 250; You cannot use keywords to get variable names

3. Let's talk about the data type of variables ：

Data type function : Let the compiler gcc Calculate the memory space to be allocated by the variable in the future

12 The basic data types of class are as follows :

（ Reading format ： Keyword name  / meaning / Allocated memory size / The range of numbers ）

char         Character type ( It is essentially a single byte integer )     1 byte        -128~127

unsigned char    Nonnegative single byte integer       1 byte         0~255

short          Double byte integer          2 byte       -32768~32767

unsigned short    Nonnegative double byte integer          2 byte         0~65535

int            Four byte integer     4 byte       -2^31~2^31-1

unsigned int       Nonnegative four byte integer          4 byte          0~2^32-1

long               Four byte integer                 4 byte        -2^31~2^31-1

unsigned long      Nonnegative four byte integer            4 byte          0~2^32-1

long long            Eight byte integers                8 byte        -2^63~2^63-1

unsigned long long Nonnegative octet integer               8 byte          0~2^64-1

float               Single-precision floating-point               4 byte              A little

double             Double precision floating point               8 byte              A little

Be careful ：

about 32 Bit system /64 Bit system , unsigned int All are 4 byte

about 32 Bit system ,unsigned long yes 4 byte

about 64 Bit system ,unsigned long yes 8 byte

You can also use keywords sizeof Be able to view the size of the allocated memory of the data type .

sizeof Keyword Syntax ∶ Memory size ( Unit is byte )= sizeof( Variable name or data type name )

for example ︰long long var = 1;

printf("%d \n",sizeof(var)); // You can print out the memory size used , Try it yourself .

4. Let's talk about integer data types

The keyword of integer data type is int

     It can be used short perhaps unsigned short perhaps long perhaps unsigned long modification

     therefore int Can represent 2 Byte or 4 byte

 int Six forms of type ：

     form 1：short(int) use 2 The byte represents , A signed

            short int a = 250; be equal to short a =250;

     form 2：long(int) use 4 The byte represents , A signed

            long int a = 250; be equal to long a = 250;

     form 3：unsigned short(int) use 2 The byte represents , Unsigned

            unsigned short int a = 250; be equal to unsigned short a = 250;

     form 4：unsigned long(int) use 4 The byte represents , Unsigned

           unsigned long int a = 250 ; be equal to unsigned long a = 250;

Integer constant ：

100：gcc The compiler defaults to int type

100L：gcc The compiler regards it as long type

100LL：gcc The compiler regards it as long long type

100U：gcc The compiler regards it as unsigned int type

100UL：gcc The compiler regards it as unsigned long type

Floating point data types ：float,double

    1.23：gcc The compiler regards it as double type

    1.23f：gcc The compiler regards it as float type

Data types and corresponding placeholders ( to printf Function USES )

    data type                 Place holder

     char                 %c , %hhd

unsigned char         %c , %hhd

     short                      %hd

unsigned short            %hu

     int                           %d

unsigned int                %u

   long                         %ld

unsigned long             %lu

   float                   %f perhaps %g  // The former retains redundant 0, The latter is not reserved

double                  %lf perhaps %lg