问:如何通过系统调用捕获错误？

Question

我正在构建一个GUI应用程序,我在其中进行系统调用并调用gnuplot来运行脚本.现在我想构建一个错误消息,说明什么时候出错(例如没有安装gnuplot或者路径错误).

所以我一直在考虑推出一个QMessageBox,但我不知道如何检查系统调用是否成功.

if(//System call didn't work)
{
    QMessageBox msgBox;
    msgBox.setWindowTitle("Error");
    msgBox.setIcon(QMessageBox::Critical);
    msgBox.setText("GNUPLOT was not installed");
    msgBox.exec();
}

我的系统调用如下所示:

system(gnuplot script.txt);

有什么建议？

Answer 1

您应该使用QProcess而不是低级系统调用,因为它是Qt代码库中的一个很好的抽象.否则,您将最终处理特定于平台的位.QProcess已经为你解决了这个问题.如果你恰好使用阻塞方法,也就是说.同步,你可以写下面的代码.

QProcess process;

process1.start("gnuplot arg1 arg2 etc");

// Wait for it to start
if(!process.waitForStarted())
    return 0;

bool retval = false;
QByteArray buffer;
while ((retval = process.waitForFinished()));
    buffer.append(process.readAll());

if (!retval) {
    qDebug() << "Process 2 error:" << process.errorString();
    msgBox.setText(buffer);
    return 1;
}

如果你不想在你的gnuplot脚本运行时阻塞,你可以将一个插槽或简单的lambda与C++ 11连接到readyRead()信号.当然,您还需要连接到error()信号.没有lambda的代码可以使用前C++ 11环境,如下所示:

GnuPlotReader::GnuPlotReader(QQProcess *process, QObject *parent)
    : QObject(parent)
    , m_process(process)
    , m_standardOutput(stdout)
{
    connect(m_process, SIGNAL(readyRead()), SLOT(handleReadyRead()));
    connect(m_process, SIGNAL(error(QProcess::ProcessError)), SLOT(handleError(QProcess::ProcessError)));
    connect(&m_timer, SIGNAL(timeout()), SLOT(handleTimeout()));

    m_timer.start(5000);
}

GnuPlotReader::~GnuPlotReader()
{
}

void GnuPlotReader::handleReadyRead()
{
    m_readData = m_process->readAll();

    if (!m_timer.isActive())
        m_timer.start(5000);
}

void GnuPlotReader::handleTimeout()
{
    if (m_readData.isEmpty()) {
        m_standardOutput << QObject::tr("No data was currently available for reading from gnuplot") << endl;
    } else {
        m_standardOutput << QObject::tr("GnuPlot successfully run")<< endl;
    }

}

void GnuPlotReader::handleError(QProcess::ProcessError processError)
{
    if (processError == QProcess::ReadError) {
        m_standardOutput << QObject::tr("An I/O error occurred while reading the data, error: %2").arg(m_process->errorString()) << endl;
        messageBox.setText(m_readData);
    }
}