Difference between function pointer and pointer function

One 、【 A function pointer 】  
    While the program is running , Function code is the algorithmic instruction part of a program , Like arrays, they take up storage space , Have corresponding addresses . You can use a pointer variable to point to the first address of the array , You can also use a pointer variable to point to the first address of the function code , Pointer variables that point to the first address of the function code are called function pointers . 

1、 Function pointer definition  

seeing the name of a thing one thinks of its function , A function pointer is a pointer , But the function that this pointer points to , It is not an ordinary basic data type or class object .

Function type （* Pointer variable name ）( Parameter list ); 
“ Function type ” Describe the return type of the function , because “()” Has a higher priority than “*”, Therefore, the parentheses outside the pointer variable name are essential , hinder “ Parameter list ” Represents the parameter list of the function pointed to by the pointer variable . 
for example ： 
int (*f)(int x); 
double (*ptr)(double x); When defining function pointers, please pay attention to ：      
The number and type of parameters of the function pointer and the function it points to should be — To ; The type of function pointer and the return value type of function must also be consistent . 
2、 Assignment of function pointer  
The function name, like the array name, represents the first address of the function code , Therefore, when assigning values , Just point the function pointer to the function name . for example , 
int func(int x);   /* Declare a function */ 
int (*f) (int x);    /* Declare a function pointer */ 
f=func;            /* take func The first address of the function is assigned to the pointer f */ 
In assignment, the function func Without brackets , And without parameters , because func Represents the first address of a function , So after assignment , The pointer f It points to the function func(x) The first address of the code . 
3、 Call function through function pointer  
Function pointer is called by function name and related parameters . 
Similar to other pointer variables , If the pointer variable pi Is to point to an integer variable i The pointer to , be *p Equal to the variable it refers to i; If pf Is to point to a floating point variable f The pointer to , be *pf It is equivalent to the variable it refers to f. similarly ,*f It's a pointing function func(x) The pointer to , be *f It represents the function it points to func. So in the implementation of f=func; after ,(*f) and func The same function . 
Because the function pointer points to a function in the storage area , Therefore, the corresponding function can be called through the function pointer . Now let's discuss how to call a function with a function pointer , It should perform the following three steps ： First , To illustrate the function pointer variable .
for example ：int (*f)(int x); 
secondly , To assign a value to a function pointer variable . 
for example ： f=func;    (func(x) There must be a definition ) 
Last , Use (* Pointer to the variable )( Parameter table ); Call function . 
for example ：    (*f)(x);(x You have to assign a value first ) 
【 example 】 Any input n Number , Find the largest number , And output the maximum value . 
main() 
{ 
    int f();         
    int i,a,b; 
    int (*p)();    /* Define function pointers */         
    scanf("%d",&a); 
    p=f;            /* Give the function pointer p assignment , Make it point to a function f */         
    for(i＝1;i<9;i++)        
    { 
        scanf("%d",&b); 
        a＝(*p)(a,b);    /* Through the pointer p Call function f */        
    }
printf("The Max Number is:%d",a) 
} 
 
f(int x,int y) 
{    
    int z;    
    z＝(x>y)?x:y;  
    return(z); 
} 
The running result is ： 
343 -45 4389 4235 1 -534 988 555 789 The Max Number is：4389 

 

A function pointer , First of all, it's a pointer , It's just that this pointer points to a function . The pointer variable can point to the address of the variable 、 Array 、 character string 、 Dynamically assign addresses , It can also point to a function , Each function is compiled , The system will assign an entry address to the function , The function name indicates the entry address , Then a pointer variable that points to a function is called a function pointer variable .

When writing an embedded driver , You will encounter the following structure ：

struct file_operations {

int (*seek) (struct inode * ,struct file *, off_t ,int);

int (*read) (struct inode * ,struct file *, char ,int);

int (*write) (struct inode * ,struct file *, off_t ,int);

int (*readdir) (struct inode * ,struct file *, struct dirent * ,int);

int (*select) (struct inode * ,struct file *, int ,select_table *);

int (*ioctl) (struct inode * ,struct file *, unsined int ,unsigned long);

int (*mmap) (struct inode * ,struct file *, struct vm_area_struct *);

int (*open) (struct inode * ,struct file *);

int (*release) (struct inode * ,struct file *);

int (*fsync) (struct inode * ,struct file *);

int (*fasync) (struct inode * ,struct file *,int);

int (*check_media_change) (struct inode * ,struct file *);

int (*revalidate) (dev_t dev);

}

The structure above file_operations The components inside are A function pointer ：int (*read) (struct inode * ,struct file *, char ,int); Be careful int (*read) It is bracketed .

Two 、【 Pointer function 】 
A function can not only bring back the value of an integer data , Character type values and real type values , You can also bring back pointer type data , Make it point to an address unit . 
        Functions that return pointers , The general definition format is ：         
        Type identifier    * Function name ( Parameter table )
        int *f(x,y); 

among x,y It's a formal parameter ,f Is the function name , The call returns an address pointer to integer data .f(x,y) Is the function , Its value is a pointer . 

Be careful , This function has a return value , The return value is int *, That is, the return value is of pointer type .

Such as ：char *ch(); A function that returns a character pointer , Look at the following example ： 
【 example 】 The string 1(str1) Copy to string 2(str2), And output string 2. 
#include "stdio.h"
 main() 
{
    char *ch(char *,char *); 
    char str1[]="I am glad to meet you!";     
    char str2[]="Welcom to study C!";     
    printf("%s",ch(str1,str2)); 
} 
char *ch(char *str1,char *str2) 
{    
    int i;     
char *p;     
p=str2      
if(*str2==NULL) exit(-1);     
do     
{  
*str2=*str1;   
str1++;  
str2++; 
}
while(*str1!=NULL);     
return(p); 
} 
Through analysis, we can conclude that ： 
A function pointer is a pointer to a function , The pointer function only indicates that it is a function whose return value is a pointer , Function pointers can be used to point to a function .