Qt源码分析--QProcess

QProcess 类用于启动外部程序并与它们进行通信。

官方调用例子：

QObject *parent;
    ...
    QString program = "./path/to/Qt/examples/widgets/analogclock";
    QStringList arguments;
    arguments << "-style" << "fusion";

    QProcess *myProcess = new QProcess(parent);
    myProcess->start(program, arguments);

可以看到最终是调用start()函数执行外部程序。

下面分析下start()函数的调用。

void start(const QString &program, const QStringList &arguments, OpenMode mode = ReadWrite);

可见最后一个参数是默认参数。

void QProcess::start(const QString &program, const QStringList &arguments, OpenMode mode)
{
    Q_D(QProcess);
    if (d->processState != NotRunning) {
        qWarning("QProcess::start: Process is already running");
        return;
    }
    if (program.isEmpty()) {
        d->setErrorAndEmit(QProcess::FailedToStart, tr("No program defined"));
        return;
    }
    d->program = program;
    d->arguments = arguments;
    d->start(mode);
}

void QProcess::start(OpenMode mode)
{
    Q_D(QProcess);
    if (d->processState != NotRunning) {
        qWarning("QProcess::start: Process is already running");
        return;
    }
    if (d->program.isEmpty()) {
        d->setErrorAndEmit(QProcess::FailedToStart, tr("No program defined"));
        return;
    }
    d->start(mode);
}

void QProcessPrivate::start(QIODevice::OpenMode mode)
{
    Q_Q(QProcess);
#if defined QPROCESS_DEBUG
    qDebug() << "QProcess::start(" << program << ',' << arguments << ',' << mode << ')';
#endif
    if (stdinChannel.type != QProcessPrivate::Channel::Normal)
        mode &= ~QIODevice::WriteOnly;     // not open for writing
    if (stdoutChannel.type != QProcessPrivate::Channel::Normal &&
        (stderrChannel.type != QProcessPrivate::Channel::Normal ||
         processChannelMode == QProcess::MergedChannels))
        mode &= ~QIODevice::ReadOnly;      // not open for reading
    if (mode == 0)
        mode = QIODevice::Unbuffered;
    if ((mode & QIODevice::ReadOnly) == 0) {
        if (stdoutChannel.type == QProcessPrivate::Channel::Normal)
            q->setStandardOutputFile(q->nullDevice());
        if (stderrChannel.type == QProcessPrivate::Channel::Normal
            && processChannelMode != QProcess::MergedChannels)
            q->setStandardErrorFile(q->nullDevice());
    }
    q->QIODevice::open(mode);
    if (q->isReadable() && processChannelMode != QProcess::MergedChannels)
        setReadChannelCount(2);
    stdinChannel.closed = false;
    stdoutChannel.closed = false;
    stderrChannel.closed = false;
    exitCode = 0;
    exitStatus = QProcess::NormalExit;
    processError = QProcess::UnknownError;
    errorString.clear();
    startProcess();
}

