Question

I have something like this, in fact more complex struct than this:

typedef struct _sample {
    unsigned char type;
    char *name;
    test *first;
} sample;
typedef struct _test {
    test *prev;
    test *next;
    char *name;
    int total;
    test_2 **list;
} test;
typedef struct _test_2 {
    char *name;
    unsigned int blabla;
} test_2;
sample *sample_var;

I want to backup this struct into a file and after restore it.

I also try with fwrite(sample_var, sizeof(sample), 1, file_handle); but the real problem is sizeof(sample) that return wrong size, not real variable size.

There is a way to save it into file & restore without knowing the size?

Answer 1

You are trying to serialize, or marshal the structure. You can’t just fwrite the data (having pointers is the most obvious stopper). The sizeof problem is really minor when compared to storing pointers in a file (a pointer is meaningless outside the program where it originated).

You will have to define your own serialization / deserialization functions. You could either use your own simple format or use JSON, XML, XDR or something like that.

Personally I would go with JSON, since it’s all the rage these days anyway.

As an aside, here is a C FAQ vaguely linked to your own question (though it discusses interoperabillity issues).