CODEWITHSUNDEEP
cmemory-managementmemory-alignmentstruct-member-alignment
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Reputation: 314

C program - Structure variable data packing and alignment

What will be the output of the program on a 32-bit machine (using GCC)? Explain.

#include<stdio.h>

int main() {

     struct node {
         int data;
         struct node *link;
     };

     struct node *p, *q;
     p = (struct node *) malloc(sizeof(struct node));
     q = (struct node *) malloc(sizeof(struct node));
     printf("%d, %d\n", sizeof(p), sizeof(q));
     return 0;
}

The output shows

4, 4.

Is the above program related to structure member alignment padding and data packing?

Upvotes: 3

Views: 175

Answers (4)

Sourav Ghosh
Sourav Ghosh

Reputation: 134286

  • Point 1 Use %zu format specifier to print the output of sizeof of type size_t .

  • Point 2 Note the type of p, it is struct node *, same as q.

So, essentially, sizeof(p) and sizeof(q) are exactly same as sizeof(struct node *), which, on your platform are 32 bit wide. As you're not considering a variable here, so the alignment and padding for the structure and members are neither relevant nor involved in this case.

That said, please see why not to cast the return value of malloc() and family in C.

Upvotes: 2

muXXmit2X
muXXmit2X

Reputation: 2765

On a 32-bit system the stored addresses are always 32 bits big. If you're printing the size of a pointer you're basically just printing the size of the address it points to (32 Bit -> 4 Byte).

If you want to know the size of the struct do something like this:

struct node p;
struct node q = {4, &p};

printf("%zu, %zu\n", sizeof(p), sizeof(q));

Upvotes: 2

timrau
timrau

Reputation: 23058

No, you are just printing the size of the pointers. It's not related to the internal member layout of structures.

Upvotes: 3

TryinHard
TryinHard

Reputation: 4118

No, it is not related to the structure member alignment padding and data packing.

Since it is a 32-bit machine the size of any pointer will be 4 bytes. Since p and q are of type struct node * i.e. pointer to struct node the result prints size of pointers p and q.

Upvotes: 0

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