Thread: [C] Pointer to a Pointer to a Structure

Thanks, i've done a try with your code and .. it compile, but .. SEGV (segmentation fault)
this is the code i've tried:

Code:

#include <stdio.h>
#include <stdlib.h>

struct book{
int number;
char *title;
};

int book_list(struct book **l, int *num_books);

int main(){
	int *books;
	int i;	
	books=(int*)malloc(sizeof(int));
	*books=3;
	struct book *list[*books];
	for(i=0;i<*books; i++){
		list[i]->number=i;
		list[i]->title= "ciao";
	}
	book_list(list, books);
		
	

return EXIT_SUCCESS;	
}


int book_list(struct book **l, int *num_books){
int i;	
	for(i = 0;i < *num_books;++i)
  printf("number: %d title: %s\n", l[i]->number, l[i]->title);
  return EXIT_SUCCESS;
  }

i think the problem is in 'list' initialization.
I have initialized 'list' as you said .. i have no idea how to do with a malloc..
What do you think about?
Thanks so much!

Code:

struct book *list[*books];

You're declaring an array of pointers to book structs. There's no actual place to store book info. You have two options:
1. Allocate space before setting the book number or title: list[i] = malloc(sizeof(*list[i]))
2. Create at array of plain old structs, instead of pointers to struct, and access members using the . operator

Code:

    struct book list[*books];  // notice I dropped the *

    for (i = 0; i < *books; i++) {
        list[i].number = i;  // . operator instead of ->
        list[i].title = "ciao";
    }
    book_list(&list, books);

I prefer method 2 since you don't seem to show need for dynamically allocating each element of the array.

I've compiled the code as you suggested me:

Code:

#include <stdio.h>
#include <stdlib.h>

struct book{
int number;
char *title;
};

int book_list(struct book **l, int *num_books);

int main(){
	int *books;
	int i;	
	books=(int*)malloc(sizeof(int));
	*books=3;
    struct book list[*books];  // notice I dropped the *

    for (i = 0; i < *books; i++) {
        list[i].number = i;  // . operator instead of ->
        list[i].title = "ciao";
    }
    book_list(&list, books);	
	

return EXIT_SUCCESS;	
}


int book_list(struct book **l, int *num_books){
int i;	
	for(i = 0;i < *num_books;++i)
  printf("number: %d title: %s\n", l[i]->number, l[i]->title);
  return EXIT_SUCCESS;
  }

But the compiler say:

Code:

gcc dplist.c -o dplist
dplist.c: In function ‘main’:
dplist.c:22:5: warning: passing argument 1 of ‘book_list’ from incompatible pointer type
dplist.c:9:5: note: expected ‘struct book **’ but argument is of type ‘struct book (*)[(unsigned int)(*books)]’

i've tried to call book_list in other two ways (all wrong):
book_list(list, books);
and
book_list(*list,books);


Any idea?

Yeah, I didn't think my response through too carefully -- it would require changing the declaration of book_list, which you can't do. Use laserlight's method.

I've rewrited the code with a better indentation, also using your method, but it still SEGV

Code:

#include <stdio.h>
#include <stdlib.h>

struct book {
    int number;
    char *title;
};

int book_list(struct book **l, int *num_books);

int main() {
    int num_books = 3;
    int i;
    struct book *list = malloc(sizeof(*list) * num_books);
    for (i = 0; i < num_books; i++) {
        list[i].number = i;
        list[i].title = "ciao";
        printf("[MAIN] number: %d title: %s\n", list[i].number, list[i].title); 
    }

    book_list(&list, &num_books);
    free(list);
    return EXIT_SUCCESS;
}

int book_list(struct book **l, int *num_books) {
    int i;
    for (i = 0; i < *num_books; ++i)
        printf("[BOOK_LIST] number: %d title: %s\n", l[i]->number, l[i]->title);
    return EXIT_SUCCESS;
}

Could you explain me, why it SEGV?
I mean, i've allocated 3 structures, i can write into these structures (the printf into the main function help me to check if it can write into the structures), but i cannot print them in stout .. i'm asking why..
Do you know the answer? Please, i know i'm boring, but i need to understand..

Great!
I really, really thanks you!
This is the right way:

Code:

int book_list(struct book **l, int *num_books) {
    int i;
    for (i = 0; i < *num_books; ++i)
        printf("[BOOK_LIST] number: %d title: %s\n", (*l)[i].number, (*l)[i].title);
    return EXIT_SUCCESS;
}

I should modify the title with "[SOLVED]"?

So you passed in &list, which is the address of a pointer to a book struct, i.e. a pointer to a pointer to a book struct. Inside book_list, it doesn't know whether you passed it the address of what is effectively an array, or the address of a pointer to a single element. You did the former, but treat it like the latter. You need to dereference l before indexing the array. You need

Code:

(*l)[i].number

*l gives you the address of the memory you allocated in main, the start of the array
(*l)[i] gives you the i'th element of that array. You need the parens because [] is higher precedence than * (dereference)
(*l)[i] is a book struct (not a pointer to a book struct), hence use of the . operator

EDIT: wow, way too slow. Beaten twice! @ laserlight: his OP said he must use that declaration of book_list, with the **.

@tabstop: Thanks for the edit!
@anduril462: exactly what i was looking for! Thanks so much for the explaination!
@laserlight: i must use it, cause my teacher wants..
Anyway, thanks to all of you.

That's a present for you:
http://1.bp.blogspot.com/-gUDVGJSZc3...ni-ricette.jpg
Try to guess where i'm writing from..