c++ memory mess in arrays

Question

Why does this happen?

u_char macDst[6], macSrc[6], ipType[2]={0x88, 0x92};
u_char header[] = {0x04,0x00,0x20,0x00,...,};
const int len = sizeof(macDst) + sizeof(macSrc) + sizeof(header) + 2;
u_char* pck[len];

int j = 0, length = sizeof(macDst);
for (j; j < length; j++)
{
    pck[j] = &macDst[j];
}
length += sizeof(macSrc);

for (j; j < length; j++)
{
    pck[j] = &macSrc[j - sizeof(macDst)];
}

pck[j++] = &ipType[0];
pck[j++] = &ipType[1];

length += sizeof(header) + 2;

for (j; j < len; j++)
{
    pck[j] = &header[j - sizeof(macDst) - sizeof(macSrc)];
}
u_char testSend[170];

memcpy(testSend, *pck, sizeof(pck));

The var testSend only has macDst values (before initialized), while in the pointer I have all of them. If I copy them one by one, I get a good result:

for (int k = 0; k < 170; k++)
{
    testSend[k] = *pck[k];
}

Any idea??

Thanks!!

Edit:

This is a physic problem:

-----ooooooooo------------aaaa----bbbb----
- Null Data

When I make the pointers array I am doing this:

oooooooooaaaabbbb

But when I sent the packet and when I did the memcpy (or memmove) I was saying this: take this pointer and this amount of memory and copy them:

----|oooooooooo----------|----aaaaa----bbbb----

being the amount the sum of the 3 memory places.

Answer 1

I find the code convoluted, and I would never write it this way. Still, the error here:

memcpy(testSend, *pck, sizeof(pck));

You are dereferencing pck - and point to the first element, which is a char* . This is what you copy using your memcpy. On the other hand, your hand-written loop does dereference the pointer before copying it.

Answer 2

Fill the testSend array directly:

u_char *p = testSend;
memcpy(p, macDst, sizeof(macDst)); p += sizeof(macDst);
memcpy(p, macSrc, sizeof(macSrc)); p += sizeof(macSrc);
memcpy(p, ipType, sizeof(ipType);  p += sizeof(ipType);
memcpy(p, header, sizeof(header));