Standard C language 6

nineteen 、 Process image

    Program ： Executable files stored on disk （ Binary 、 Script files ）

    process ： Procedures in progress in the system , Load data into memory and run

    Process image ： It refers to the distribution of process memory

        text   Code segment         Store binary instructions 、 const data   By const Decorated initialized global variables and static local variables

                              Permissions are read-only

        data   Data segment         Initialized global variables ; Initializing static local variables

        bss   Static data segment     Uninitialized global variables , The memory of a given segment will be automatically cleaned up as 0;

                            Uninitialized static local variables , Default cleanup is 0;

        stack Stack             local variable 、 Block variable （ Operating system management , It will automatically apply for 、 Free memory , Small memory ）

        heap   Pile up             It is managed manually by programmers （ Large memory ,）

    Global and local variables

    local variable ： Variables defined in functions

        Storage location ：stack Stack memory

        Life cycle ： From the beginning of the function call to the end of the function

        Scope of action ： Can only be used within a function

    Global variables ： Variables defined outside the function

        Storage location ：data（ initialization ）、bss（ uninitialized ）

        Life cycle ： From the beginning of the program to the end of the program

        Scope of action ： It can be used anywhere in the program

    Block variable ： Variables defined in a statement block （if/for/while）

        Storage location ：stack Stack memory

        Life cycle ： From the beginning of the function call to the end of the function

        Scope of action ： Can only be used within a statement block

   

    Be careful ： When a local variable has the same name as a global variable , Local variables in functions mask global variables ;

        Block variables mask global and local variables （ Global variables are capitalized ,）

twenty 、 Type qualifier

    (auto)       Define automatic memory allocation 、 Free variables in memory （ local variable ）

                Global variables cannot be used auto,C11 The standard is used to automatically identify types ;

    extern       Declare variables （ Global variables ）（extern Type name Variable name ）,

                Tell the compiler that this variable has been defined elsewhere . Let the compilation pass temporarily , Link not found , Will report a mistake

                Cannot assign values when declaring variables

                In multi file programming , hypothesis a.c Global variables are defined ,b.c To use , Need .

    static       Static variables （）

                    Change storage location ;

                        Change the storage location of local variables    stack ->  data / bss（ Initialize to 0, Store in bss）

                        static Modified local variables It's called Static local variables

                    Extending the life cycle ;

                        Prolongs the life cycle of local variables ： The end of the function will not be destroyed , Until the end of the process

                    Limit the scope of action ;

                        Limit global variables 、 The scope of the function , Use only in this document .

                        Can prevent global variables 、 Function name naming conflict ; Prevent other documents from using

   

    const       “ Protect ” The value of a variable cannot be changed explicitly , But you can modify the value by accessing memory

                But if you decorate the initialized global variables 、 Initialized static local variables ,

                Then the variable will change from data Change it to text, Turned into “ Constant ” Cannot force modification

    volatile     If the value of the variable is not displayed and modified , Then when using this variable, it will not be read from memory

                Instead, continue to use the last read result , This process is called value optimization , It's usually done ;

                Variable is volatile After retouching , The compiler does not optimize the value of this variable , Every time

                Reread from memory .

                General hardware programming 、 Multi line programming will use ;

    register     register , Apply to change the storage medium of variables from memory to registers , Because of the limited number of registers

                Not necessarily successful .

                Be careful ： Register variables cannot take addresses

        storage medium ： Hard disk -> Memory -> Advanced caching -> register

    typedef     Type redefinition ,

                When defining variables , If you add typedef, The variable name becomes the new type name of this type

                Be careful ：typedef Not a replacement type

The 21st 、 Function recursion

    This behavior of function calling itself is called function recursion , Improper use will lead to dead circulation

    Recursion is an algorithm that can realize divide and conquer , Put a complex big problem , Break it down into several small problems , Until all the problems are solved

    How to write recursion well ：

        1. Find the exit

        2. Solve a small problem

        3. Call yourself

    Each call of the recursive function will produce its own copy in the stack memory , Until the implementation to the export

    Will release this layer of recursive functions , therefore , Compared with the cycle , Recursion is very memory intensive , slowly ,

    If you can use functions, don't use recursion

    advantage ：

        1. A good understanding 、 have a lucid brain

        2. Good solution to nonlinear problems

    shortcoming ：

        1. Memory consumption , slowly