Problem with struture size

My problem: this structure that was suposed to be 10 bytes big, is in fact 12... At least that's what the printf tells... I really need to resize the structure to exactly 10 bytes. The problem is that in this case 3 + 2 + 1 + 4 = 12. ;)

Code:

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#include<stdio.h>
#include<conio.h>

typedef struct{
        char        tagid  [ 3];        //0-2  TAG identifier. It contains of string "ID3"
        short        tagver :16 ;        //3-4  TAG version. Can be eg. 03 00
        char        flags  : 8 ;        //5                Flags
        long        size  :32 ;        //6-9  Size of TAG
} MP3ID3TAG2;

main(){
        printf("?: %d\n", sizeof(MP3ID3TAG2));
        getch();
}

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What's going on here??
P.S.: I tried both Visual C++ and GCC, they both do that same thing.

well if you really need it to be 10 bytes change the long to a short

[edit]whoops just looked that will mess you up but that would be the only way to size it down[/edit]

If you add the sizes well: 3 bytes from the id string, 1 byte from flags, 2 bytes from tag version, and 4 bytes from size, all added up does 10 bytes. But the printf presents a 12 byte size. Plus when doing freads from file data 12 bytes are placed inside the structure but I can't have access those extra 2. I did something raw like

Code:

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        MP3ID3TAG2 tag;
        //read file data to structure
        char *s = (char*)&tag;
        for(i=0;i<sizeof(MP3ID3TAG2);i++)
                printf("%2x", s[i]);

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Worked fine. The 12 bytes were there, but that's not the point. The problem is the compiler is assuming I have a 12 byte structure, but I only want 10, forcefully.

It's being aligned on word boundaries. In other words, it's being padded by your compiler.

On an aside, you are only supposed to use the bitfield specifier on integers. That is to say, "unsigned", "unsigned int", "signed int", or just "int". Anything else is undefined. C99 allows bit fields of type _Bool also.

Quzah.

Quote:

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Originally Posted by quzah

It's being aligned on word boundaries. In other words, it's being padded by your compiler.

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Yes I noticed something like that... So how can I undo that, or prevent it?? Are there any pre-compiler directives I may use??

Read a bit on bitfields. You may also want to look here at the #pragma pack directive.

Quzah.

Bah, pragma pack didn't worked.... but I choped the size field in 2 words, and worked fine. Thanks anyway.
Cheers

Quote:

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Originally Posted by xErath

<...> this structure that was suposed to be 10 bytes big, is in fact 12. <...> I really need to resize the structure to exactly 10 bytes. The problem is that in this case 3 + 2 + 1 + 4 = 12. ;)

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This is a design error. You should never write code based on such an assumption, because there is no way in C to satisfy it in a portable way.

The good way is to work 'by hand' bytes by bytes with an array of unsigned char, which exactly what a stream is.

However, the structure can be useful internally, but never as an interface with a stream or the like.

Note that the padding bytes are not the only problem. You also must consider endianness with objects bigger than one byte holding a numerical value (int, double etc.).

the reason will be word allignnent and padding problem.

if you deffine structure like this it will do the job i.e 10 bytes

typedef struct{
char tagid [ 3]; //0-2 TAG identifier. It contains of string "ID3"
char flags : 8 ; //5 Flags
short tagver :16 ; //3-4 TAG version. Can be eg. 03 00
long size :32 ; //6-9 Size of TAG
} MP3ID3TAG2;

regards

Bleh - see my reply to your other thread on the same subject