Allocate memory for struct array element (string)

Question

I am trying to read from keyboard and store the info in my struct book. The user is asked to enter the array size and I dynamically allocate the size to my book array. But when n > 1, the run time error is exc_bad_access. I don't know why it works when n = 1 and it doesn't work when n > 1.

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

struct book
{
    char *title;
    char *author;
    double price;
};

int main()
{
    int n, i = 0;
    printf("please enter the value for n\n");
    scanf("%d", &n);

    struct book *stack = malloc(sizeof(struct book) * n);
    stack->author = malloc(sizeof(char) * 100);
    stack->title = malloc(sizeof(char) * 100);
    //allocate memory for book and struct members
    while (i < n )
    {
        printf("Please enter the title\n");
        scanf("%s", stack[i].title);
        printf("Please enter the author\n");
        scanf("%s", stack[i].author);
        printf("Please enter the price\n");
        scanf("%lf", &stack[i].price);
        i ++;
    }
    free(stack);
    return 0;
}

My thought here is be able to set the upper limit for the input in terms of char number, so I think it is necessary to keep it in place right? — angerhang
– angerhang, Commented Nov 2, 2014 at 11:53
use a length specifier with %s to prevent buffer overflow. Also you may need to reconsider the choice of input method if there are any books with multiple words in their title or author name! — M.M
– M.M, Commented Nov 2, 2014 at 12:17
sizeof (char) * 100 equals 1 * 100 equals 100. So what is the need for using sizeof (char)? Coding 100 would do the job. — alk
– alk, Commented Nov 3, 2014 at 11:28

Marco A. · Accepted Answer · 2014-11-02 12:07:45Z

3

You're allocating enough structures for any size of elements you input, but not the space for the buffers:

struct book *stack = malloc(sizeof(struct book) * n);
stack->author = malloc(sizeof(char) * 100);
stack->title = malloc(sizeof(char) * 100);
// what about stack[1].author, stack[2].author and the rest??

This will work instead:

struct book *stack = malloc(sizeof(struct book) * n);
for (int i = 0; i < n; ++i) {
    stack[i].author = malloc(sizeof(char) * 100);
    stack[i].title = malloc(sizeof(char) * 100);
}

And remember to free your memory as well.

edited Nov 2, 2014 at 12:07

answered Nov 2, 2014 at 11:29

Marco A.

43.8k27 gold badges146 silver badges263 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

alk Over a year ago

In C casting the result of malloc/calloc/realloc isn't necessary nor is it recommended. Just do not do this.

Vincent · Accepted Answer · 2014-11-02 11:29:45Z

2

For each book in the stack, you have to allocate memory for the author and the title.

for(int i = 0; i < n; i++)
{
    stack[i].author = malloc(sizeof(char) * 100);
    stack[i].title = malloc(sizeof(char) * 100);
}

You are only allocating memory for the first element.

answered Nov 2, 2014 at 11:29

Vincent

6463 silver badges9 bronze badges

1 Comment

Vincent Over a year ago

stack->author = malloc(sizeof(char) * 100); stack->title = malloc(sizeof(char) * 100);

l0gg3r · Accepted Answer · 2014-11-02 11:30:39Z

0

You need to allocate memory for

char *title;
char *author;

When you do struct book *stack = malloc(sizeof(struct book) * n); memory is allocated for char* pointers (4 or 8 bytes depending on platform you're building).

So you need to allocate some memory for inside values

while (i < n )
{
    char *title = malloc(255 * sizeof(char));
    printf("Please enter the title\n");
    scanf("%s", title);
    stack[i].title = title;
    .....
    i ++;
}

answered Nov 2, 2014 at 11:30

l0gg3r

8,9343 gold badges30 silver badges46 bronze badges

1 Comment

alk Over a year ago

Why the intermediate title? Why not allocated to and read into the struct's member directly?

Collectives™ on Stack Overflow

Allocate memory for struct array element (string)

3 Answers 3

1 Comment

1 Comment

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related