What's this: __attribute__((packed))

frankli · 06-14-2004, 08:46 PM

I see some code about Hostap project,in these codes,I found some structure which have such signs: __attribute__((packed)),I don't know what does this mean. So soon I found more such signs in Linux Kernel Souce code.Who can tell me what does this mean exactly and how I use it?Thanks

eric.r.turner · 06-14-2004, 09:59 PM

Here is how I think it works (please correct me if I'm wrong!)

__attribute__((packed)) ensures that structure fields align on one-byte boundaries. If you want to ensure that your structures have the same size on all processors, the packed attribute is how you tell gcc.

As an example, let's define this structure:

Code:

struct s {
   char aChar;
   int    anInt;
};

A processor that aligns on eight-byte boundaries may compile this so that aChar is in the first byte, followed by seven bytes of unused space, then starting anInt in the ninth byte.

A processor that aligns on four-byte boundaries may compile this so that aChar is in the first byte, followed by three bytes of unused space, then starting anInt in the fifth byte.

To force anInt to begin immediately after aChar, you would define the structure like this:

Code:

struct s {
   char aChar;
   int anInt __attribute__((packed));
};

To test these ideas out, I ran this code on an old Pentium 166:

Code:

#include <stdio.h>

struct s1 {
   char a;
   int  i;
};

struct s2 {
   char a;
   int i __attribute__((packed));
};

int main( int argc, char* argv[] ) {

  struct s1 s_1;
  struct s2 s_2;

  printf( "sizeof s1 is %d\n" , sizeof(s_1) );
  printf( "sizeof s2 is %d\n" , sizeof(s_2) );

  return( 0 );
}

And got these results:

Code:

eric.r.turner@turing:~/lab/packed$ ./foo
sizeof s1 is 8
sizeof s2 is 5

Looks like this processor aligns on four-byte boundaries.

frankli · 06-14-2004, 11:58 PM

thanks a lot!

frankli · 06-15-2004, 12:54 AM

But when I wirte the progarm like this:
struct s1 {
void *a;
char b[2];
int i;
};

struct s2 {
void *a
char b[2];
int i ;
}__attribute__((packed));

and got the result like this:
sizeof s1 is 12
sizeof s2 is 10
WHY? I think sizeof s2 should be 11!

Dark_Helmet · 06-15-2004, 01:56 AM

Why would it be 11?

void *a => 32-bit processor means this is 32 bits wide (or 4 bytes)
char b[2] => a char is typically 1 byte. You have two of them. So b occupies 2 bytes
int i => most 32-bit machines also default to 32 bits for plain integers (again, 4 bytes)

So sizeof(a) + sizeof(b) + sizeof(i) = 4 + 2 + 4 = 10

For your non-packed structure, you processor is aligning to 4-byte boundaries. Thus, a fills one full 4-byte block, b fills two bytes of the next block (leaving 2 other bytes unused), and i occupies the next full 4-byte block. So, in that case:

sizeof(a) + sizeof(b) + sizeof(wasted space) + sizeof(i) = 4 + 2 + 2 + 4 = 12

frankli · 06-15-2004, 03:20 AM

thanks,I made a mistake.

vijayviji · 03-16-2011, 02:50 AM

struct s2 {
char a;
int i __attribute__ ((packed));
char b;
int i1;
};

(on 32-bit machine)
Its showing the size of this structure is 12. Can you plz tell me why ?

vijayviji · 03-16-2011, 03:00 AM

Yup.. I found it..

Its allocating as follows,

1 byte + 4 bytes + 1 byte + 2 bytes + 4 bytes
char a, int i, char b, zeroes, int i1

eric.r.turner · 03-16-2011, 07:16 AM

Quote:

Originally Posted by vijayviji

Yup.. I found it..

LOL, this thread is nearly seven years old!

james strouth · 03-16-2011, 03:52 PM

I found this thread helpful. Thanks for the __attribute__ ((packed)) explaination.

rajesh180186 · 09-11-2012, 01:52 AM

hi ,

struct s2 {
char a;
int i __attribute__ ((packed));
char b;
int i1;
};

here sizeof(s2) = 12.

But actually size should be 10 ( 1 byte+4 bytes + 1 byte + 4 bytes ),how ?

how it would be ( 1 byte+4 bytes + 1 byte + 2 bytes + 4 bytes ) =
sizeof(a)+ sizeof(i) +sizeof(b)+ 2 zeros + sizeof(i1))
here what is this another 2 bytes ( 2 zeros ) ? Can anyone ,please explain ?

Thanks in advance .

regards,
Rajesh.

NevemTeve · 09-11-2012, 02:47 AM

You might want to use two packed's:

Code:

struct s2 {
    char a;
    int i __attribute__ ((packed));
    char b;
    int i1 __attribute__ ((packed));
};

But please note that this packed stuff:

- is a non-standard extension
- slows down the processing
- should be avoided

eric.r.turner · 09-11-2012, 08:32 AM

Quote:

Originally Posted by rajesh180186

hi ,

struct s2 {
char a;
int i __attribute__ ((packed));
char b;
int i1;
};

here sizeof(s2) = 12.

But actually size should be 10 ( 1 byte+4 bytes + 1 byte + 4 bytes ),how ?

how it would be ( 1 byte+4 bytes + 1 byte + 2 bytes + 4 bytes ) =
sizeof(a)+ sizeof(i) +sizeof(b)+ 2 zeros + sizeof(i1))
here what is this another 2 bytes ( 2 zeros ) ? Can anyone ,please explain ?

Thanks in advance .

regards,
Rajesh.

The processor is adding an extra two bytes after b so that the next integer starts on an 8 byte boundary. If you want the integer to start immediately after the char, you have to tell the processor to pack it.

NevemTeve · 09-11-2012, 09:03 AM

it's four, actually, not eight

rstewart · 09-11-2012, 09:37 AM

Why not use the pack pragma to set the alignment around the structure? IE:

Code:

#pragma pack(1)

struct s2 {
    char a;
    int i;
    char b;
    int i1;
};

#pragma pack()

sizeof(struct s2): 10

Whereas:

Code:

struct s2 {
    char a;
    int i;
    char b;
    int i1;
};

sizeof(struct s2): 16