Fundamentals of C language -- 2-5 points of knowledge about pointers and functions

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Pointer and function pass parameter

Ordinary variables as function parameters

1、 When a function passes parameters , When ordinary variables are used as parameters , Formal and argument names can be the same or different , In fact, it is the argument that replaces the corresponding formal parameter .
2、 Inside the subfunction , The value of the formal parameter is equal to the argument . The reason is that the value of the actual parameter is assigned to the formal parameter when the function is called —— Value transfer call

Array as function parameter

1、 When the array name is passed as a formal parameter , What is actually passed is not the entire array , It's the first address of the first element of the array .（ That is, the first address of the entire array ） So inside the subfunction , The array name passed in is equal to a pointer to the first address of the array
2、 The first address of the first element of the array obtained by passing parameters in the sub function , The value of the first address of the first element of the array obtained from the outside is the same .

Pointer as function parameter

It is the same as array as function parameter
Structure variables as function parameters
1、 It is the same as ordinary variables .
2、 Because structures are usually large , So if you directly use structure variables to transfer parameters , Then the efficiency of function call will be very low . Solution ： Do not pass variables , Pass the pointer address of the variable into .

Value passing and address calling （ Classic example swap function ）

stay C There are no two ways of transmitting value and address in language ,C The language itself always passes values when calling functions , But the value passed can be variable name , It can also be a pointer to a variable .

Input type parameter and output type parameter

1、 The function name is a symbol , Represents the first address of the entire function code segment , In essence, it is a pointer constant , So the function name used in the program is used as the address , Used to call functions .
2、 Function body is the key of function , By a pair { } Cover up , Including many sentences of code , The function body is what the function actually does .
3、 Formal parameter list and return value . The formal parameter is the input part of the function , The return value is the output of the function .
4、 There is no parameter list and return value , Functions can also be used . Parameters can be global variables .

Used in function arguments const The pointer

const Generally used in functions , Usage is const int *p;
const Used to modify pointers to pass parameters to functions , The function is to declare that the content pointed to by this pointer will not be changed inside the function , So pass an immutable pointer to the function （char *p = “linux”） The error will not be triggered , And one that is not declared const Function of the pointer to , When you pass him an unalterable pointer, be careful of mistakes

void func_1(char *p)
{
    
    *p = 'a';
}
void main(void)
{
    
    char *a = "linux";            //  There was a paragraph error , as a result of ： This string exists in the code segment that cannot be changed 
    char a[] = "linux";           //  Can run , The result is “aiunx”
    func_1(a);
    
    return 0;
}

Generally speaking, the input parameters are input parameters , The output parameters are output parameters . Function parameters can have many , There is only one return value . What if the function needs to return many values ？
In programming, the input and output of functions depend on function parameters , The return value is only used to indicate whether the result of function execution is success or failure
If this parameter is used for input , It is called input parameter ; If the purpose of this parameter is output , It is called output parameter .
The output parameter is used to make the data output from the function to the outside

When you see the prototype of a function , How to see which parameters do input and which do output at a glance ？
If the function parameter is a common variable, it must be an input parameter ;
If you pass the pointer, there are two possibilities ：
If this parameter is input （ Usually, the input only needs to read this parameter inside the function without changing it ） Just add const To modify
If the function parameter is a pointer variable and has not been added const, Then it means that this parameter is used as an output parameter .