CODEWITHSUNDEEP
androidc++visual-c++structclang
Max Raskin
Max Raskin

Reputation: 1052

Clang and Visual C++ struct alignment compatibility issues

We're having a problem struct memory packing and alignment.

Android is not honoring the #pragma pack(push, <n>) which is in several hundred places in our code base. This is causes segfault.

The Android Clang compiler requires an __ attribute __ decorator on the struct or class, for example:

struct __attribute__((packed, aligned(8))) Test
{
    char a;
    char b;
    double d;
};

As opposed to this for Visual C++ that honors the pragma:

#pragma pack(push, 8)
struct  Test
{
       char a;
       char b;
       double d;
};
#pragma pack(pop)

Since the use of #pragma pack is so widespread, it will be a time-consuming task to fix.

We tried using the -mms-bitfields compiler flag which sets the default structure layout to be compatible with the Microsoft compiler standard (i.e. it honors the #pragma pack). However this only works for trivial structs and not classes with base classes or virtual functions. We get the following error with these type of classes.

“error : ms_struct may not produce Microsoft-compatible layouts for classes with base classes or virtual functions [-Wincompatible-ms-struct]”

How can we mitigate this issue - is there any workaround to make #pragma pack work for non-trivial structs/classes other than to go over all the classes/struct between push and pop pragmas and add the packed attribute?

Thanks

Upvotes: 2

Views: 1448

Answers (1)

cdonat
cdonat

Reputation: 2822

First of all, I have the impression, that you are doing something fundamentally wrong, when you have "several hundred places" in your code, where you need to define alignment to prevent a segfault. This pragma is non standard and it is not widespread to use it. Most noticeably it is not widespread to use it as extensive as you do. It is not in the standard as well.

Anyway, since clang will ignore the pragma and msvcc will ignore the attributes, I'd put both in the code. You might use e.g. grep and sed to prevent a lot of manual work.

Upvotes: 2

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