structure padding alignment?

Question

typedef struct structc_tag
{
   char        c;
   double      d;
   int         s;
} structc_t;

Applying same analysis, structc_t needs sizeof(char) + 7 byte padding + sizeof(double) + sizeof(int) = 1 + 7 + 8 + 4 = 20 bytes but the sizeof(structc_t) will be 24 bytes.I didn't understood the alignment.Any one can help me?

This calculation is correct unless you pack the structures and you are on the 64 bit system which requires access to be aligned — 0___________, Sep 23 '17 at 08:14
sizeof(char) + 7 byte padding + sizeof(double) + sizeof(int) + 4 byte padding = 24 — zappy, Sep 23 '17 at 08:16
@zappy Is it because for 64-bit and 32-bit systems the memory needs to be aligned on a 4-byte boundary? — Gaurav Pathak, Sep 23 '17 at 08:57
The size needs to be 24 to ensure that the second element in an array is properly aligned. — Jonathan Leffler, Sep 23 '17 at 08:58
@GauravPathak Do you understand why 7 bytes are padded after first char? The largest type in the struct is double which is of size 8 bytes — zappy, Sep 23 '17 at 09:01
@zappy Yes, I understand that. It's a bit unclear to me regarding 4-bytes padding after the last integer. — Gaurav Pathak, Sep 23 '17 at 09:59
Got it. It is because of alignment rules `https://stackoverflow.com/questions/4374276/alignment-rules`. — Gaurav Pathak, Sep 23 '17 at 10:09
BTW, where you have the choice, putting your fields in decreasing size order minimizes the padding (e.g., moving ‘c` to be after `s` here. — John Hascall, Sep 23 '17 at 10:34

score 1 · Accepted Answer · answered Sep 23 '17 at 09:24

Your struct looks like this, if we write it all out:

typedef struct structc_tag
{
   char        c;
   char        pad1[7];
   double      d;
   int         s;
   char        pad2[4];
} structc_t;

The pad2 is what you don't want. It comes into play because the compiler assumes you may want to make an array of these things, in which case every d has to be aligned, so sizeof(structc_t) is 24.

You can enable struct packing using a compiler-specific extension like this:

typedef struct structc_tag
{
   char        c;
   char        pad1[7];
   double      d;
   int         s;
} __attribute__((packed)) structc_t;

Now pad1 is needed (otherwise d will immediately follow c which you don't want). And pad2 is no longer implicitly added, so the size should now be 20.

Does `__attribute__((packed)) structc_t` is used to avoid padding at the end of the structure? — Gaurav Pathak, Sep 23 '17 at 10:03
@GauravPathak: Yes. It also disables internal padding (which we add back in with `pad1`). — John Zwinck, Sep 23 '17 at 15:09

structure padding alignment?

1 Answers1