C programm hangs when recv() not expected amount of bytes - c

Question

I have server side app in C that must read data and process it. Now it handles two messages: send screenshot to client and click mouse. When debugging I found that it receives 16 bytes when client ask a screenshot, but when I send click mouse message, server receives 32 bytes and hangs. Why is that happening and how to process all incoming packets correctly? I'am pretty new at winsock and there not much explaning informations or example how to handle data in wild. Here's the main code of server:
struct coord
{
int x;
int y;
};
struct send_packet
{
int magic;
int cmd;
coord Coords;
};
struct recv_packet
{
int magic;
int code;
int length;
BYTE body[0];
};
do
{
ZeroMemory(&buff, BUFLEN);
int numRcvBytes = 0;
while (numRcvBytes < sizeof(send_packet))
{
iResult = recv(ClientSocket, buff + numRcvBytes, BUFLEN, 0);
//if (iResult <= 0)
// closesocket(ClientSocket);
if (iResult == SOCKET_ERROR)
{
printf("Error %d", WSAGetLastError());
closesocket(ClientSocket);
WSACleanup();
}
numRcvBytes += iResult;
}
numRcvBytes = 0;
if (iResult > 0)
{
send_packet *pkt;
pkt = (send_packet *)buff;
printf("magc %d cmd %d\n", pkt->magic, pkt->cmd);
printf("Received %d %s\n", iResult, buff);
printf("m:%d c:%d x:%d y:%d\n", pkt->magic, pkt->cmd, pkt->Coords.x, pkt->Coords.y);
int command = pkt->cmd;
recv_packet *rcv_pkt;
int buff_size = 0;
BYTE *data_buffer = NULL;
int size;
int coordX;
int coordY;
wchar_t message[512];
switch (command)
{
case GET_SCREEN:
data_buffer = GetScreeny(75, &buff_size);
rcv_pkt = (recv_packet *)malloc(sizeof(recv_packet)+buff_size);
rcv_pkt->magic = MAGIC;
rcv_pkt->code = 0;
rcv_pkt->length = buff_size;
memcpy(rcv_pkt->body, data_buffer, buff_size);
size = sizeof(rcv_pkt->magic) + sizeof(rcv_pkt->code) + sizeof(rcv_pkt->length) + buff_size;
if (send(ClientSocket, (char *)rcv_pkt, size, 0) == SOCKET_ERROR)
{
printf("Error %d\n", WSAGetLastError());
closesocket(ClientSocket);
//WSACleanup();
//return 1;
}
free(rcv_pkt);
break;
case CLICK_MOUSE:
coordX = pkt->Coords.x;
coordY = pkt->Coords.y;
//wsprintf(message, L"x:%d y:%d", coordX, coordY);
//MessageBox(NULL, message, NULL, MB_OK);
MoveMouse(coordX, coordY);
ClickMouse();
break;
default:
break;
}
/*
char send_buff[1024+1] = "";
ZeroMemory(&send_buff, 1025);
memset(send_buff, 'A', 1024);
recv_packet *rcv_pkt = (recv_packet *)malloc(sizeof(recv_packet)+1024+1);
//recv_packet rcv_pkt = { 0 };
rcv_pkt->magic = MAGIC;
rcv_pkt->code = 0;
rcv_pkt->length = strlen(send_buff)+1;
memcpy(rcv_pkt->body, send_buff, 1025);
int size = sizeof(rcv_pkt->magic) + sizeof(rcv_pkt->code) + sizeof(rcv_pkt->length) + 1024 + 1;
//printf("%d", size);
//getchar();
//return 0;
*/
}
} while (iResult > 0);
I would appreciate any help! Thank you.

Answer 1

You should only read one message at a time. If there are 2 messages available in the socket, your current code will read both but only handle the first.
Change your read to:
iResult = recv(ClientSocket, buff + numRcvBytes, sizeof(send_packet) - numRcvBytes, 0);
Even better would be to extract the receiving to a separate function:
int recvBytes(int socket, size_t numBytes, void *buffer) {
int numRcvBytes = 0;
while (numRcvBytes < numBytes)
{
iResult = recv(socket, buff + numRcvBytes, numBytes - numRcvBytes, 0);
if (iResult == SOCKET_ERROR)
{
// Leave cleanup to caller
return SOCKET_ERROR;
}
numRcvBytes += iResult;
}
return numRcvBytes;
}