'一元*'的无效类型参数（具有'int'）。 基本上我在指针和结构的逻辑上遇到麻烦

Question

I have this code here and it's giving me compiler errors and I understand that there are dozens of other questions on here with a title basically the exact same; I've read through quite a few of them and haven't had all my questions answered. I've read a lot about pointers and structures in the past few days on stackoverflow as well as teaching sites, again dozens of them and I feel I just need a physical example in front of me to dissected and explained. I feel like I know most of this stuff but I quite more unsure of myself quite more than I'd like to be. If someone could help me out to understand what Is going on. I've posted a numbered list below the tell you what I think is happening and lets hope I'm right for the most part, less explaining for you to have to do haha just kidding. Well here is the code.

struct foo{
    int num;
    char  *word;
    struct foo *ptr;
};

void func1(struct foo*);
void func2(struct foo);

int main() {

    struct foo a;
    a.num = 20;
    a.word = "wordOne";
    func1(&a);
    printf("%d %s", a.num, a.word);       // refer to #3 below.

    a.ptr = &a;
    a.num = 55;
    a.word = "thirdToLastWord";
    func2(a);
    printf("%d %s\n", a.num, a.word);
}

void func1(struct foo *a)
{
    while(*(a->word) != '\0')
        {
            putchar(*(a->word));
            a->word++;
        }
    putchar('\n');
    if(a->num % 10 != 0)
        { a->num *= 2; }
    a->word--;
    printf("num equals %d\n", (*a).num);
}

void func2(struct foo a)
{
    if((*a).num == a->ptr->num)               //compiler error
        {(*a).num = (*((*a).ptr)).num +1; }   //compiler error
    else    {   a->num = 200;   }
    a->word = "wordsix";
    a->ptr->word = "wordseven";
    printf("function 2 %d %s\n", (*a).num, (*((*a).ptr)).word);}
}

Ok. So I think I understand the first function. But please correct me on any of the below if I am wrong or it seems like I have a faulty understanding of why its doing what it does.

1)Before main func1 declares the struct "foo" as a pointer. When func1 is called we pass in the address of "a" so we know what address to point to.

2)Everything within func1 will change the original struct. The word is decremented down to only show the last letter "e" when printed out outside of func1.

3) on the line I commented #3 that is printing out the original struct foo's .num not the pointers version (even though they would be the same thing).

I hope I haven't messed up so far.

4) Ok so the a.ptr = &a Sets the struct var "struct foo *ptr" with in the original foo structure equal to the address of a; which happens to be the exact struct that the pointer "struct foo *ptr" finds himself in. Right?

5) And we are referring to him as a member of a (the main/orig structure) he has already been established as a pointer above main.

6) So does this make an infinite loop of structures within structures that are linked together by a pointer within themselves?

Where I commented compiler error is where I'm positive I'm having problems.

7) First off, after setting a.ptr = &a; we pass a copy of struct, with the initialized pointer "ptr", into the function func2, Yes?

8) In the first if stmt it looks like we are comparing a->word to a->ptr->num. Which I assume is meaning to say: IF { the member "word" belonging to the struct that "a" is pointing to is equal to member "num", belonging to the struct that pointer "ptr" is pointing to and "ptr is again being pointed to by a pointer "a" } That seems right but I don't think it is....

9) If that is true then the line below say the exact same thing with a +1 added on.

10) And I suppose finally the final printf within main prints out either what num and word were before they went into func2 or some pointer(s) within func2 have changed their values.

I would greatly appreciate all your feedback and suggestions of what I should do to ingrain this pointer && structure ptr->member and/or structure.member stuff in my head.

Also, I'm not sure if this came up in my problem but I am fairly shaky with double pointers as well. If its not too much trouble could someone show me how a double pointer could have integrated itself into my code here?

Thank you

Answer 1

a is not a pointer. You are using -> operator to access non-pointer struct members on many occasions which is giving you the errors.

There are many errors in your code. For example, you are not allocating memory for *word . But storing a value in it:

a.word = "wordOne";

You should access struct member num as a.num .

Here's a tutorial on using pointers to structures which might help you understand it.

Answer 2

There really are quite a few errors in this code, I'll attempt to go down through your list for clarity's sake.

1.) Yes, you can pass a pointer here, but it is necessary to check for NULL before dereferencing a pointer parameter. If you get into the habit of doing this now, you'll save yourself countless hours tracking down the source of segmentation faults.

2.) func1 doesn't change a->num

if(a->num % 10 != 0)
    { a->num *= 2; }
// a->num % 10 = 20 % 10 = 0, and 0 != 0 is false

Also, Blue Moon is correct about a->word , this could be done correctly one of two ways.

With a global:

const char *global = "wordOne";

int main(int argc, char *argv[]) {
  a.work = (char*)global;
  /*...*/
}

Or the preferred way that Blue Moon is referring to:

const char *global = "wordOne";

int main(int argc, char *argv[]) {
  size_t sz = sizeof(global);
  a.work = (char*)malloc(sz);
  for(int i = 0; i < sz; i++) { a.work[i] = global[i]; }
  /*...*/
}

Note: With the second method you will eventually need to free() that memory.

Read a bit more on pointers and you'll understand why

In regards to func2() , this function doesn't use a pointer argument, so dereferencing it is sure to throw a compilation error.

void func2(struct foo);
// struct foo is passed 'by value'

I really hope that helps clear things up! :-)