sizeof pointer to a structure in c

Question

This is the structure declaration

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

And when I execute the following line, shouldn't the size of ptr2 be 12 bytes(I use 64-bit machine)?

struct node *ptr;
  struct node ptr2;
  printf("%u\n%u\n",sizeof(ptr),sizeof(ptr2));

Th output is as follows:

kv@kv:~/Desktop$ ./a.out
8
16

also [*beware of pragma pack*](http://stackoverflow.com/questions/8568432/is-gccs-attribute-packed-pragma-pack-unsafe) — Antti Haapala -- Слава Україні, Apr 07 '16 at 05:47
@AnttiHaapala I do not code in `64 bit` compiler ... then is all OK 32 bit int + 64 bit pointer is 12 Bytes. — Spektre, Apr 07 '16 at 05:51
Possible duplicate of [Structure padding and packing](http://stackoverflow.com/questions/4306186/structure-padding-and-packing) — Ilja Everilä, Apr 07 '16 at 07:15

Spektre · Accepted Answer · 2016-04-07T10:14:06.213

that depends on what align settings you have in your compiler. For align crucial struct's (for example for file-format decoding/encoding purposes) try this (in C++ language if you have different see the docs how to achieve similar functionality):

#pragma pack(1)
struct node
 {
 int data;
 struct node *next;
 };
#pragma pack()

In your case it looks like you have set align to 8 or 16 bytes. This property is used to align data to speed up memory access by better addressing math and less cache misses ...

#pragma pack(x) temporarily sets align property to x bytes
#pragma pack() restore previous align setting.

You can look at these as a push and pop.

The align tells compiler on what size should align any struct/class/union so they always start/end at multiple of align property address. This is important for pointer logic. For example if you have align to 8 or 16 and your structure is raw size 12 Bytes the compiler fills the rest with empty space to match multiple of align size (in your case add 4 bytes). This will lead to less CACHE misses (if align is big enough for your current CPU). Some compilers can also take advantage of this (in case of that size is power of 2) and change pointer math to bit shifts.

Try to search in your IDE for things like: Project->Options->Compiler->Align

But as few of you pointed out in some architectures un-aligned memory access is prohibited (like SPARC) and usually ends up with crash/exception.

This worked(ptr2 had 12 bytes). Can you please elaborate what exactly #pragma pack() does? — , Apr 07 '16 at 05:31

sizeof pointer to a structure in c

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