[cfe-users] Clang Sizeof give diff value for Microsoft and Linux

[David Blaikie via cfe-users]

On Wed, Feb 17, 2021 at 12:08 AM Vivek Pandey <
vivek.pandey at tallysolutions.com> wrote:

> I think Clang, on/for windows, should give a compile time flag/option that
> can be used to control it (A flag when set make compile-time operator like
> sizeoff to behave like MSVC or non-MSVC)
>

But that would break the ability for that code to call existing libraries
(including the MS runtime), I think - which would be quite broken/unusable,
so far as I know.


> As in the current form it is breaking building of cross-platform
> application.
>

What dependence does this application have on the size of certain
structures? That seems quite not-cross-platform to me & the code probably
should be changed to be flexible to the different size of layouts on
different platforms.


>
>
> With C++11 onward many things are incorporated in standard so that one
> code base can be used across platform. Now if compiler is blocking the flow
> then it seems moving back to pre C++11.
>
>
>
> *From:* David Blaikie <dblaikie at gmail.com>
> *Sent:* Friday, January 29, 2021 3:23 AM
> *To:* Vivek Pandey <vivek.pandey at tallysolutions.com>
> *Cc:* cfe-users at lists.llvm.org; Manu Agarwal <
> manu.agarwal at tallysolutions.com>; Tuhin Sengupta <
> tuhin.sengupta at tallysolutions.com>
> *Subject:* Re: [cfe-users] Clang Sizeof give diff value for Microsoft and
> Linux
>
>
>
> Clang on Windows is designed to be compatible with MSVC - which has
> different layout requirements than the Itanium ABI/GCC on Linux. I don't
> think there's a way to use the same ABI on both platforms - especially not
> if you are interacting with any code compiled by another compiler on both
> platforms (existing/foregin C++ precompiled libraries).
>
>
>
> On Thu, Jan 28, 2021 at 1:45 PM Vivek Pandey via cfe-users <
> cfe-users at lists.llvm.org> wrote:
>
> Hi Team,
>
>
>
> We are using Clang 11 for our product that has common C++ code base for
> Windows, Linux, Macintosh, ….
>
>
>
> We observed that sizeof operator gives different value on Windows and
> Linux/OSX, when the inheritance is from a common base class:
>
>
>
> Example Sample:
>
>
>
> #include <cassert>
>
>
>
> struct Base {}; // empty class
>
>
>
> struct Derived1 : Base {
>
>     int i;
>
> };
>
>
>
> struct Derived2 : Base {
>
>     Base c; // Base, occupies 1 byte, followed by padding for i
>
>     int i;
>
> };
>
>
>
> struct Derived3 : Base {
>
>     Derived1 c; // Derived1 is too derived from same Base class
>
>     int i;
>
> };
>
>
>
> int main()
>
> {
>
>     assert
> <https://ind01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fen.cppreference.com%2Fw%2Fcpp%2Ferror%2Fassert&data=04%7C01%7Cvivek.pandey%40tallysolutions.com%7C1a2156b9e1d146b7360608d8c3d71ae2%7C66bcd9b727254893bb969ae424774af6%7C0%7C0%7C637474675935486094%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C2000&sdata=NYeOWjC%2B0xhCxaFWPvJTpcN9xvz8uCZDV5gAKEan8J0%3D&reserved=0>(sizeof(Derived2)
> == 2*sizeof(int));
>
>
>
>     assert
> <https://ind01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fen.cppreference.com%2Fw%2Fcpp%2Ferror%2Fassert&data=04%7C01%7Cvivek.pandey%40tallysolutions.com%7C1a2156b9e1d146b7360608d8c3d71ae2%7C66bcd9b727254893bb969ae424774af6%7C0%7C0%7C637474675935496090%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C2000&sdata=ymboJjAMIu8lm26ZdA7L%2FArWNyxIUUu96c89R5f4oR8%3D&reserved=0>
> (sizeof(Derived3) == 3*sizeof(int));
>
> }
>
>
>
>               When we compile above program using Clang 11 and run it on
> windows and Linux  sizeof(Derived3) give different value.
>
>
>
> We don’t want a work-around via making first data member of derived class
> of type different from class that also is derived from same empty base
> class.
>
> Is there any flag that we can use so that it gives same result on all
> platform (Windows/Linux/OSX/Android/iOS/….)
>
> Or another way to solve this gracefully.
>
>
>
> Thank you!
>
>
>
> Best Regards,
>
> Vivek
>
>
>
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