QProcess Class

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

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

      QProcess gzip;
      gzip.start("gzip", QStringList() << "-c");
      if (!gzip.waitForStarted())
          return false;

      gzip.write("Qt rocks!");
      gzip.closeWriteChannel();

      if (!gzip.waitForFinished())
          return false;

      QByteArray result = gzip.readAll();

      QProcess process;
      process.setCreateProcessArgumentsModifier([] (QProcess::CreateProcessArguments *args)
      {
          args->flags |= CREATE_NEW_CONSOLE;
          args->startupInfo->dwFlags &= ~STARTF_USESTDHANDLES;
          args->startupInfo->dwFlags |= STARTF_USEFILLATTRIBUTE;
          args->startupInfo->dwFillAttribute = BACKGROUND_BLUE | FOREGROUND_RED
                                             | FOREGROUND_INTENSITY;
      });
      process.start("C:\\Windows\\System32\\cmd.exe", QStringList() << "/k" << "title" << "The Child Process");

Member Function Documentation

QProcess::QProcess(QObject *parent = Q_NULLPTR)

Constructs a QProcess object with the given parent.

[virtual] QProcess::~QProcess()

Destructs the QProcess object, i.e., killing the process.

Note that this function will not return until the process is terminated.

QStringList QProcess::arguments() const

Returns the command line arguments the process was last started with.

This function was introduced in Qt 5.0.

See also setArguments() and start().

[virtual] bool QProcess::atEnd() const

Reimplemented from QIODevice::atEnd().

Returns true if the process is not running, and no more data is available for reading; otherwise returns false.

[virtual] qint64 QProcess::bytesAvailable() const

Reimplemented from QIODevice::bytesAvailable().

[virtual] qint64 QProcess::bytesToWrite() const

Reimplemented from QIODevice::bytesToWrite().

[virtual] bool QProcess::canReadLine() const

Reimplemented from QIODevice::canReadLine().

This function operates on the current read channel.

See also readChannel() and setReadChannel().

[virtual] void QProcess::close()

Reimplemented from QIODevice::close().

Closes all communication with the process and kills it. After calling this function, QProcess will no longer emit readyRead(), and data can no longer be read or written.

void QProcess::closeReadChannel(ProcessChannel channel)

Closes the read channel channel. After calling this function, QProcess will no longer receive data on the channel. Any data that has already been received is still available for reading.

Call this function to save memory, if you are not interested in the output of the process.

See also closeWriteChannel() and setReadChannel().

void QProcess::closeWriteChannel()

Schedules the write channel of QProcess to be closed. The channel will close once all data has been written to the process. After calling this function, any attempts to write to the process will fail.

Closing the write channel is necessary for programs that read input data until the channel has been closed. For example, the program "more" is used to display text data in a console on both Unix and Windows. But it will not display the text data until QProcess's write channel has been closed. Example:

  QProcess more;
  more.start("more");
  more.write("Text to display");
  more.closeWriteChannel();
  // QProcess will emit readyRead() once "more" starts printing

The write channel is implicitly opened when start() is called.

See also closeReadChannel().

CreateProcessArgumentModifier QProcess::createProcessArgumentsModifier() const

Returns a previously set CreateProcess modifier function.

Note: This function is available only on the Windows platform.

This function was introduced in Qt 5.7.

See also setCreateProcessArgumentsModifier() and QProcess::CreateProcessArgumentModifier.

QProcess::ProcessError QProcess::error() const

Returns the type of error that occurred last.

See also state().

[signal] void QProcess::errorOccurred(QProcess::ProcessError error)

This signal is emitted when an error occurs with the process. The specified error describes the type of error that occurred.

This function was introduced in Qt 5.6.

[static] int QProcess::execute(const QString &program, const QStringList &arguments)

Starts the program program with the arguments arguments in a new process, waits for it to finish, and then returns the exit code of the process. Any data the new process writes to the console is forwarded to the calling process.

The environment and working directory are inherited from the calling process.

Argument handling is identical to the respective start() overload.

If the process cannot be started, -2 is returned. If the process crashes, -1 is returned. Otherwise, the process' exit code is returned.

See also start().

[static] int QProcess::execute(const QString &command)

This is an overloaded function.

Starts the program command in a new process, waits for it to finish, and then returns the exit code.

Argument handling is identical to the respective start() overload.

After the command string has been split and unquoted, this function behaves like the overload which takes the arguments as a string list.

See also start().

int QProcess::exitCode() const

Returns the exit code of the last process that finished.

This value is not valid unless exitStatus() returns NormalExit.

QProcess::ExitStatus QProcess::exitStatus() const

Returns the exit status of the last process that finished.

On Windows, if the process was terminated with TerminateProcess() from another application, this function will still return NormalExit unless the exit code is less than 0.

This function was introduced in Qt 4.1.

[signal] void QProcess::finished(int exitCode, QProcess::ExitStatus exitStatus)

This signal is emitted when the process finishes. exitCode is the exit code of the process (only valid for normal exits), and exitStatus is the exit status. After the process has finished, the buffers in QProcess are still intact. You can still read any data that the process may have written before it finished.

Note: Signal finished is overloaded in this class. To connect to this one using the function pointer syntax, you must specify the signal type in a static cast, as shown in this example:

  connect(process, static_cast<void(QProcess::*)(int, QProcess::ExitStatus)>(&QProcess::finished),
      [=](int exitCode, QProcess::ExitStatus exitStatus){ /* ... */ });

See also exitStatus().

