Pointer to Structure error explanation

I have modified some tutorial code to get input from the keyboard, and then call a function with a pointer to a structure as input.

I have got it working but I don't understand why the first time I tried it it didn't work, it only worked when I used & in scanf for the integer of age.

Code:

---

#include <stdio.h>
#include <string.h>

struct tag{                    /* the structure type */
    char lname[20];            /* last name */
    char fname[20];            /* first name */
    int age;                    /* age */
    float rate;                /* e.g. 12.75 per hour */
};

struct tag my_struct;          /* define the structure */
void show_name(struct tag *p);  /* function prototype */

int main(void)
{
    struct tag *st_ptr;        /* a pointer to a structure */
    st_ptr = &my_struct;        /* point the pointer to my_struct */
   
    printf("\n What is your first name?");
    scanf("\n %s", st_ptr->fname);
    printf("\n What is your surname?");
    scanf("\n %s", st_ptr->lname);
    printf("\n What is your age?");
    scanf("\n %d", &my_struct.age);

    show_name(st_ptr);          /* pass the pointer */
    /* return 0; */
}

void show_name(struct tag *p)
{
    printf("\n %s ", p->fname);  /* p points to a structure */
    printf(" \n %s ", p->lname);
    printf("%d\n", p->age);
}

---

In red, if I try without the & it gives me:

Quote:

---

pointerstruct.c: In function ‘main’:
pointerstruct.c:24: warning: format ‘%d’ expects type ‘int *’, but argument 2 has type ‘int’

---

I know for scanf the & is not necessary for fname and lname as they are arrays and have constant pointers to the first element of the array anyway. But with the age I can't see why I get this message when use st_ptr->age, as I am using a pointer to the address at my_struct, which is what the error message is complaining of, it wants an address, but I am giving it an address.

Also I have seen in tutorials that they use st_ptr->age or my_struct.age without the & in scanf.

Why would you? So &my_struct.age, or &st_ptr->age will give you the address of the element age - a pointer to an integer, which is what scanf should have. If you do not use the & operator, you get the integer age passed to scanf - and unless that HAPPENS to be a valid address, it would cause an invalid memory access (segfault for example).

So, whether you use a structure or pointer to structure, you need to get the ADDRESS of the age element in the structure.

--
Mats

my_struct.age is an int not a pointer so you need & with it.

Quote:

---

Originally Posted by hellogamesmaste

Also I have seen in tutorials that they use st_ptr->age or my_struct.age without the & in scanf.

---

Then they are wrong.

Is it true then, that whenever your pointer points to a structure address, it does not point to all the components, so you have to use & for age as if it was just a normal variable? otherwise if you leave out the & it points to the value of int rather than the address, so it won't compile?

Damn tutorials online, maybe I should get a book.

It points to the structure. Using the -> notation follows the pointer to the actual structure itself, so the result is never a pointer (unless the field itself is a pointer).