Switching between packed/unpacked structs

Question

I have a lot of configuration data in EEPROM (4KB) which I read out in packed structures. Throughout my firmware I need to read/change these values very frequently, and performance of packed structures is not optimal, so I have a second struct which is exactly the same, only not packed. I have to copy each value one by one from the packed struct to the unpacked one, which is error-prone, because when I add a value to the packed structure I also have to remember to add it to the copy-function.

Is there a smarter way to do this?

Answer 1

This is the over-smart method of doing this, using the precompiler:

mydata-fields.h

FIELD(int, one)
FIELD(char, two)
FIELD(long, three)

mydata.h

#define FIELD(t, n) t n;
struct MyData
{
#include "mydata-fields.h"
};
struct __attribute__((packed)) MyDataPacked /*or whatever your compiler needs */
{
#include "mydata-fields.h"
};
#undef FIELD

#define FIELD(t, n) a->n = b->n;
static inline void Unpack(MyData *a, MyDataPacked *b)
{
#include "mydata-fields.h"
}
static inline void Pack(MyDataPacked *a, MyData *b)
{
#include "mydata-fields.h"
}
#undef FIELD

UPDATE: With a bit of imagination you can even write a generic packed.h that receives the names of the struct and the fields file as parameters:

extract of packed.h

/* ... */
struct UNPACKED
{
#include FIELDS_H
};
/* and so on... */

And then simply in the real file:

mydata.h

#define UNPACKED MyData
#define PACKED MyDataPacked
#define FIELDS_H "mydata-fields.h"
#include "packed.h"
#undef FIELDS_H
#undef PACKED
#undef UNPACKED

This will be useful if you have a lot of packed/unpacked structures and want to avoid some typing.

A not so evil solution would be to write a script (in Python!) that generates the code from the fields specification in some input text file.