passing a struct argument in a struct function

Question

I am testing passing of struct array argument in struct function "push" but getting an error message of

"passing 'shelf' (aka 'struct shelf') to the parameter of incompatible type 'shelf *' (aka 'struct shelf *');".  

How can I fix this error? I don't know what needs to be done in order to make it work. I have declared the structs and push function in the header file.

Main purpose of this program is to access the values of book struct in shelf struct.

Here are my code snippets.

header file

#ifndef __POINTERS_H_
#define __POINTERS_H_

typedef struct book
{
    char *b_title;
    int b_pages;
}book;

typedef struct shelf
{
    char *s_title;
    int s_pages;
}shelf;

book book_details[100];
shelf shelf_item[100];

shelf push(shelf item[100]);


#endif // __POINTERS_H_

c file:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "pointers.h"

int main(void)
{
    book book_details[100];
    shelf shelf_item[100];

    book_details[0].b_title = "c++";
    book_details[0].b_pages = 200;

    push(shelf_item[0]);
    printf("Shelf Item's title is: %s\n", shelf_item[0].s_title);
    printf("Shelf Item's title is: %i\n", shelf_item[0].s_pages);

    return 0;
}

shelf push(shelf item[100])
{
    strcpy(item[0].s_title, book_details[0].b_title);
    item[0].s_pages = book_details[0].b_pages;

    return item[0];
}

Answer 1

1. The arguments do not match.

shelf item[100] when passed as an argument will decay to shelf *item.

As you are passing a shelf object as an argument(the first element of the array), that won't work.

---

2. Note that this statement:

book_details[0].b_title = "c++"

Leads to undefined behavior, b_title struct member is an uninitialized pointer, as is s_title, you cannot store anything in it until it points to a valid memory location, alternatively you can use a char array:

char b_title[100];

And

char s_title[100];

To assign a string to it you then need to use strcpy or a similar method.

---

3. There is also the fact that you declare book book_details[100]; and shelf shelf_item[100]; twice, at global scope and again in main.

---

Assuming global shelf_item and book_detais arrays you can do something like:

Header

typedef struct book {
    char b_title[100]; //b_title as char array, alternatively you can allocate memory
    int b_pages;
}book;

typedef struct shelf {
    char s_title[100]; //s_title as char array
    int s_pages;
}shelf;

book book_details[100];
shelf shelf_item[100];

void push(); //as shelf is global there is no need to pass it as an argument

C file

int main(void) {
    strcpy(book_details[0].b_title ,"c++"); //strcpy
    book_details[0].b_pages = 200;

    push();

    printf("Shelf Item's title is: %s\n", shelf_item[0].s_title);
    printf("Shelf Item's title is: %i\n", shelf_item[0].s_pages);
    return 0;
}

void push() {
    strcpy(shelf_item[0].s_title, book_details[0].b_title);
    shelf_item[0].s_pages = book_details[0].b_pages;
}

---

Assuming local shelf_item and book_detais arrays you can do something like:

Header

#define SIZE 100

typedef struct book {
    char *b_title;
    int b_pages;
}book;

typedef struct shelf {
    char *s_title;
    int s_pages;
}shelf;

void push(book *b, shelf *s); //pass both arrays as arguments

C file

int main(void) {

    book book_details[SIZE];
    shelf shelf_item[SIZE];

    book_details[0].b_title = malloc(SIZE); //allocate memory for b_title
 
    if(book_details[0].b_title == NULL){ /*deal with error*/}

    strcpy(book_details[0].b_title ,"c++");
    book_details[0].b_pages = 200;

    push(book_details, shelf_item);

    printf("Shelf Item's title is: %s\n", shelf_item[0].s_title);
    printf("Shelf Item's title is: %i\n", shelf_item[0].s_pages);
    return 0;
}

void push(book *b, shelf *s) {

    s[0].s_title =  malloc(SIZE); //allocate memory for s_title
    if(s[0].s_title == NULL){ /*deal with error*/}

    strcpy(s[0].s_title, b[0].b_title);
    s[0].s_pages = b[0].b_pages;
}

Answer 2

There are several issues. Here's the first one which causes the issue:

push(shelf_item[0]);

The error message is that you are passing shelf while shelf * is expected (array). Simply pass the address of the item (the start of the array): &shelf_item[0]

The prototype

shelf push(shelf item[100]);

does not make any sense if you just want to push one item.

Also

book book_details[100];
shelf shelf_item[100];

are declared twice.

And it is "bad style" (TM) if a function got only one argument passed but uses a hidden/global variable to also work on.

And your print statement does not print the values of the returned item.