Find packed size of a structure

Question

Is there a way to find the packed size of a structure defined and declared without packed attribute in GCC compiler?

Example:

struct Name
{
   int a;
   char ch;
}

any function or macro like get_packed_size(Name) should return 5

No it shouldn't return 5. The size includes alignment and [padding](http://en.wikipedia.org/wiki/Sizeof#Structure_padding). It should return 8 on most platforms. — user207421, May 07 '14 at 07:30

score 2 · Answer 1 · answered May 07 '14 at 07:52

Define your struct using a macro that provides the required information. For example (though there are other possible implementations):

#define DEFINE_STRUCT_WITH_KNOWN_PACKED_SIZE(StructName, StructBody)\
struct StructName StructBody\
struct __attribute__ ((__packed__)) StructName##_packed StructBody

#define GET_PACKED_SIZE(StructName) sizeof (struct StructName##_packed)

DEFINE_STRUCT_WITH_KNOWN_PACKED_SIZE(Name, {
   int a;
   char ch;
};)

#include <stdio.h>

int main() {
    printf("%lu", GET_PACKED_SIZE(Name));
}

score 0 · Accepted Answer · answered May 07 '14 at 07:31

0

No, there's no way... you only have sizeof(), and you need to take care of the padding...

answered May 07 '14 at 07:31

Wagner Patriota

5,494
26
49

1

Obligatory: `sizeof` is not a function. :) – unwind May 07 '14 at 07:39

Find packed size of a structure

2 Answers2