Sending Struct over UDP

[[email protected]]

I want to send a STRUCT over UDP using CAsyncSocket, based on a demo class I found called CUdpAsySk. Sending strings work nicely. Then creating a struct:

//struct defined in CUdpAsySk.h
struct StoSend
{
int16 version;
double gps_lat;
double gps_long;
}
class CUdpAsySk : public CAsyncSocket
{
public:
StoSend*pStoSend;
StoSend*pStoReceive;
...

}
//Send struct in void CUdpAsySk:oAsyncSendBuff()
m_strRemote="192.168.1.5";
m_nPort=4000;
dwBytes = SendTo((const char *)&pStoSend, sizeof(*pStoSend), m_nPort,m_strRemote);


//Receive
nRead = ReceiveFrom((char *)&pStoReceive, 699999, m_strRemote, m_nPort);

This work on local computer when bytes sent are equal or less then 20 bytes, from a remote computer no struct is received. When is failes I receive a:
Unhandled exception at 0x7c1772af (mfc71d.dll) in Device.exe: 0xC0000005: Access violation reading location 0x00000f94.

Anyone here that can help me with this problem?

[MrViggy]

Break into the debugger, and see where you are in the stack. There is not enough code provided to say. Also, please use code tags when posting code.

Viggy

[ahoodin]

Well just looking at your send command, you are sending a reference to a pointer. Should be:

dwBytes = SendTo((const char *)pStoSend, sizeof(*pStoSend), m_nPort,m_strRemote);

dwBytes = SendTo((const char *)&pStoSend, sizeof(*pStoSend), m_nPort,m_strRemote);


//Receive
nRead = ReceiveFrom((char *)&pStoReceive, 699999, m_strRemote, m_nPort);

Anyone here that can help me with this problem?

[[email protected]]

Do I need to convert the struct into bytes before sending it or is this the correct way of sending a struct over UDP?

[ahoodin]

Its fine.

Is your server echoing the bytes back?
nRead = ReceiveFrom((char *)&pStoReceive, sizeof(StoSend), m_strRemote, m_nPort);

[[email protected]]

dwBytes = SendTo((const char *)&pStoSend, sizeof(*pStoSend), m_nPort,m_strRemote);

nRead = ReceiveFrom((char *)&pStoReceive, sizeof(StoSend), m_strRemote, m_nPort);

Sending a struct of 24 bytes local the pStoReceive is received correctly, however sending more then 24 bytes I get a error msg: Unhandled exception at 0x7c1772af (mfc71d.dll) in Device.exe: 0xC0000005: Access violation reading location 0x00000f94.

using: LAN PC to PC no data is transmitted correctly?

using: dwBytes = SendTo((const char *)pStoSend, sizeof(*pStoSend), m_nPort,m_strRemote); no data is transmitted correctly?

[ahoodin]

If you are going to send an array you are going to have to multiply and reflect that in your sendto and recvfrom.

sizeof(StoSend) * nArrayElements

[[email protected]]

Finally found the correct way, the problem is unneeded casting and incorrect handling of pointers

dwBytes = SendTo(pStoSend, sizeof(*pStoSend), m_nPort,m_strRemote);

nRead = ReceiveFrom(pStoReceive, sizeof(*pStoReceive), m_strRemote, m_nPort);

[ahoodin]

Well you really shouldn't use sizeof on a dereferenced pointer, you should use sizeof on a type.

Hence:

dwBytes = SendTo(pStoSend, sizeof(StoSend), m_nPort,m_strRemote);

nRead = ReceiveFrom(pStoReceive, sizeof(StoReceive), m_strRemote, m_nPort);