How to initialize an array of struct in a struct? - c

Question

I have a struct with an array of another struct inside it and I am having trouble initialising the struct.
typedef struct stack * Stack;
typedef struct book * Book;
struct book {
char *title;
int pages;
};
struct stack {
int num_books;
Book array[50]
};
What I am trying to do is to create an empty stack with zero books but I keep getting segmentation faults on everything I tried.
Here is my initialization function:
Stack create_stack(void) {
Stack s = malloc(sizeof(struct stack) * 50);
s->num_books = 0;
// s->array[0]->title = Null;
// s->array[0]->pages = 0;
// the above 2 lines give a seg fault: 11
// I also tried:
// s->array = s->array = malloc(sizeof(struct book) * 50);
// Which gives the error that array type 'Book [50]' is not assignable
return s;
}
How can I create an empty stack with zero books?

Answer 1

You haven't allocated memory for struct book objects. The struct:
struct stack {
int num_books;
Book array[50];
};
defines array member as 50 elements array of pointers to book struct (that is, Book is synonym to struct book *). These are still "wild" pointers, and you need to assign them with allocated struct objects. In other words, by calling:
Stack s = malloc(sizeof(struct stack) * 50);
you have made a room for fifty objects of type struct stack, but inside each of these structs, there is room for struct book pointers, not objects itself.
Like mentioned in comments, typedefing a pointer type is an easy way to obfuscate the code.

Answer 2

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define SIZE 2
typedef struct book {
char * title ;
int pages;
} Book;
typedef struct stack {
int num_book;
Book book_arr[SIZE];
} Stack;
//------------------------------------------------
int main (void ){
Stack s1;
printf("Enter Number of Books : " );
scanf("%d",&s1.num_book);
getchar();
//BOOK
for( size_t j = 0 ; j < s1.num_book ; j++ ){
char temp[100];
printf("Enter the Book Title for %zd Book : ", (j+1) );
fgets(temp,100,stdin);
strtok(temp,"\n"); // for removing new line character
s1.book_arr[j].title = malloc ( sizeof(temp) +1 );
strcpy(s1.book_arr[j].title,temp);
// puts(s1.book_arr[j].title );
printf("Enter Pages for %zd Book : ",(j+1) );
scanf("%d",&s1.book_arr[j].pages); getchar();
}
//PRINT
size_t count = 0 ;
for( size_t i = 0 ; i < s1.num_book ; i++ ){
while(count < SIZE ) {
printf("Book Title : %s\nBook pages : %d\n",s1.book_arr[count].title, s1.book_arr[count].pages );
free(s1.book_arr[count].title );
count++;
}
}
return 0;
}
Is that what you were trying to achieve ?