CODEWITHSUNDEEP
clinux
raja
raja

Reputation: 13

structure padding alignment?

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?

Upvotes: 1

Views: 477

Answers (1)

John Zwinck
John Zwinck

Reputation: 249642

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.

Upvotes: 1

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