C 中的增量问题（指针）

Question

I can only change the content of the "birthday" function, the rest must stay the same.

I am having a silly problem trying to increment the age variable...It does get incremented if I use the ++ operator but not by decimal 1, it seems to increment by 4 which is the sizeof an int. What am I doing wrong here?

    #include <stdio.h>

typedef struct {
    int* age;
    const char* name;
} Person;

void birthday(Person* person);


int main()
{
    void(*birthday_ptr)(Person* person) = &birthday;
    Person me = {30,"foobar"};
    (*birthday_ptr)(&me);
    return 0;
    }

void birthday(Person* person)
{
    printf("%p\n", person);
    printf("%d\n", person->age);
    person->age = 31; // this works...it sets the value pointed to by the ptr to 31...
    printf("%d\n", person->age);
    person->age += 1; // this compiles, but it seems to increment by 4 the value pointed to by the ptr, not 1 (relates to int size ?!)
    printf("%d\n", person->age);
    int* silly = (int*)(person);
    (*silly)++; // this works and increments age to 36...
    (*person->age)++; // this causes an access violation...why ?! 
    printf("silly: %d\n", person->age);
}

Answer 1

You must dereference a pointer to change the value it's pointing to. But I reckon the age variable should not be a pointer at all. It should be an int :

typedef struct {
    int age;
    const char* name;
} Person;

This solves many of your problems, including the fact that you cast 30 to a pointer, and use %d to print the pointer, which causes undefined behavior.

Answer 2

The correct solution is to modify your struct:

typedef struct {
    int age;
    const char* name;
} Person;

indeed, it is more logical that age is an int and not an int* (age is an integer not an adress of this form 0x1F8C45 ).

With such a change, your code:

    (person->age)++; // this causes an access violation...why ?!
    printf("silly: %d\n", person->age);

is correct.

If indeed, you would like to keep your struct as it currently is, you need to increment the value of member of the struct int * age but you can only change the content of the "birthday" function.

Here is my proposition.

However, in my opinion, this is not a good C language nor is it a good practice. It is just a hack to bypass the problems'constraints (not to touch the structure) ==> You should be careful concerning the warning (also, your code does not compile if you use -Werror flag with gcc and consider the response of @Ayxan Haqverdili .

Create a pointer pointing to the address of your age member

// a pointer pointing to the address of person->age
 int *p = (&person->age);
 // increment the value pointed by the pointer (which is supposed to be age)
 ++*p; // person->age is 32
 ++*p; // person->age is 33
 (*p)++; // the same -- increment to 34 etc etc

The whole birhday() function is below

void birthday(Person* person)
{
    printf("%d\n", person->age);
    person->age = 31; // this works...it sets the value pointed to by the ptr to 31...
    printf("%d\n", person->age);

    int *p = NULL;
    p = (int*)(&person->age);
    (*p)++; // age =32
    printf("value of p=%d\n", *p);
    printf("person->age=%d\n", person->age);

    (*p)++; // person->age is 33
    printf("value of p=%d\n", *p);
    printf("person->age=%d\n", person->age);


}

The ouput of the execution displays the following results:

30
31
value of p=32
person->age=32
value of p=33
person->age=33