as far as I know gcc uses double-word alignment as default. Therefore your Value3 which only is a short will increase the struct size up the the next value dividable by 4.
If you wish to have 1 byte alignment us the __attribute__ compiler directive and play around with it a little.
//---------------------------------------------------------------------------
#define _ALIGN1_ __attribute__ ((aligned (1))) #define _ALIGN2_ __attribute__ ((aligned (2))) #define _ALIGN4_ __attribute__ ((aligned (4))) #define _PACKED_ __attribute__ ((packed))
typedef struct
{
unsigned char sync[ 2 ];
unsigned char clss;
unsigned char id;
unsigned short len;
} _PACKED_ sUbxTgmHeader;//---------------------------------------------------------------------------
For more details please see the Cynus gcc compilers manual.
Cheers,
Roman
Hello,
Who can tell me why sizeof(Test.Value) gives 12 bytes? I expected 10 bytes. Due to allignment in my 32bit target (both intel and arm) two bytes are added before 'Value4', and sizeof(TestStruct) is 16bytes Here is my test program:
test.cpp: -------- #include "stdio.h"
int main()
{ struct TestStruct
{
struct
{
unsigned long Value1;
unsigned long Value2;
unsigned short Value3;
} Value;
unsigned long Value4;
} Test; printf("\r\n" "Sizeof Test: %d", sizeof(Test));
printf("\r\n" "Sizeof Value: %d", sizeof(Test.Value));
printf("\r\n" "Expected size of Value: 10");
}
----------target: intel P3, gcc 3.2 20020927 (prerelease) - the target was build under cygwin: 'gcc test.cpp' and tested: './a.exe'. result: ----------- $ ./a.exe
Sizeof Test: 16 Sizeof Value: 12 Expected size of Value: 10 ----------- On my ARM target (gcc 3.3) I detected the same problem.
Eric
