How to fix C++ gcc compile warning for padding of struct

user_1035897 0
Viewed 5296

I have the following innocuous looking code:

void myFunc(){
    struct stance {
        long double interval;
        QString name;
    };
    // [...]
}

When I build this using standard version of gcc on Ubuntu 18.04 I get a warning like this:

MySource.cpp:12: warning: padding size of 'stance' with 8 bytes to alignment boundary (-wpadded)

I know that this warning shows up because the compiler needs to adjust the padding for my struct to something that I might not have expected and is kind enough to warn me as the user about this.

However, I am trying to have a warning-free build and so the question is, how can I make it explicit in my code in a standard compliant way that the compiler does not need to issue this warning?

So to be clear, I don't want to suppress warnings in my build script, nor using #pragma or similar either. I want to change the code of this struct so that my alignment expectations are explicit and match whatever the compiler wants to do hence not needing the warning to be displayed.

gcc-warning compiler-warnings suppress-warnings c++
2 Answers

Just disable the warning (or - well, don't enable it, I don't know of any warning set like -Wall or -Wextra that includes it). -Wpadded is not meant to be always enabled, unless you want to always manually specify the necessary padding explicitly.

-Wpadded

Warn if padding is included in a structure, either to align an element of the structure or to align the whole structure. Sometimes when this happens it is possible to rearrange the fields of the structure to reduce the padding and so make the structure smaller.

(emphasis added)

This is one case where it's not possible. long double is 10 bytes, and it requires 16 bytes alignment (4 on x86 Linux); QString is effectively a pointer, so it needs 8 bytes alignment (4 on 32 bit Linux). You can swap them however you want, but if you want to keep natural alignment (and thus best performance) you'll either get 6 + 8 bytes of padding or 8 + 6 bytes of padding.

In general, adding padding is not a problem, happens all the time and there are cases such as this when it's unavoidable. The general rule to keep it at a minimum is to place elements in order of decreasing alignment requirements, but again, it cannot always be avoided.

As mentioned above, the only alternative (keeping good alignment) is making the padding explicit, but it doesn't make much sense (unless you are designing a file format or something and you want to make everything explicit, but in that case you wouldn't use a QString and would pack to 1 byte).

struct stance {
    long double interval;
    char unused0[6];
    QString name;
    char unused1[8];
};
user_2146710
user_8411080
Related