why malloc doesnot work in my c program?

Question

struct BOOK
{
    char name[120];
    char author[120];
    int  year[50];
};

int main (void)
{
    int          i;
    int          number;
    struct BOOK* books;

    number = 50000;

    printf("before \nsizeofbooks %d \n sizeofBOOK %d\n",
           sizeof(books), sizeof(struct BOOK));

    books = (struct BOOK*)malloc(sizeof(struct BOOK) * number);

    printf("sizeofbooks %d \n sizeofBOOK %d\n",
           sizeof(books), sizeof(struct BOOK));

    free(books);
    return 0;
}

the output is:

before
sizeofbooks 4
sizeofBOOK 440
after
sizeofbooks 4
sizeofBOOK 440

It always outputs 4, even if I write to a different array, but I would expect it to change. What am I doing wrong?

My os is winxp 32 bit and I use codeblocks.

[You should not cast the return of malloc](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) — Eregrith, Dec 20 '13 at 13:16
You're not doing anything wrong, you just misunderstand how things work. Pointers are not arrays. — William Pursell, Dec 20 '13 at 13:21
@JeffRSon: I preferred that dupe since it's more similar to this question (and the answers apply perfectly). — Matteo Italia, Dec 20 '13 at 13:27
Note that `typedef struct { ... } Book;` would prevent having to write `struct` all the time. And it's nice to treat it as a 'class'. — meaning-matters, Dec 20 '13 at 14:15

score 7 · Answer 1 · answered Dec 20 '13 at 13:16

7

books is a pointer, who's size is 4. You can't read the size of your dynamically created "array".

You'll see that malloc works, when it doesn't return NULL.

answered Dec 20 '13 at 13:16

JeffRSon

10,404
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score 3 · Answer 2 · answered Dec 20 '13 at 13:19

3

It prints 4 because that is the size of books (i.e. size of a pointer).

answered Dec 20 '13 at 13:19

Eutherpy

4,471
7
40
64

score 0 · Answer 3 · answered Dec 20 '13 at 13:22

0

alternative : You can use a array of struct BOOK, i.e. struct BOOK books[5000];

answered Dec 20 '13 at 13:22

baliman

588
2
8
27

so how will i cast books[]=malloc(sizeof(struct BOOK)*no); – heidi boynww Dec 20 '13 at 13:28
you don't have to do that because books[5000] will allocate the memory – baliman Dec 20 '13 at 13:39

why malloc doesnot work in my c program?

3 Answers3