struct padding influence in C struct serialization

问题:

I have the following structs in C:

typedef struct sUser {
    char name[nameSize];
    char nickname[nicknameSize];
    char mail[mailSize];
    char address[addressSize];
    char password[passwordSize];
    int totalPoints;
    PlacesHistory history;
    DynamicArray requests;
}User;

typedef struct sPlacesHistory {
    HistoryElement array[HistorySize];
    int occupied;
    int last;
}PlacesHistory;

and the functions:

void serializeUser( User * user, FILE * fp ) {
    fwrite( user, nameSize + nicknameSize + mailSize + addressSize + passwordSize + sizeof( int ) + sizeof( PlacesHistory ), 1, fp );
    serializeDynamicArray( user -> requests, fp );
}

User * loadUser( FILE * fp ) {
    User * user = malloc( sizeof( User ) );
    fread( user, nameSize + nicknameSize + mailSize + addressSize + passwordSize + sizeof( int ) + sizeof( PlacesHistory ), 1, fp );
    user -> requests = loadDynamicArray( fp );

    return user;
}

When I load the struct User, and I print that user (loaded from file), the field "last" of placesHistory has the value of 255 or -1, depending on the order of the fields of the PlacesHistory structure. But The User I saved had -1 on that member.

So when i get 255, it is obviously wrong.. I suspect this has to do about struct padding.

How can I do this in such a way that the order of fields in the structure doesn't matter?
Or which criteria do I need to follow to make things work right?
Do I need to fwrite/fread one member at a time? ( I would like to avoid this for efficiency matters )
Do I need to serialize to an array first instead of a file? (I hope not .. because this implicates to know the size of all my structures beforehand because of the mallocated array- which means extra work creating a function for every non simple structure to know it's size)

Note: *Size are defined constants
Note2: DynamicArray is a pointer to another structure.

回答1:

Yes, it probably has to do with padding in front of either totalPoints or history.

You can just write out sizeof(User) - sizeof(DynamicArray) and read back in the same. Of course this will only be compatible as long as your struct definitions and compiler don't change. If you don't need serialized data from one version of your program to be compatible with another version, then the above should work.

回答2:

Why are you adding up all elements individually? That's just adding a lot of room for error. Whenever you change your structure, your code might break if you forgot to change all the places where you add the size up (in fact, why do you add it up each time?).

And, as you suspected, your code doesn't account for structure padding either, so you may be missing up to three bytes at the end of your data block (if your largest element is 4 bytes).

Why not sizeof(User) to get the size of the amount of data you're reading/writing? If you don't want parts of it saved (like requests), then use a struct inside a struct. (EDIT: Or, like rlibby suggested, just subtract the sizeof of the part you don't want to read.)

My guess is that your strings sizes are not divisible by 4, so you are 3 bytes short, and as such, it's possible that you were supposed to read "0xffffffff" (=-1) but ended up just reading "0xff000000" (=255 when using little endian, and assuming that your structure was zeroed out initially).

回答3:

padding may be your problem because

nameSize + nicknameSize + mailSize + addressSize + passwordSize + sizeof( int ) + sizeof( PlacesHistory ) != sizeof( User ) 

so la last member (and last in struct) remain unitialized. To check this do a memset(,0,sizeof(User)) before reading from file.

To fix this use #pragma pack(push,1) before and #pragma pack(pop) after