Padding in gcc

Padding in gcc

[Rajat Jain]

Hi,

Perhaps a little offtopic but no harm in trying ...

Can we always rely on the fact that gcc will pad the following structure?

typedef struct {
    int descrp;      SYSCALL64_STRUCT_ATTR;
    (Automatic padding here?)
    long long pos;   SYSCALL64_STRUCT_ATTR;
    int mode;        SYSCALL64_STRUCT_ATTR;
} lseek64_t;

What is the correct way to make it portable? Explicitly added padding fields?

Thanks,

Rajat
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Re: Padding in gcc

[pradeep singh]

On 7/9/07, Rajat Jain <Rajat.Jain@infogain.com> wrote:
> Hi,
>
> Perhaps a little offtopic but no harm in trying ...
>
> Can we always rely on the fact that gcc will pad the following structure?
>
> typedef struct {
>     int descrp;      SYSCALL64_STRUCT_ATTR;
>     (Automatic padding here?)
>     long long pos;   SYSCALL64_STRUCT_ATTR;
>     int mode;        SYSCALL64_STRUCT_ATTR;
> } lseek64_t;
>
> What is the correct way to make it portable? Explicitly added padding fields?
may be i am missing something but why is this not portable? And how
does padding makes this portable?
Please enlighten me.
Thanks
>
> Thanks,
>
> Rajat
>
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>


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RE: Padding in gcc

[Rajat Jain]

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Hi,
 
>>
>> Can we always rely on the fact that gcc will pad the following structure?
>>
>> typedef struct {
>>     int descrp;      SYSCALL64_STRUCT_ATTR;
>>     (Automatic padding here?)
>>     long long pos;   SYSCALL64_STRUCT_ATTR;
>>     int mode;        SYSCALL64_STRUCT_ATTR;
>> } lseek64_t;
>>
>> What is the correct way to make it portable? Explicitly added padding fields?

>may be i am missing something but why is this not portable? And how
>does padding makes this portable?

Hi ... sorry I could not ask very well, what I wanted to. I have a piece of code that ASSUMES that the above structure is padded (and I do not want to change that code). So how can I make sure that this structure is always padded (Is there any other way apart from adding explicit padding field?) 
 
So like we have a "packed" gcc attribute, do we have a "padded" gcc attribute or something like that (that ensures that gcc will always pad this structure.).
 
Thanks,
Rajat
 
 

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Re: Padding in gcc

[pradeep singh]

On 7/9/07, Rajat Jain <Rajat.Jain@infogain.com> wrote:
>
>
>
> Hi,
>
> >>
> >> Can we always rely on the fact that gcc will pad the following structure?
> >>
> >> typedef struct {
> >>     int descrp;      SYSCALL64_STRUCT_ATTR;
> >>     (Automatic padding here?)
> >>     long long pos;   SYSCALL64_STRUCT_ATTR;
> >>     int mode;        SYSCALL64_STRUCT_ATTR;
> >> } lseek64_t;
> >>
> >> What is the correct way to make it portable? Explicitly added padding
> fields?
>
> >may be i am missing something but why is this not portable? And how
> >does padding makes this portable?
>
> Hi ... sorry I could not ask very well, what I wanted to. I have a piece of
> code that ASSUMES that the above structure is padded (and I do not want to
> change that code). So how can I make sure that this structure is always
> padded (Is there any other way apart from adding explicit padding field?)
>
> So like we have a "packed" gcc attribute, do we have a "padded" gcc
> attribute or something like that (that ensures that gcc will always pad this
> structure.).
GCC will always pad it for best alignment depending on the
architecture. There is attribute to avoid this padding  though.
But padding is done by default depending on architecture for which you
compile your program.
So as long as cross compiliation process is correct, it should work i guess.

OTOH do you want to add just a fixed number of bytes irrespective of
the architecture?

Thanks
[PS : psbl has blacklisted me ,please CC to newbies too, just in case]

>
> Thanks,
> Rajat
>
>


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Re: Padding in gcc

[Rodrigo Rubira Branco]

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On Mon, 2007-07-09 at 17:10 +0530, Rajat Jain wrote:
> Hi,
> 
> Perhaps a little offtopic but no harm in trying ...
> 
> Can we always rely on the fact that gcc will pad the following structure?
> 
> typedef struct {
>     int descrp;      SYSCALL64_STRUCT_ATTR;
>     (Automatic padding here?)
>     long long pos;   SYSCALL64_STRUCT_ATTR;
>     int mode;        SYSCALL64_STRUCT_ATTR;
> } lseek64_t;
> 
> What is the correct way to make it portable? Explicitly added padding fields?
> 
> Thanks,
> 
> Rajat
> -
> To unsubscribe from this list: send the line "unsubscribe linux-newbie" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.linux-learn.org/faqs

Unless you explicitly say this structure is packed
(__attribute__((packed))) gcc will align it... for sure you need to take
care for yourself when you are trying to use different cache lines for
members of this structure.

Anyway, the code itself is portable, since you are just using C.


cya,


rodrigo (BSDaemon).

-- 
Rodrigo Rubira Branco
Software Engineer 
Advanced Linux Response Team (ALRT) / Linux on Power Toolchain
IBM Linux Technology Center (IBM/LTC)
rrbranco@br.ibm.com

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