Strange struct padding?

**cyberfish** · 12-14-2008

I understand that the standard doesn't specify padding etc etc, but this is a "real world" question.

I have a struct

Code:

struct S {
        long long int a;
        int b;
        int c;
        char d;
        char e;
};

and I expect its size to be 18 bytes, since no padding is needed (ints already aligned to 4-bytes boundaries, and chars don't matter).

But gcc 4.2.3 on x86-64 Linux gives me a size of 24. How does that work?

*edit* is it so that if I stack them up in an array, the long long member will be aligned to 8-bytes boundaries? */edit*

**C_ntua** · 12-14-2008

edit: meeh, I am not sure will let someone else give a better answer

**tabstop** · 12-14-2008

I believe your edit is the correct reason.

**Elysia** · 12-14-2008

Both chars are apparently padded. Makes sense, since otherwise the "e" member would not be aligned. And if the last member isn't 4 bytes, then arrays won't be aligned.

**CornedBee** · 12-14-2008

Structs are typically end-padded to the size divisible by the alignment requirements of the most demanding member. Your guess is exactly the correct reason.

**cyberfish** · 12-14-2008

I see.

Many thanks.

**Sandhya Reddy** · 07-12-2011

Structure padding is done by the compilers and this depends on the architectures. Some architectures cannot access the data which will be stored on the odd addresses or they may find difficult to access it. This is the reason for padding extra bytes.
Structure padding in C

**cyberfish** · 07-12-2011

Stop bumping 4 years old threads!! Look at the date of the last post.

**laserlight** · 07-12-2011

Originally Posted by cyberfish

Stop bumping 4 years old threads!! Look at the date of the last post.

Quoted For Truth

*thread closed*

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