Thread: memcpy - & or not?

Hi,

I want to use memcpy to copy parameters. I am not sure whether my usage of & is correct or not? Could someone help me please?

Code:

    memcpy(m_msgParms.m_mqprReplyQueue, msgDescriptor.ReplyToQ, sizeof(m_msgParms.m_mqprReplyQueue)); //MQCHAR48
    //m_mqprReplyQueue[48];

    memcpy(m_msgParms.m_mqprMessageId, msgDescriptor.MsgId, sizeof(m_msgParms.m_mqprMessageId)); //MQBYTE24
     //char m_mqprMessageId[24];

    memcpy(m_msgParms.m_mqprFormat, msgDescriptor.Format, sizeof(m_msgParms.m_mqprFormat)); //MQCHAR8
     //char m_mqprFormat[4];

    memcpy(m_msgParms.m_mqprPersistency, &msgDescriptor.Persistence, sizeof(m_msgParms.m_mqprPersistency)); //MQLONG
     //char m_mqprPersistency[4];

My struct-members (m_msgParms) are char arrays. The data types after // are the data types from the source (MsgDescriptor)

Ok thank you very much.If my second Argument is pointer, I have to use &,correct?
const char * data = buffer.getBuffer();

memcpy(m_mqprMessageP, &data, sizeof(m_mqprMessageP));

I want to set a Pointe to the buffer.

Ignore my message in #2. I realised I'm wrong in my explanation as I confused myself a bit... Sorry about that.

Let's say you have an array A.

Code:

int A [] = { 1 , 2 , 3 , 4 , 5 , 6 , 7 };
cout << A; // Some memory location 0xCCCCCCCC
cout << *A; // 1
cout << &A; // Will also give you 0xCCCCCCCC contrary to what I said in #2 (confused myself a teeny bit...) but remember this: A is a pointer to the first element so you can do something like:
cout << *(A + 1); // which yields you 2, but you cannot do this:
cout << &A + 1; // as &A is a pointer to your entire array, by pointer arithmetic, you end up at a location (7 * sizeof(int)) ahead of where A points to which is out of the bound of your array

So, in your example, you can just pass 'data' as it's a pointer itself. Since, &data will point to the entire array of char, you could pass that too as source, as both hold the same memory location in case of arrays.

[EDIT]
Also, it might be worth noting that an array knows its size in general but a pointer doesn't
[/EDIT]

Originally Posted by Zeus_

Your ReplyToQ, MsgID and Format are all char arrays, and you haven't passed them as their & equivalent so it only makes sense to do the same for Persistence. As everything is already a pointer, and memcpy expects a void* and a const void*, pass them without the &.

> Is that correct?

Yep

This is my complete memcpy-part. In that case, it should work now (all of my members are char [] and HyqcmqprParms is my struct):

Code:

const BASBuffer & buffer = msg->buffer(); 
    const char * data = buffer.getBuffer(); 
    MQLONG dataLength = buffer.tell();    


    m_msgParms = new HyqcmqprParms(); //create new object on heap
    memset(m_msgParms, 0, sizeof (*m_msgParms)); // set all members to 0


    memcpy(m_msgParms.m_eyeCatcher, validEyeCatcher, sizeof(m_msgParms.m_eyeCatcher)); //const char

    memcpy(m_msgParms.m_version, version_1_0, sizeof(m_msgParms.m_version)); //const char

    memcpy(m_msgParms.m_mqprQueueMgr, msgDescriptor.ReplyToQMgr, sizeof(m_msgParms.m_mqprQueueMgr));

    memcpy(m_msgParms.m_mqprQueue, m_queueinfo.m_queueName, sizeof(m_msgParms.m_mqprQueue)); //const char

    memcpy(m_msgParms.m_mqprReplyQueue, msgDescriptor.ReplyToQ, sizeof(m_msgParms.m_mqprReplyQueue));
 
    memcpy(m_msgParms.m_mqprMessageP, &data, sizeof(m_msgParms.m_mqprMessageP));
    
    memcpy(m_msgParms.m_mqprMessageL, dataLength, sizeof(m_msgParms.m_mqprMessageL));

    memcpy(m_msgParms.m_mqprMessageId, msgDescriptor.MsgId, sizeof(m_msgParms.m_mqprMessageId));

    memcpy(m_msgParms.m_mqprCorrelId, msgDescriptor.CorrelId, sizeof(m_msgParms.m_mqprCorrelId));

    memcpy(m_msgParms.m_mqprCommitScope, putMsgOptions.Options, sizeof(m_msgParms.m_mqprCommitScope));

    memcpy(m_msgParms.m_mqprFormat, msgDescriptor.Format, sizeof(m_msgParms.m_mqprFormat));

    memcpy(m_msgParms.m_mqprPersistency, msgDescriptor.Persistence, sizeof(m_msgParms.m_mqprPersistency));

    memcpy(m_msgParms.m_mqprPriority, msgDescriptor.Priority, sizeof(m_msgParms.m_mqprPriority));

    memcpy(m_msgParms.m_mqprExpiry, msgDescriptor.Expiry, sizeof(m_msgParms.m_mqprExpiry));

    memcpy(m_msgParms.m_mqprTargetEntity, ' ', sizeof(m_msgParms.m_mqprTargetEntity));

    memcpy(m_msgParms.m_mqprSwid, ' ', sizeof(m_msgParms.m_mqprSwid));

    memcpy(m_msgParms.m_reserved1, ' ', sizeof(m_msgParms.m_reserved1));