What will be the output of this ?
struct abc{
char i[5];
char b[8];
int j ;
};
struct abc s1;
According to me it should be 1 * 5 + 1 * 8 + 4 bytes = 17 bytes
When I do sizeof(s1) it shows 20 ! Why?
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Anirudh _k
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2Possibly because there is padding between members to have them properly aligned. `<[iiiii][bbbbbbbb]###[jjjj]>` – pmg Jan 20 '21 at 13:48
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The size and alignment requirement for `int` depends on the platform. `sizeof(int)` isn't necessarily 4. – François Andrieux Jan 20 '21 at 13:48
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1Try putting the int first. In general, always list the largest members in a union first, then next biggest etc. – Arthur Kalliokoski Jan 20 '21 at 13:49
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@ArthurKalliokoski: possibly (with members ordered by size) the padding goes to the end of the struct rather than the middle :-) – pmg Jan 20 '21 at 13:50
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@ArthurKalliokoski that seems like an arbitrary choice to me. What advantage would following that rule have? – couka Jan 20 '21 at 13:51
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@FrançoisAndrieux Thanks , I believe its due to padding of different compilers – Anirudh _k Jan 20 '21 at 13:51
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@pmg how did you decide that only ### 3 padds will be added ? – Anirudh _k Jan 20 '21 at 13:53
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Because you said `sizeof (struct abc)` is `20` – pmg Jan 20 '21 at 13:53
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oh okay . I thought you computed something @pmg – Anirudh _k Jan 20 '21 at 13:54
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@pmg can we connect on linkedIn ? – Anirudh _k Jan 20 '21 at 13:56
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1To the people replying to my comment about putting ints first, you want ints to be aligned to the word size of your machine so the bus doesn't have to make two or more accesses, but chars are accessed byte by byte so alignment doesn't matter for them. Somewhat similar for short ints. – Arthur Kalliokoski Jan 20 '21 at 13:56
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Maybe you would ask "why put padding between `b` and `j`?" ... Well because `b` has underlying `char` type and doesn't require alignment, and `j` requires alignment on address multiple of `4`. – pmg Jan 20 '21 at 13:56
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nope @Anirudh_k, I have no account on linkedin. – pmg Jan 20 '21 at 13:57
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@pmg makes sense !! Great – Anirudh _k Jan 20 '21 at 13:57
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@pmg any social media account ? – Anirudh _k Jan 20 '21 at 13:57
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Stack Overflow is my best social media available :-) – pmg Jan 20 '21 at 13:58
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@couka that'll minimize the padding because larger members have alignment requirement not smaller than smaller members, so internal paddings won't be necessary. See [The Lost Art of Structure Packing](http://www.catb.org/esr/structure-packing/) – phuclv Jan 20 '21 at 14:16