Two way communication using sockets - c++

Question

I am trying to implement two way communication using sockets and not quite sure where I'm going wrong. I have an application that launches a child application, the child application then tries to communicate with the application that launched it, but I am not getting anything.
In the application that launches the child:
int clsSocketThread::initialiseSocket(bool blnIsModule, QString strPurpose) {
const char* cpszLocalHost = "localhost";
//Get the socket
int intSocket = socket(AF_INET, SOCK_STREAM, 0);
if ( intSocket == 0 ) {
clsDebugService::exitWhenDebugQueueEmpty("Failed to create socket!");
}
struct hostent* pHostEntry = gethostbyname(cpszLocalHost);
if ( pHostEntry == nullptr ) {
clsDebugService::exitWhenDebugQueueEmpty("Unable to resolve ip address!");
}
//Initliase and get address of localhost
struct sockaddr_in srvAddr;
bzero((char*)&srvAddr, sizeof(srvAddr));
//Set-up server address
memcpy(&srvAddr.sin_addr, pHostEntry->h_addr_list[0], pHostEntry->h_length);
srvAddr.sin_family = AF_INET;
srvAddr.sin_port = htons(clsSocketThread::mscuint16Port);
char* pszIP = inet_ntoa(srvAddr.sin_addr);
if ( pszIP != nullptr ) {
qdbg() << "Setting up socket on ip: " << pszIP
<< ", port: " << clsSocketThread::mscuint16Port
<< ((strPurpose.isEmpty() == true) ? "" : strPurpose);
}
socklen_t tSvrAddr = sizeof(srvAddr);
int intRC;
#if !defined(STANDALONE)
if ( blnIsModule == true ) {
intRC = inet_pton(srvAddr.sin_family, pszIP, &srvAddr.sin_addr);
if ( intRC <= 0 ) {
clsDebugService::exitWhenDebugQueueEmpty("Invalid address not supported!");
}
intRC = ::connect(intSocket, (const struct sockaddr*)&srvAddr, tSvrAddr);
} else
#endif
{
intRC = bind(intSocket, (const struct sockaddr*)&srvAddr, tSvrAddr);
}
if ( intRC < 0 ) {
clsDebugService::exitWhenDebugQueueEmpty("Socket operation failed!");
}
if ( blnIsModule != true && listen(intSocket, 5) < 0 ) {
clsDebugService::exitWhenDebugQueueEmpty("Cannot listen to socket!");
}
return intSocket;
}
This function is used by both the launcher and the child, when the child calls it the first parameter is true. I've run both in debuggers and all the function calls are successful and there are no errors.
In the launching application I have a thread:
void clsSocketThread::serverSocketBody() {
if ( mintSocket == 0 ) {
mintSocket = clsSocketThread::initialiseSocket();
}
QByteArray qarybytBuffer;
char arycBuffer[2048];
int intNewSocket = 0;
size_t tBufferSize = sizeof(arycBuffer);
QJsonObject objJSON;
while( mpThread != nullptr ) {
if ( intNewSocket <= 0 ) {
struct sockaddr_in cliAddr;
socklen_t tCliLen = sizeof(cliAddr);
intNewSocket = accept(mintSocket, (struct sockaddr*)&cliAddr, &tCliLen);
if ( intNewSocket < 0 ) {
continue;
}
}
//Read from the other socket!
bzero(arycBuffer, tBufferSize);
ssize_t tRead = read(intNewSocket, arycBuffer, tBufferSize);
if ( tRead <= 0 ) {
continue;
}
qarybytBuffer = QByteArray(arycBuffer, (int)tRead);
int intIdx = qarybytBuffer.indexOf(clsJSON::msccOpenCurlyBracket)
,intIdx2 = qarybytBuffer.lastIndexOf(clsJSON::msccCloseCurlyBracket);
if ( intIdx >= 0 && intIdx2 > intIdx ) {
qarybytBuffer = qarybytBuffer.mid(intIdx, intIdx2 - intIdx + 1).trimmed();
QJsonObject objJSON(clsJSON(&qarybytBuffer).toQJsonObject());
if ( objJSON.contains(clsJSON::mscszMsgType) == true ) {
qdbg() << "[RX]Data: " << arycBuffer;//HACK
clsJSON::blnDecodeAccordingToType(&objJSON);
}
}
}
}
I'm not receiving any messages from the child. Both applications are set-up to communicate on localhost:8123

Answer 1

This is operating system specific.
For Linux, read Advanced Linux Programming then syscalls(2), socket(7), unix(7), fifo(7), pipe(7)
With Qt, consider using QSocketNotifier in your main thread.
You might also want to use POCO, ONCRPC, JSONRPC, Wt (perhaps with libcurl) or libonion.
You could get some inspiration by studying the C++ source code of Qt, of POCO, of VMIME.
Be aware that in many (but not all) cases, a single send(2) -or write(2)- on emitter side may correspond to several recv(2) -or read(2)- on the receiving side (and vice versa), at least with TCP on different machines. So you need some event loop (often around poll(2)...) and documented conventions on application-level message formats. Then SMTP or HTTP could be inspirational (and in some cases, useful).