cody cody - 9 days ago 9
C Question

how to serialize a struct in c?

I have a struct object that comprises of several primitive data types, pointers and struct pointers. I want to send it over a socket so that it can be used at the other end. As I want to pay the serialization cost upfront, how do I initialize an object of that struct so that it can be sent immediately without marshalling? For example

struct A {
int i;
struct B *p;
};

struct B {
long l;
char *s[0];
};

struct A *obj;

// can do I initialize obj?
int len = sizeof(struct A) + sizeof(struct B) + sizeof(?);
obj = (struct A *) malloc(len);
...

write(socket, obj, len);

// on the receiver end, I want to do this
char buf[len];

read(socket, buf, len);
struct A *obj = (struct A *)buf;
int i = obj->i;
char *s = obj->p->s[0];
int i obj.i=1; obj.p.


Thank you.

Answer

This answer is besides the problems with your malloc.

Unfortunately, you cannot find a nice trick that would still be compatible with the standard. The only way of properly serializing a structure is to separately dissect each element into bytes, write them to an unsigned char array, send them over the network and put the pieces back together on the other end. In short, you would need a lot of shifting and bitwise operations.

In certain cases you would need to define a kind of protocol. In your case for example, you need to be sure you always put the object p is pointing to right after struct A, so once recovered, you can set the pointer properly. Did everyone say enough already that you can't send pointers through network?

Another protocolish thing you may want to do is to write the size allocated for the flexible array member s in struct B. Whatever layout for your serialized data you choose, obviously both sides should respect.

It is important to note that you cannot rely on anything machine specific such as order of bytes, structure paddings or size of basic types. This means that you should serialize each field of the element separately and assign them fixed number of bytes.

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