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#include <stdio.h>
#include <stdlib.h>

typedef struct A
{
   char a;
   int b;
   char c;
} A;

typedef struct B
{
   char a __attribute__ ((packed));
   int b __attribute__ ((packed));
   char c __attribute__ ((packed));
} B;

typedef struct C
{
   char a;
   int b;
   char c;
} C __attribute__ ((packed));

#pragma pack(1)
typedef struct D
{
   char a;
   int b;
   char c;
} D;
#pragma pack()

int main(int argc, char *agrv[])
{
   printf("A=%i\n", sizeof(A));
   printf("B=%i\n", sizeof(B));
   printf("C=%i\n", sizeof(C));
   printf("D=%i\n", sizeof(D));
}

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/* The Multiboot information.  */
typedef struct multiboot_info
{
  unsigned long flags;
  unsigned long mem_lower;
  unsigned long mem_upper;
  unsigned long boot_device;
  unsigned long cmdline;
  unsigned long mods_count;
  unsigned long mods_addr;
  union
  {
    aout_symbol_table_t aout_sym;
    elf_section_header_table_t elf_sec;
  } u;
  unsigned long mmap_length;
  unsigned long mmap_addr;
} multiboot_info_t;

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sgeorge@yakumo ~/source
$ gcc -O2 structs.c -o structs.exe

sgeorge@yakumo ~/source
$ gcc -v
Reading specs from /usr/lib/gcc-lib/i686-pc-cygwin/3.3.1/specs
Configured with: /GCC/gcc-3.3.1-3/configure --with-gcc --with-gnu-ld --with-gnu-
as --prefix=/usr --exec-prefix=/usr --sysconfdir=/etc --libdir=/usr/lib --libexe
cdir=/usr/sbin --mandir=/usr/share/man --infodir=/usr/share/info --enable-langua
ges=c,ada,c++,f77,pascal,java,objc --enable-libgcj --enable-threads=posix --with
-system-zlib --enable-nls --without-included-gettext --enable-interpreter --enab
le-sjlj-exceptions --disable-version-specific-runtime-libs --enable-shared --dis
able-win32-registry --enable-java-gc=boehm --disable-hash-synchronization --verb
ose --target=i686-pc-cygwin --host=i686-pc-cygwin --build=i686-pc-cygwin
Thread model: posix
gcc version 3.3.1 (cygming special)

sgeorge@yakumo ~/source
$ ./structs
A=12
B=6
C=12
D=6

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union x
{
    double d;
    int i;
}

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union y
{
    int i[2];
    double d;
}

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   union record {
       struct person_record person;
       struct enterprise_record enterprise;
   };