Standard C language learning summary 7

One , Problems needing attention when using pointers :

    Null pointer : The value is NULL The pointer variable of is called null pointer ,

            If the null pointer is dereferenced, a segment error will occur

           NULL Generally as an error sign , When the return value of a function is a pointer type , have access to NULL

            As the return result of function execution error

            How to avoid segment errors caused by null pointers :

            Make a judgment before using an unknown pointer

            if(NULL == p)

            1, When the parameter of a function is a pointer , The pointer passed to you by others may be a null pointer

            2, When the return value obtained from the function is a pointer type , A null pointer may be returned

        Be careful :NULL In most systems 0, Individually 1

    Wild pointer :

        Pointers to uncertain memory spaces are called wild pointers

        Consequences of dereferencing the field pointer :

            1, Everything is all right

            2, Segment error

            3, Dirty data

        Wild pointers are more harmful than null pointers , Because it can't judge ,

        And it may be a hidden mistake , Not exposed for a short time

        All wild pointers are made by programmers themselves , How to avoid generating wild pointers

            1, Be sure to initialize when defining pointer variables

                int* p = NULL;

            2, Function does not return stack memory ( Local variables in a function ) The address of

            3, After the memory pointed to by the pointer is released , Pointer variables should be set to null in time NULL

Two , The operation of the pointer

    Pointer variables store integers , Theoretically, all operators that can be used by integers can be used ,

    But most operators are meaningless

    The pointer + n The pointer + Pointer type width *num     It's equivalent to moving forward n Elements

    The pointer - n The pointer - Pointer type width *num     It's equivalent to retreating n Elements

    The pointer - The pointer ( The pointer - The pointer )/ Pointer type width   Calculate how many pointer elements are directly separated between two pointers

3、 ... and , The pointer and const

    When we use pointers as function parameters to improve the efficiency of parameter transfer , The efficiency of parameter transmission is improved

    But there is a risk that variables will be shared and modified , have access to const Protect the memory pointed to by the pointer

    const int* p;   Protect the memory pointed to by the pointer from being modified

    int const *p;   ditto

    int* const p;   Protect pointer variables from modification

    const int* const p; Neither the pointer variable nor the memory pointed to by the pointer can be modified

    int const * const p;     ditto

Four , Pointer arrays and array pointers

    Pointer array :

        The array composed of pointer variables is called pointer array , Its members are pointer variables of the same type

        type * arr[ length ];

        int* arr[10];

    Array pointer :

        Is a pointer to an array

        type (*arrp)[ length ];

        int (*arrp)[10];

5、 ... and , Array name and pointer :

    Array names are special pointers

    The array name is a constant , You can't change its value , The array name does not have its own storage space between it and the first address of the array

    Is a mapping relationship

    Array name == & Array name

    Pointer variables have their own storage space , It and Institute Directional relation

    When the pointer variable points to the first address of the array , Pointers can be used as array names , The array name can also be used as a pointer

        Array name [i] == *( Array name +i)

        *(p+i) == p[i]

    Be careful : When an array is used as a parameter of a function, it becomes a pointer , So the length is lost

6、 ... and , The secondary pointer

    The secondary pointer is the pointer to the pointer , What is stored inside is the address of the pointer variable

    Definition :   Type name ** Variable name _pp

    assignment :   Variable name _pp = & Pointer to the variable ;

    Quoting :

        * Variable name _pp <==> Pointer to the variable

        ** Variable name _pp <==> * Pointer to the variable <==> data

    Be careful : When you need to share pointer variables between functions , Pass the address of the pointer ( The secondary pointer )

7、 ... and , A function pointer

    The function name is an address , The function name represents the entry position of the function in the code segment

    A function pointer is a pointer to a function , It stores the entry location address of the function in the code segment

    return type  (*p)( type 1, type 2,...);

    int (*funcp)(int,double);   //funcp Specifically point to func Function pointer of

    You can use function pointers , Pass a function as an argument to another function , This method is called function callback mode