GCC - packed structures

 

GCC allows you to specify attributes of variables and structures using the keyword__attribute__, the syntax of which is __attribute__((attribute list)). One such attribute is __packed__ which specifies that

a variable or structure field should have the smallest possible alignment--one byte for a variable, and one bit for a field, unless you specify a larger value with the aligned attribute.
GCC Manual

which means that GCC will not add any of the zero's for padding (for memory alignement) and make variables or fields immediately next to each other. For example, here are some things I tried out -- I created a C source file - test.c

struct test_t {

  int  a;

  char b;

  int  c;

} ;

 

struct test_t test = { 10, 20, 30};

And compiled it with the -S option (ie to generate the assembly equivalent of the code generated).

$gcc -c test.c -S -o test.s

The file test.s -

        .file   "test.c"

.globl test

        .data

        .p2align 2

        .type   test,@object

        .size   test,12

test:

        .long   10

        .byte   20

        .zero   3

        .long   30

        .ident  "GCC: (GNU) 3.2.2 [FreeBSD] 20030205 (release)"

Notice the emphasized code. You can see that the structure "test" is being declared. First the field "a" (int) as .long 10 followed by "b" (char) as .byte 20. To keep the fields' word alignment, notice that GCC has added 3 zero bytes (.zero 3) before field "c" (int) which is declared as .long 30. This makes the effective sizeof struct test_t as 12 instead of the expected 9. Then I tried with the __packed__ attribute -

struct test_t {

  int  a;

  char b;

  int  c;

} __attribute__((__packed__));

 

struct test_t test = { 10, 20, 30};

and the "-S" output I got after compiling was

        .file   "test.c"

.globl test

        .data

        .type   test,@object

        .size   test,9

test:

        .long   10

        .byte   20

        .long   30

        .ident  "GCC: (GNU) 3.2.2 [FreeBSD] 20030205 (release)"

in which the zeros are missing making the sizeof structure test_t = 9. Always remember that memory alignment is *good* even if it compromises space, so think twice before using this attribute. It is generally useful when you want to assign a structure to a block of memory and manipulate it through the fields of a structure.

LINKS

-Structure packing.
-Specifying attributes of variables.

By vivek on 2004-03-16