C / C++ / MFC

mo_nica881 wrote:

what is the role of #pragma pack(1).

mo_nica881 wrote:

in some program i always see "#pragma pack(1)" included in Header. what is the role of #pragma pack(1).

struct aligntest
{
    char aChar;
    int nNum;
};

#pragma pack( push, 1 ) // Set alignment to 1
struct IntParts
{
   BYTE byPart1;
   BYTE byPart2;
   BYTE byPart3;
   BYTE byPart4;
};
#pragma pack( pop ) // Restore previous settings

int nSplit = 123213;
IntPart ip = { 0 };
memcpy( &ip, &nSplit, sizeof( ip ));

Nibu babu thomas wrote:

Normally sizeof( aligntest ) will return 8 but the actual size is 5. Extra 3 bytes is used for padding. This is done for efficiency sake.

#pragma pack( push, 1 ) // Set alignment to 1
struct IntParts
{
   BYTE byPart1;
   BYTE byPart2;
   BYTE byPart3;
   BYTE byPart4;
};
#pragma pack( pop ) // Restore previous settings

int nSplit = 123213;
IntPart ip = { 0 };
memcpy( &ip, &nSplit, sizeof( ip ));

sk8er_boy287 wrote:

I'm pretty much sure (from experience) that sizeof always returns the size of the type of structure (5 for your code sample). sizeof doesn't care about memory alignment.

sk8er_boy287 wrote:

...(5 for your code sample).

#pragma pack( push, 1 ) // Set alignment to 1
struct IntParts1
{
   BYTE byPart1;
   BYTE byPart2;
   BYTE byPart3;
};
#pragma pack( pop ) // Restore previous settings

struct IntParts2
{
   BYTE byPart1;
   BYTE byPart2;
   BYTE byPart3;
};
IntParts1 ip1 = {0, 1, 2};
IntParts2 ip2 = {0, 1, 2};
memcpy( &ip1, &ip2, sizeof( ip1 ));

sk8er_boy287 wrote:

I wonder what this code would do (try to copy 8 bytes into 3) :

#pragma pack( push, 1 ) // Set alignment to 1
struct IntParts1
{
   char byPart1;
   int byPart2;
};
#pragma pack( pop ) // Restore previous settings

struct IntParts2
{
   char byPart1;
   int byPart2;
};

IntParts1 ip1 = {0};
IntParts2 ip2 = {0};
memcpy( &ip1, &ip2, sizeof( ip1 ) );
printf( "%d, %d\n", sizeof( IntParts1 ), sizeof( IntParts2 ) );

sk8er_boy287 wrote:

Here's another example which should copy 8 bytes into 5:

#pragma pack( push, 1 ) // Set alignment to 1
struct IntParts1
{
   char byPart1;   // offset 0, size 1 byte
   int byPart2;    // offset 1, size 4 bytes  (sizeof(IntParts1) == 5)
};
#pragma pack( pop ) // Restore previous settings
 
struct IntParts2
{
   char byPart1;   // offset 0, size 1 byte
   int byPart2;    // offset 4, size 4 bytes  (sizeof(IntParts2) == 8)
};

maxmaven wrote:

CWndDerived