copying struct to char

Hello..

Im working on a dns resolver program and Im having some problems with making the request for dns server..

the header structure is:

Code:

---

typedef struct {
        unsigned short id;
        unsigned short flags;
        unsigned short qdc;
        unsigned short anc;
        unsigned short nsc;
        unsigned short arc;
} dnsheader;

int somefunc() {
        dnsheader *head = makeheader();        //this will create the header, set the values and pass the pointer to it

        //now i want to copy header to char
        char *buf = new char[1024];
        memset(buf, 0, sizeof(buf));
        memcpy(buf, &head, sizeof(dnsheader));

        char *ptr = buf + sizeof(dnsheader);

        //here i will put the rest of data in the buf char
        return 0;
}

---

What am I doing wrong?

Thanks a lot for help

Without knowing any exact compiler errors or which lines numbers errors occur, I think this could be a problem:

Code:

---

char *ptr = buf + sizeof(dnsheader);

---

you can do this..

Code:

---

char *ptr = buf;

---

or this:

Code:

---

memcopy(ptr, buf, sizeof(dnsheader));

---

> What am I doing wrong?
All sorts of things potentially.

1. Assuming that your struct is packed perhaps.

2. makeheader();
Does this return a pointer to local memory, or something more permanent (like allocated memory)?

3. memcpy(buf, &head, sizeof(dnsheader));
You probably want to copy what head pointed to (drop the &)

4. memset(buf, 0, sizeof(buf));
Tells you the size of the pointer (probably 4), not the amount it points to (1024)

Quote:

---

Originally Posted by Salem

1. Assuming that your struct is packed perhaps.

---

Some other code that I took a look at had packed struct, but I dont know what that means. Why would I need that? Where could I read more about it?

Thanks

Essentially, a compiler is free to add padding between struct members to better optimise the alignment of each data item (in memory) for efficient access.
http://c-faq.com/struct/padding.html

This comes as a surprise when people expect a struct to have a certain size. Mostly they print sizeof(myStruct) and ask why it isn't the same as the sum-of-sizes of the members of the struct.

Packing is a very compiler specific way of squeezing out the padding, at the expense of more complicated code (generated by the compiler).

Another issue is http://en.wikipedia.org/wiki/Endian