Extra variables lead to segmentation fault

Question

In the following code, I have two structs.

The first one is book which describes the number of pages of the book using page.

The second one is library which holds all the books using a pointer books, with the parameter num_book which tells the total number of books of the library.

The program can be compiled and run perfectly fine, and the printf result is OK.

But when I added the extra variable (e.g. int x = 1;) as shown in the code. I can still compile the program, but running the executable gives segmentation fault.

I have no idea why it is the case since everything seems to initialized properly. Thanks.

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

typedef struct {
    int page;
} book;

typedef struct {
    int num_book;
    book *books;
} library;


int main() {
    library *my_library;
    int n = 5; // number of books in the library

    // extra variable not used
    // uncomment it gives segmentation fault
    // int x = 1;

    my_library->num_book = n;

    my_library->books = (book *) malloc( (my_library->num_book) * sizeof(book) );

    for(int i = 0; i < my_library->num_book; i++){
        my_library->books[i].page = i+10;
        printf("Book %d\n"
               "Number of pages = %d\n",
               i, my_library->books[i].page);
    }

    return 0;
}

"everything seems to initialized properly" ironically the problem is exactly with uninitialized variable. — bolov, Dec 11 '18 at 11:13
@K_inverse it's not that you are stupid. Far from it. It's that I found it ironically funny. — bolov, Dec 11 '18 at 11:20
But, why the program can still run when I comment `int x = 1` in the above flaw code? — K_inverse, Dec 11 '18 at 12:00
What happens in the flawed code is undefined behavior, so there's no guarantee about what happens. It's possible that the `int x = 1` (or lack thereof) changed the stack in a way that causes/avoids the memory corruption leading to the crash. — Blaze, Dec 11 '18 at 12:27

score 2 · Answer 1 · answered Dec 11 '18 at 11:11

2

Add this line after declaration of my_library

my_library = malloc(sizeof(*my_library));

answered Dec 11 '18 at 11:11

awakened

192
9

score 2 · Answer 2 · edited Dec 11 '18 at 11:17

In C, you must allocate memory for your struct manually using malloc.

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

typedef struct {
    int page;
} book;

typedef struct {
    int num_book;
    book *books;
} library;


int main() {
    library *my_library = (library *) malloc(sizeof(library));
    int n = 5; // number of books in the library

    // extra variable not used
    // uncomment it gives segmentation fault
    int x = 1;

    my_library->num_book = n;

    my_library->books = (book *) malloc( (my_library->num_book) * sizeof(book) );

    for(int i = 0; i < my_library->num_book; i++){
        my_library->books[i].page = i+10;
        printf("Book %d\n"
               "Number of pages = %d\n",
               i, my_library->books[i].page);
    }

    return 0;
}

[Don't cast the result of malloc](https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) — pmg, Dec 11 '18 at 11:16

score 2 · Answer 3 · answered Dec 11 '18 at 11:12

2

In

    library *my_library;
    /* ... */
    my_library->num_book = n;
 // ^^^^^^^^^^ junk here

my_library has not been assigned (or initialized with) a usable value.

answered Dec 11 '18 at 11:12

pmg

106,608
13
126
198

Extra variables lead to segmentation fault

3 Answers3