调用startProcess()函数时会根据操作系统的不同有不同的实现。

我们看下在linux系统下的实现。

void QProcessPrivate::startProcess()
{
    Q_Q(QProcess);
#if defined (QPROCESS_DEBUG)
    qDebug("QProcessPrivate::startProcess()");
#endif
    // Initialize pipes
    if (!openChannel(stdinChannel) ||
        !openChannel(stdoutChannel) ||
        !openChannel(stderrChannel) ||
        qt_create_pipe(childStartedPipe) != 0) {
        setErrorAndEmit(QProcess::FailedToStart, qt_error_string(errno));
        cleanup();
        return;
    }
    if (threadData->hasEventDispatcher()) {
        startupSocketNotifier = new QSocketNotifier(childStartedPipe[0],
                                                    QSocketNotifier::Read, q);
        QObject::connect(startupSocketNotifier, SIGNAL(activated(int)),
                         q, SLOT(_q_startupNotification()));
    }
    // Start the process (platform dependent)
    q->setProcessState(QProcess::Starting);
    // Create argument list with right number of elements, and set the final
    // one to 0.
    char **argv = new char *[arguments.count() + 2];
    argv[arguments.count() + 1] = 0;
    // Encode the program name.
    QByteArray encodedProgramName = QFile::encodeName(program);
#ifdef Q_OS_MAC
    // allow invoking of .app bundles on the Mac.
    QFileInfo fileInfo(program);
    if (encodedProgramName.endsWith(".app") && fileInfo.isDir()) {
        QCFType<CFURLRef> url = CFURLCreateWithFileSystemPath(0,
                                                          QCFString(fileInfo.absoluteFilePath()),
                                                          kCFURLPOSIXPathStyle, true);
        {
            // CFBundle is not reentrant, since CFBundleCreate might return a reference
            // to a cached bundle object. Protect the bundle calls with a mutex lock.
            static QBasicMutex cfbundleMutex;
            QMutexLocker lock(&cfbundleMutex);
            QCFType<CFBundleRef> bundle = CFBundleCreate(0, url);
            // 'executableURL' can be either relative or absolute ...
            QCFType<CFURLRef> executableURL = CFBundleCopyExecutableURL(bundle);
            // not to depend on caching - make sure it's always absolute.
            url = CFURLCopyAbsoluteURL(executableURL);
        }
        if (url) {
            const QCFString str = CFURLCopyFileSystemPath(url, kCFURLPOSIXPathStyle);
            encodedProgramName += (QDir::separator() + QDir(program).relativeFilePath(QString::fromCFString(str))).toUtf8();
        }
    }
#endif
    // Add the program name to the argument list.
    argv[0] = nullptr;
    if (!program.contains(QLatin1Char('/'))) {
        const QString &exeFilePath = QStandardPaths::findExecutable(program);
        if (!exeFilePath.isEmpty()) {
            const QByteArray &tmp = QFile::encodeName(exeFilePath);
            argv[0] = ::strdup(tmp.constData());
        }
    }
    if (!argv[0])
        argv[0] = ::strdup(encodedProgramName.constData());
    // Add every argument to the list
    for (int i = 0; i < arguments.count(); ++i)
        argv[i + 1] = ::strdup(QFile::encodeName(arguments.at(i)).constData());
    // Duplicate the environment.
    int envc = 0;
    char **envp = 0;
    if (environment.d.constData()) {
        envp = _q_dupEnvironment(environment.d.constData()->vars, &envc);
    }
    // Encode the working directory if it's non-empty, otherwise just pass 0.
    const char *workingDirPtr = 0;
    QByteArray encodedWorkingDirectory;
    if (!workingDirectory.isEmpty()) {
        encodedWorkingDirectory = QFile::encodeName(workingDirectory);
        workingDirPtr = encodedWorkingDirectory.constData();
    }
    // Start the process manager, and fork off the child process.
    pid_t childPid;
    forkfd = ::forkfd(FFD_CLOEXEC, &childPid);
    int lastForkErrno = errno;
    if (forkfd != FFD_CHILD_PROCESS) {
        // Parent process.
        // Clean up duplicated memory.
        for (int i = 0; i <= arguments.count(); ++i)
            free(argv[i]);
        for (int i = 0; i < envc; ++i)
            free(envp[i]);
        delete [] argv;
        delete [] envp;
    }
    // On QNX, if spawnChild failed, childPid will be -1 but forkfd is still 0.
    // This is intentional because we only want to handle failure to fork()
    // here, which is a rare occurrence. Handling of the failure to start is
    // done elsewhere.
    if (forkfd == -1) {
        // Cleanup, report error and return
#if defined (QPROCESS_DEBUG)
        qDebug("fork failed: %ls", qUtf16Printable(qt_error_string(lastForkErrno)));
#endif
        q->setProcessState(QProcess::NotRunning);
        setErrorAndEmit(QProcess::FailedToStart,
                        QProcess::tr("Resource error (fork failure): %1").arg(qt_error_string(lastForkErrno)));
        cleanup();
        return;
    }
    // Start the child.
    if (forkfd == FFD_CHILD_PROCESS) {
        execChild(workingDirPtr, argv, envp);
        ::_exit(-1);
    }
    pid = Q_PID(childPid);
    // parent
    // close the ends we don't use and make all pipes non-blocking
    qt_safe_close(childStartedPipe[1]);
    childStartedPipe[1] = -1;
    if (stdinChannel.pipe[0] != -1) {
        qt_safe_close(stdinChannel.pipe[0]);
        stdinChannel.pipe[0] = -1;
    }
    if (stdinChannel.pipe[1] != -1)
        ::fcntl(stdinChannel.pipe[1], F_SETFL, ::fcntl(stdinChannel.pipe[1], F_GETFL) | O_NONBLOCK);
    if (stdoutChannel.pipe[1] != -1) {
        qt_safe_close(stdoutChannel.pipe[1]);
        stdoutChannel.pipe[1] = -1;
    }
    if (stdoutChannel.pipe[0] != -1)
        ::fcntl(stdoutChannel.pipe[0], F_SETFL, ::fcntl(stdoutChannel.pipe[0], F_GETFL) | O_NONBLOCK);
    if (stderrChannel.pipe[1] != -1) {
        qt_safe_close(stderrChannel.pipe[1]);
        stderrChannel.pipe[1] = -1;
    }
    if (stderrChannel.pipe[0] != -1)
        ::fcntl(stderrChannel.pipe[0], F_SETFL, ::fcntl(stderrChannel.pipe[0], F_GETFL) | O_NONBLOCK);
    if (threadData->eventDispatcher.loadAcquire()) {
        deathNotifier = new QSocketNotifier(forkfd, QSocketNotifier::Read, q);
        QObject::connect(deathNotifier, SIGNAL(activated(int)),
                         q, SLOT(_q_processDied()));
    }
}

先把程序名也加到参数列表中，然后Fork出一个子进程，调用子进程execChild(workingDirPtr, argv, envp);