what is the sizeof this structure ? and why?

Question

the following program gives output 4. I thought it will output 8 sizeof(int) + sizeof(unsigned int)

#include<stdio.h>
#include<stdlib.h>
int main()
{
        struct node
        {
                int a;
                struct node * next;
        };
        struct node * p= (struct node *) malloc( sizeof( struct node));
        printf("%d", sizeof( p));
}

You're taking the size of the pointer instead of the object itself. — Mysticial, Jun 23 '13 at 06:27
It's also important to know if you're compiling this for 32-bits or 64-bits, as that makes a difference as to how large the `int` and pointers are. — Raceimaztion, Jun 23 '13 at 06:30

Lee Daniel Crocker · Answer 1 · 2013-06-23T06:39:41.690

In this code, p is a pointer, so you're just printing the size of a pointer (which is apparently 4 bytes in your compiler/OS combination). If you want the size of the structure, you need to print sizeof(*p). Also, as was pointed out, using "%d" for a size_t won't necessarily work ("%zu" is correct, although %d will on most compilers/OSs in the real world). Also, you shouldn't assume that the size of the structure "should" be 8. Pointers might be bigger, or the compiler might want to pad or align the structure in some odd way.

score 1 · Answer 2 · edited Jun 23 '13 at 06:37

1

p is just a pointer. Its size depends on ABI, which is in your case 4.

edited Jun 23 '13 at 06:37

Maroun

94,125
30
188
241

answered Jun 23 '13 at 06:28

Atmaram Shetye

993
7
15

1

No, it will be the size of a pointer, which may or may not be the size of an unsigned int. – Lee Daniel Crocker Jun 23 '13 at 06:29
You're right that it's the size of the pointer. You're wrong to assume what that size is. – Yevgeny Simkin Jun 23 '13 at 06:30

score 1 · Answer 3 · answered Jun 23 '13 at 06:30

1

And the correct answer: your output, if any, is indeterminate, because you're using the wrong format specifier in the call to printf(). You should have used %zu instead - sizeof() yields an object of type size_t.

Thus, your program invokes undefined behavior, and it's free to do (and print) anything it wants to.

answered Jun 23 '13 at 06:30

I believe converting an unsigned int to int will not result in undefined behavior. It might convert some thing like 255 into something like -127 on 8 bit systems. Not sure though. A source to the undefined behaviour would be greatly appreciated! – Atmaram Shetye Jun 23 '13 at 06:34
1

The point is that `sizeof(x)` doesn't yield an `unsigned int`, it yields a `size_t`. On most systems, that's the same as an `unsigned int`, but it's not required to be. `%zu` is the proper format specifier for a `size_t`, whereas `%d` may cause terrible things to happen. – amalloy Jun 23 '13 at 06:44

what is the sizeof this structure ? and why?

3 Answers3