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

Answer 1

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

Answer 2

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));
}