Standard C language learning summary 5

One , Function arguments

    1, The shape parameter , The variables defined in the function only belong to the function it is in ,

    This function cannot be used anymore

    2, Ordinary real participants transfer data between formal parameters by assignment ( One way value passing )

    3,return It's actually storing data in a public area ( Functions can access ), If you don't write return sentence ,

    Then it will read that the area originally refers to , Get a piece of junk data

    4, Array as a function parameter is , The length in brackets will be lost

    You need to add an extra variable to pass the length of the array

    5, When an array is passed as a function parameter , Pass is the first address of the array , be called " Address delivery " Functions and callers of functions can share the same array

1, A function is best to solve a problem , Reduce the error rate , Readability improvement

        2, Try to reduce the number of dependency layers between functions ( Reduce coupling )

        3, The data is provided by the caller , The result is returned to the caller ( Improve versatility )

        4, Consider the illegal parameters of the function , You can tell the caller that the parameter is incorrect by returning a value ,

          You can also write the situation in the form of notes ( Improve the robustness of functions )

Process image

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

    process : Programs running in the system

    Process image : It refers to the process memory distribution

            text     Code segment       Store binary instructions , const data , Permission is read-only ,

                                Forced modification will produce segment errors

            data     Data segment       Initialized global variables , Initialized static local variables

            bss     Static data segment   Uninitialized global variables , This section of memory will be automatically cleaned up as 0

                                Uninitialized static local variables ,

            stack   Stack           local variable , Block variable Managed by the operating system ,

                                It will automatically apply for , Free memory Small memory

            heap     Pile up           Manually managed by the programmer , advantage : Large enough

Local and global variables :

        Global variables : Variables defined outside the function

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

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

            Scope of action : Anywhere in the program

        local variable : Variables defined in functions

            Storage location :stack Stack memory

            Life cycle : From function call definition to function end

            Scope of action : Can only be used within a function

        Block variable : Variables defined in a statement block if\for\while

            Storage location :stack Stack memory

            Life cycle : From function call definition to function end

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

        Be careful : Local variables can have the same name as global variables , Local variables in functions will mask global variables with the same name ,

            Block variables will mask the global with the same name in the statement block , local variable , Therefore, it is recommended that global variables be capitalized

  3、 ... and , Type qualifier

        auto Used to define automatic memory allocation , Free variables in memory ( local variable ), Not adding means adding

        Be careful : Global variables cannot use auto Embellished

        stay C11 For automatic identification of types

extern

            Declare variables :   extern Type name   Variable name ;

                Tell the compiler that this variable has been defined elsewhere , Please feel free to use

                But you can only let the compilation pass temporarily , But if you can't find the definition that can't be changed when linking , Still report a mistake

                You can no longer assign values when declaring variables

                In multi file programming , hypothesis a.c Define global variables in N, Want to be in b.c Use in N,

                You need to declare this variable before using

        static"

            Change storage location :

                Change the storage location of local variables , from stack Change it to data perhaps bss( Depends on whether to initialize )

                The local variables modified by it are called static local variables

            Extending the life cycle :

                Prolongs the life cycle of local variables , Until the end of the process

            Limit the scope of action :

                Limit the scope of global variables , Only in this document

                Can prevent global variables , Function naming conflict , It can also prevent being used by other files "

        const

            " Protect " The value of a variable cannot be changed explicitly

            But you can modify the value by accessing memory

            But if you decorate an initialized global variable , 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 , General variables will be .

            Variable is volatile After modification , The compiler does not optimize the value of this variable , It will be re read from memory every time

            General hardware programming , Multithreaded programming will use

            volatile int num = 10;

            if(num == num)  // It may be false

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

        register

            Apply to change the storage medium of variables from memory to registers , Due to the limited number of registers, the application may not be successful

            Be careful : Register variables cannot take addresses

        typedef

            Type redefinition ,

            When defining variables , If you add typeodef, Then the variable name becomes the new type name of this type

            typedef int num;

            int a;

            num b;

            Be careful :typedef It's not a replacement relationship