Why does this c struct work？

Question

I see flow c language code in the windows driver kit(WDK) sample：

typedef struct _COMMON_DEVICE_DATA
{
    PDEVICE_OBJECT  Self;

    BOOLEAN         IsFDO;
    ......................


} COMMON_DEVICE_DATA, *PCOMMON_DEVICE_DATA;


typedef struct _PDO_DEVICE_DATA
{
    COMMON_DEVICE_DATA;

    PDEVICE_OBJECT  ParentFdo;

    .................

} PDO_DEVICE_DATA, *PPDO_DEVICE_DATA;

But when I want to test the smillar code , it build error.

test.c:14:6: error: ‘AA’ has no member named ‘flag’
     a.flag = 1;

Tesing code as fllow:

typedef struct __COMMON_DATA{     
    int flag;
}COMMON_DATA;

typedef struct __AA{
    COMMON_DATA;
    int x;
    int y;
}AA;

int main(int argc, char *argv[])
{
    AA a;
    a.flag = 1;
    return 0;
}

All seem same as the window sample code, But where is it wrong?

don't use reserved name in your code base (double underscore, underscore followed by capital letter) — Danh, Jan 06 '17 at 08:14
*"All seem same as the window sample code."* - In one code you use `a.flag`, in the other (as currently provided), you only declare some structure... The code are clearly different. — Holt, Jan 06 '17 at 08:17
use vistual studio build (the file extension is cpp), error: C2039: 'flag' : is not a member of '__AA'. — jim ying, Jan 06 '17 at 08:21
to Holt: I just want to know how can the wdk code works and why simple test code build error. — jim ying, Jan 06 '17 at 08:23

score 4 · Answer 1 · edited May 23 '17 at 12:07

4

The syntax used in the WDK code sample is a Microsoft extension for structure inheritance in C. But your error message looks like a gcc error message.

If you are actually using gcc, you could try building with -fms-extensions.

Link to related answer

edited May 23 '17 at 12:07

Community

1
1

answered Jan 06 '17 at 08:19

M.M

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A side note: http://softwareengineering.stackexchange.com/questions/134097/why-should-i-prefer-composition-over-inheritance . How can an AA be COMMON_DATA? It may have COMMON_DATA though. – user877329 Jan 06 '17 at 08:28
I build it use gcc, also can use vistual studio. I have built it with vs2010, also error. – jim ying Jan 06 '17 at 08:29
@user877329 the thread I linked to discusses that in more detail, I don't want to rehash in comments – M.M Jan 06 '17 at 08:29
@M.M I mean that with or without language support, inheritance is to the right thing here – user877329 Jan 11 '17 at 20:11

score 0 · Answer 2 · answered Jan 06 '17 at 08:18

0

there is no member named flag because AA has no member named flag

typedef struct __AA{
    COMMON_DATA aa_flag;
    int x;
    int y;
}AA;

You should access actual flag in the COMMON_DATA struct not in AA struct

a.aa_flag.flag = 1;

answered Jan 06 '17 at 08:18

cokceken

2,068
11
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Yes, I know it. But i want to know why the windows code works – jim ying Jan 06 '17 at 08:25

score 0 · Answer 3 · answered Jan 06 '17 at 08:21

If you've compile this with -Wall -Wextra -pedantic, you can see this warning before error:

./struct.c:6:16: warning: declaration does not declare anything
     COMMON_DATA;

It can be fixed with addition name of variable to code. This works fine:

typedef struct __COMMON_DATA{     
    int flag;
}COMMON_DATA;

typedef struct __AA{
    COMMON_DATA cd;
    int x;
    int y;
}AA;

int main(int argc, char *argv[])
{
    AA a;
    a.cd.flag = 1;
    return 0;
}

Why does this c struct work？

3 Answers3