Qt 5.6 使用 QLocalServer 和 QLocalSocket 传递文件描述符

Question

我正在学习如何在 Ubuntu 14.04 上使用带有 Qt 5.6 的 Unix 域套接字传递文件描述符。从文档中可以看出，执行此操作的方法是使用 QLocalServer 和 QLocalSocket classes。

我创建了 2 个简单的应用程序，1 个服务器和 1 个客户端。它们每个只有 1 个 class，我已将整个 class 粘贴在下面。

我可以接收整数形式的 'file descriptor'，但是当我尝试以追加模式打开它以便在客户端进程中写入时，出现以下错误：

Cannot open existing file handle:  "Unknown error"
QIODevice::write: device not open
FD Received:  20

我做错了什么？

客户：

#include <QtWidgets>
#include <QtNetwork>

#include "localclient.h"

Client::Client(QObject *parent)
{

    socket = new QLocalSocket(this);
    connect(socket, SIGNAL(readyRead()), this, SLOT(readFd()));
    connect(socket, SIGNAL(error(QLocalSocket::LocalSocketError)), this, SLOT(displayError(QLocalSocket::LocalSocketError)));

    requestFd();

}

void Client::requestFd()
{

    socket->abort();
    socket->connectToServer("mysocket");

}

void Client::readFd()
{

    QDataStream in(socket);
    in.setVersion(QDataStream::Qt_4_0);

    if (socket->bytesAvailable() < (int)sizeof(quint16))
        return;

    if (in.atEnd())
        return;

    int nextFd;
    in >> nextFd;

    QFile rxFile;
    if ( !rxFile.open(nextFd,QIODevice::Append) ) {
        qDebug() << "Cannot open existing file handle: " << rxFile.errorString();
    }
    rxFile.write("hello");

    qDebug() << "FD Received: " << nextFd;

}

void Client::displayError(QLocalSocket::LocalSocketError socketError)
{
    switch (socketError) {
    case QLocalSocket::ServerNotFoundError:
        tr("The host was not found. Please check the "
           "host name and port settings.");
        break;
    case QLocalSocket::ConnectionRefusedError:
        tr("The connection was refused by the peer. "
           "Make sure the fortune server is running, "
           "and check that the host name and port "
           "settings are correct.");
        break;
    case QLocalSocket::PeerClosedError:
        break;
    default:
        tr("The following error occurred: %1.").arg(socket->errorString());
    }

}

服务器：

#include <QtWidgets>
#include <QtNetwork>

#include <stdlib.h>

#include "localserver.h"
#include <qlocalserver.h>
#include <qlocalsocket.h>

Server::Server(QObject *parent)
{

    server = new QLocalServer(this);
    if (!server->listen("mysocket")) {
        qDebug() << QString("Unable to start the server: %1.").arg(server->errorString());
        return;
    }

    qDebug() << tr("The server is running");

    connect(server, SIGNAL(newConnection()), this, SLOT(sendFd()));

    fileToSend = new QFile("/home/me/Desktop/test.bin");
    if ( !fileToSend->open(QIODevice::WriteOnly) ) {
        qDebug() << "Unable to open file to send";
    }

}

void Server::sendFd()
{

    QByteArray block;
    QDataStream out(&block, QIODevice::WriteOnly);
    out.setVersion(QDataStream::Qt_4_0);
    out.device()->seek(0);
    qDebug() << "Sending this file handle: " << fileToSend->handle();
    out << fileToSend->handle();

    QLocalSocket *clientConnection = server->nextPendingConnection();
    connect(clientConnection, SIGNAL(disconnected()),
            clientConnection, SLOT(deleteLater()));

    clientConnection->write(block);
    clientConnection->flush();
    clientConnection->disconnectFromServer();

}

Answer 1

在 linux 上，使用类型 SCM_RIGHTS 的辅助数据使用 Unix 域套接字发送文件描述符。 Qt好像不支持使用QLocalSocket和QLocalServer这样的数据，有个老QTBUG在讲这个问题。而且好像还没解决

你可以使用 Qt D-Bus where you can send file descriptors using QDBusUnixFileDescriptor class.Or I'm afraid you may have to use Unix domain sockets to implement that yourself, there is a good example here.

但是您确定您真的需要在应用程序之间传递文件描述符吗？对于学习以外的目的，我认为总有办法克服这个限制。

P.S。您正在使用 QDataStream 序列化套接字文件描述符（就好像它是一个普通的 int）然后发送序列化数据以供其他进程也像普通的 int 一样读取它。这将允许您读取文件描述符的整数值，但您不会允许您与它进行任何交互，因为文件描述符值在它们所属的进程之外没有意义。

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