using QProcess->setReadChannelMode(QProcess::MergedChannels) and using QProcess->readall() - c++

Question

It's my first time to ask question on stackoverflow. And I'm a chinese girl, if my description about this problem has so much grammar error that you can't understand it easyly, I'm so sorry.
Below is my question:
headerfile:
class AdbDriver : public QObject
{
Q_OBJECT
private:
QString PnPutilPath_;
QProcess *process_;
public:
explicit AdbDriver(QObject *parent = 0);
~AdbDriver();
void installDriver();
};
AdbDriver::AdbDriver(QObject *parent):QObject(parent){
PnPutilPath_ = qgetenv("WINDIR") + "\\sysnative\\pnputil.exe";
process_ = new QProcess();
process_->setReadChannelMode(QProcess::MergedChannels);
process_->setStandardOutputFile("E:/log.txt");
}
sourcefile:
AdbDriver::~AdbDriver(){
delete process_;
}
void AdbDriver::installDriver(){
QFile file(PnPutilPath_);
if(file.exists()){
qDebug()<<"pnputil.exe exist";
QString generaladbDriver = "E:/driver_androidusb/generaladb.inf";
qDebug()<<"the programming include driver："<<generaladbDriver;
QFile file(generaladbDriver);
if(file.exists()){
qDebug()<<"yes, the driver is right in bihu package";
}
else{
qDebug()<<"loss driver in bihu package";
}
QStringList arguments;
arguments<<"-i"<<"-a"<<generaladbDriver;
process_->start(PnPutilPath_, arguments);
while(!process_->waitForStarted()){
qDebug()<<"wait";
}
qDebug()<<"while out";
process_->waitForReadyRead();
qDebug()<<"start";
// qDebug()<<process_->readAll();
process_->close();
}
else{
qDebug()<<"sorry, your computer has no tool pnputil.exe.";
}
}
when i Commented out code
qDebug()<<process_->readAll();
and use
process_->setReadChannelMode(QProcess::MergedChannels);
process_->setStandardOutputFile("E:/log.txt");
it works properly.But if i use
qDebug()<<process_->readAll();
instead of
process_->setReadChannelMode(QProcess::MergedChannels);
process_->setStandardOutputFile("E:/log.txt");
it will be wrong. what's the reason?

Answer 1

According to Qt document, both setReadChannelMode and setStandardOutputFile have to be called before QProcess::start to take effect, so replacing
qDebug() << process_->readAll();
with
process_->setReadChannelMode(QProcess::MergedChannels);
process_->setStandardOutputFile("E:/log.txt");
is same as just commenting out qDebug() << process_->readAll();.
So I guess the child process does not output anything, so process_->readAll() will block, and the program halts.
（你看一下E:/log.txt有没有内容，估计是process_->readAll()阻塞了）