Allign it or you loose it

[baray98]

i guys i have a struct defined as below

typedef struct s_myStruct
    {
        uint8          A;       
        uint8          B;   
        uint8          C;        
        uint8          D; 
        uint32         E; 
        uint16         F;     
    }   t_myStruct;

typedef struct s_myStruct
    {
        uint8          A;       
        uint8          B;   
        uint8          C;        
        uint8          D; 
        uint32         E; 
        uint16         F;     
    }   t_myStruct;

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by the way i have it defined somewhere these uint stuff as

typedef char int8;
typedef unsigned char uint8;
typedef short int int16;
typedef long int  int32;
typedef unsigned short int uint16;
typedef unsigned long int uint32;

typedef char int8;
typedef unsigned char uint8;
typedef short int int16;
typedef long int  int32;
typedef unsigned short int uint16;
typedef unsigned long int uint32;

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Now, my question is that when i do a

sizeof (t_myStruct)

sizeof (t_myStruct)

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it will come back as 12 , I was expecting 10.

what did i do wrong please guide me ...
baray98

[jacek]

Because this structure was aligned to word (i.e. "double word" on IA-32) boundary. If you want to make it smaller, you have to tell your compiler to pack it.

In GCC/MinGW you can do this using attributes:

struct Foo
{
   ...
} __attribute__((packed));

struct Foo
{
   ...
} __attribute__((packed));

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(of course it won't work with other compilers).

[baray98]

jacek,

in my apps i have so many struct that i am expecting it to allign in bytes (8 bit) is there any other option in the compiler so that the whole app will allign in bytes i am using GCC/MingW by the way .

thanks for the quick reply

baray98

[jacek]

You can use -fpack-struct, but I'm not sure if you really want this.

Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface.