C++ sending a packet over socket - c++

Question

I'm trying to communicate with a server program I installed. The server sends and receives all data in the form of constructed packets that follow the setup of:
int int int string nullbyte
like this:
little endian signed int -> The size of the ID (4 bytes) + size of Type (4 bytes) + size of Body (minimum 1 for null terminator) + null byte for a minimum of 10 as the value;
little endian signed int -> id
little endian signed int -> packet type
Null terminiated ascii string -> body
null byte
I've managed to read the packets just fine but when I try to send the packet with the password, the server completely ignores it which means the packet is wrong some how. I construct the packet like this:
void Packet::build(){
/*
* Create unsigned char vector to store
* the data while we build the byte array
* and create a pointer so the byte array can
* be modified by the required functions.
*/
std::vector<unsigned char> packet(m_size);
unsigned char *ptr = packet.data();
/*
* Convert each of the three integers as well
* as the string into bytes which will be stored
* back into the memory that ptr points to.
*
* Packet data follows format:
* int32 -> Size of ID + Server Data + body (minimum of 10).
* int32 -> ID of request. Used to match to response.
* int32 -> Server Data type. Identifies type of request.
* String -> Minimum of 1 byte for null terminator.
* String -> Null terminator.
*/
storeInt32Le(ptr, m_sizeInPacket);
storeInt32Le(ptr, m_requestID);
storeInt32Le(ptr, m_serverData);
storeStringNt(ptr, m_body);
/*
* Store the vector in member variable m_cmdBytes;
*
*/
m_cmdBytes = packet;
}
storeInt32Le:
void Packet::storeInt32Le(unsigned char* &buffer, int32_t value) {
/*
* Copy the integer to a byte array using
* bitwise AND with mask to ensure the right
* bits are copied to each segment then
* increment the pointer by 4 for the next
* iteration.
*/
buffer[0] = value & 0xFF;
buffer[1] = (value >> 8) & 0xFF;
buffer[2] = (value >> 16) & 0xFF;
buffer[3] = (value >> 24) & 0xFF;
buffer += 4;
}
storeStringNt:
void Packet::storeStringNt(unsigned char* &buffer, const string &s) {
/*
* Get the size of the string to be copied
* then do a memcpy of string char array to
* the buffer.
*/
size_t size = s.size() + 1;
memcpy(buffer, s.c_str(), size);
buffer += size;
}
And finally, I send it with:
bool Connection::sendCmd(Packet packet) {
unsigned char *pBytes = packet.bytes().data();
size_t size = packet.size();
while (size > 0){
int val = send(m_socket, pBytes, size, 0);
if (val <= 0) {
return false;
}
pBytes += val;
size -= val;
}
return true;
}
Packet::bytes() just returns m_cmdBytes

Answer 1

As stated in comments, you are:
making assumptions about the byte size and endian of your compiler's int data type. Since your protocol requires a very specific byte size and endian for its integers, you need to force your code to follow those requirements when preparing the packet.
using memcpy() incorrectly. You have the source and destination buffers reversed.
are not ensuring that send() is actually sending the full packet correctly.
Try something more like this:
void store_uint32_le(unsigned char* &buffer, uint32_t value)
{
buffer[0] = value & 0xFF;
buffer[1] = (value >> 8) & 0xFF;
buffer[2] = (value >> 16) & 0xFF;
buffer[3] = (value >> 24) & 0xFF;
buffer += 4;
}
void store_string_nt(unsigned char* &buffer, const std::string &s)
{
size_t size = s.size() + 1;
memcpy(buffer, s.c_str(), size);
buffer += size;
}
...
std::vector<unsigned char> packet(13 + m_body.size());
unsigned char *ptr = packet.data(); // or = &packet[0] prior to C++11
store_uint32_le(ptr, packet.size() - 4);
store_uint32_le(ptr, m_requestID);
store_uint32_le(ptr, m_serverData);
store_string_nt(ptr, m_body);
...
unsigned char *pBytes = packet.data(); // or = &packet[0] prior to C++11
size_t size = packet.size();
while (size > 0)
{
int val = send(m_socket, pBytes, size, 0);
if (val < 0)
{
// if your socket is non-blocking, enable this...
/*
#ifdef _WINDOWS // check your compiler for specific defines...
if (WSAGetLastError() == WSAEWOULDBLOCK)
#else
if ((errno == EAGAIN) || (errno == EWOULDBLOCK) || (errno == EINTR))
#endif
continue;
*/
return false;
}
if (val == 0)
return false;
pBytes += val;
size -= val;
}
return true;