C Valgrind - Source and destination overlap in memcpy

Question

I'm new to c programming and I'm writing a simple client server application. I get this message:

 Source and destination overlap in memcpy(0x41f0beb, 0x41f0258, 69141077)
    ==9522==    at 0x402D9A9: memcpy (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
    ==9522==    by 0x8049C13: message_to_buffer (message.c:92)

Here is the specific code:

case CT_ENTRY://100
{
    int c=(2+2+4+strlen(msg->content.entry->key)-1+4+4+strlen(msg->content.entry->value->data));
    char *offset=malloc(c);
    *msg_buf=offset;
    memcpy(offset,&opcode,2);
    offset+=2;
    memcpy(offset,&ctype,2 );
    offset+=2;
    int ks=strlen(msg->content.entry->key);
    int ksc=host_to_net(ks);
    memcpy(offset,&ksc,4);
    offset+=4;
    memcpy(offset, msg->content.entry->key, ks);
    offset+=ks;
    int l=host_to_net(get_time());
    memcpy(offset,&l,4);
    offset+=4;
    int ds=host_to_net(msg->content.entry->value->datasize);
    memcpy(offset,&ds,4);
    offset+=4;

    // this line here!
    memcpy(offset,msg->content.entry->value->data, msg->content.value->datasize);

    return c;
    break;

The offending line is

memcpy(offset,msg->content.entry->value->data, msg->content.value->datasize);

Can anyone please explain why this is happening? Thanks

Answer 1

You have to use memmove instead of memcpy if the source and destination memory blocks overlap.