QLocalServer emits readyRead when bytes are not available

Question

I'm trying to start QLocalServer in my local OSX 10.11.

I have client which tries connection to server in loop:

int connect(const char* filename) {
  int sock;
  struct sockaddr_un serv_addr;

  memset(&serv_addr, 0x00, sizeof(serv_addr));
  serv_addr.sun_family = AF_LOCAL;
  strncpy(serv_addr.sun_path, filename, sizeof(serv_addr.sun_path) - 1);

  if ((sock = socket(AF_UNIX, SOCK_STREAM, 0)) == -1) {
    return sock;
  }

  if (connect(sock, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) == -1) {
    close(sock);
    sock = -1;
    return sock;
  }

  return sock;
}

int main() {
  int sock;
  while((sock = connect("my_socket_server")) == -1) {
    usleep(3000);
  }

  // The code never reaches this line

  const char* buffer = "hello";
  if (send(sock, buffer, strlen(buffer), 0) < 0) {
    exit(1);
  }

  return 0;
}

When this code is running, I try to start QLocalServer in another application:

// Starting server:

QString socket_path = "my_socket_server";
QLocalServer::removeServer(socket_path);
if (!server.listen(socket_path)) {
  return false;
}

connect(&server, &QLocalServer::newConnection, this, &MyServerClass::newConnection);

...

void MyServerClass::newConnection() {
  socket = server.nextPendingConnection(); // socket - member of MyServerClass
  connect(socket, &QLocalSocket::disconnected, socket, &QLocalSocket::deleteLater);
  connect(socket, &QLocalSocket::readyRead, this, &MyServerClass::readyRead);
}

...

void MyServerClass::readyRead() {
  if (!socket->bytesAvailable()) {
    exit(1); // THIS CODE WAS CALLED. WHY?
  }

  ...
}

Why when readyRead was called, bytes are not available?

Answer 1

You might be getting multiple connections, in which case the socket variable might be pointing to a different object from the one which emits the readyRead signal. Either use QObject::sender to get the correct object in the slot, or use the QSignalMapper. If you are using Qt5, you can also use a lambda function and capture the socket object.