InputChannelMode QProcess::inputChannelMode() const

Returns the channel mode of the QProcess standard input channel.

This function was introduced in Qt 5.2.

See also setInputChannelMode() and InputChannelMode.

[virtual] bool QProcess::isSequential() const

Reimplemented from QIODevice::isSequential().

[slot] void QProcess::kill()

Kills the current process, causing it to exit immediately.

On Windows, kill() uses TerminateProcess, and on Unix and macOS, the SIGKILL signal is sent to the process.

See also terminate().

QString QProcess::nativeArguments() const

Returns the additional native command line arguments for the program.

Note: This function is available only on the Windows platform.

This function was introduced in Qt 4.7.

See also setNativeArguments().

[static] QString QProcess::nullDevice()

The null device of the operating system.

The returned file path uses native directory separators.

This function was introduced in Qt 5.2.

See also QProcess::setStandardInputFile(), QProcess::setStandardOutputFile(), and QProcess::setStandardErrorFile().

[virtual] bool QProcess::open(OpenMode mode = ReadWrite)

Reimplemented from QIODevice::open().

Starts the program set by setProgram() with arguments set by setArguments(). The OpenMode is set to mode.

This method is an alias for start(), and exists only to fully implement the interface defined by QIODevice.

See also start(), setProgram(), and setArguments().

ProcessChannelMode QProcess::processChannelMode() const

Returns the channel mode of the QProcess standard output and standard error channels.

This function was introduced in Qt 4.2.

See also setProcessChannelMode(), ProcessChannelMode, and setReadChannel().

QProcessEnvironment QProcess::processEnvironment() const

Returns the environment that QProcess will pass to its child process, or an empty object if no environment has been set using setEnvironment() or setProcessEnvironment(). If no environment has been set, the environment of the calling process will be used.

This function was introduced in Qt 4.6.

See also setProcessEnvironment(), setEnvironment(), and QProcessEnvironment::isEmpty().

qint64 QProcess::processId() const

Returns the native process identifier for the running process, if available. If no process is currently running, 0 is returned.

This function was introduced in Qt 5.3.

QString QProcess::program() const

Returns the program the process was last started with.

This function was introduced in Qt 5.0.

See also setProgram() and start().

QByteArray QProcess::readAllStandardError()

Regardless of the current read channel, this function returns all data available from the standard error of the process as a QByteArray.

See also readyReadStandardError(), readAllStandardOutput(), readChannel(), and setReadChannel().

QByteArray QProcess::readAllStandardOutput()

Regardless of the current read channel, this function returns all data available from the standard output of the process as a QByteArray.

See also readyReadStandardOutput(), readAllStandardError(), readChannel(), and setReadChannel().

ProcessChannel QProcess::readChannel() const

Returns the current read channel of the QProcess.

See also setReadChannel().

[virtual protected] qint64 QProcess::readData(char *data, qint64 maxlen)

Reimplemented from QIODevice::readData().

[signal] void QProcess::readyReadStandardError()

This signal is emitted when the process has made new data available through its standard error channel (stderr). It is emitted regardless of the current read channel.

Note: This is a private signal. It can be used in signal connections but cannot be emitted by the user.

See also readAllStandardError() and readChannel().

[signal] void QProcess::readyReadStandardOutput()

This signal is emitted when the process has made new data available through its standard output channel (stdout). It is emitted regardless of the current read channel.

Note: This is a private signal. It can be used in signal connections but cannot be emitted by the user.

See also readAllStandardOutput() and readChannel().

void QProcess::setArguments(const QStringList &arguments)

Set the arguments to pass to the called program when starting the process. This function must be called before start().

This function was introduced in Qt 5.1.

See also start(), setProgram(), and arguments().

void QProcess::setCreateProcessArgumentsModifier(CreateProcessArgumentModifier modifier)

Sets the modifier for the CreateProcess Win32 API call. Pass QProcess::CreateProcessArgumentModifier() to remove a previously set one.

Note: This function is available only on the Windows platform and requires C++11.

This function was introduced in Qt 5.7.

See also createProcessArgumentsModifier() and QProcess::CreateProcessArgumentModifier.

void QProcess::setInputChannelMode(InputChannelMode mode)

Sets the channel mode of the QProcess standard input channel to the mode specified. This mode will be used the next time start() is called.

This function was introduced in Qt 5.2.

See also inputChannelMode() and InputChannelMode.

void QProcess::setNativeArguments(const QString &arguments)

This is an overloaded function.

Sets additional native command line arguments for the program.

On operating systems where the system API for passing command line arguments to a subprocess natively uses a single string, one can conceive command lines which cannot be passed via QProcess's portable list-based API. In such cases this function must be used to set a string which is appended to the string composed from the usual argument list, with a delimiting space.

Note: This function is available only on the Windows platform.

This function was introduced in Qt 4.7.

See also nativeArguments().

void QProcess::setProcessChannelMode(ProcessChannelMode mode)

Sets the channel mode of the QProcess standard output and standard error channels to the mode specified. This mode will be used the next time start() is called. For example:

  QProcess builder;
  builder.setProcessChannelMode(QProcess::MergedChannels);
  builder.start("make", QStringList() << "-j2");

  if (!builder.waitForFinished())
      qDebug() << "Make failed:" << builder.errorString();
  else
      qDebug() << "Make output:" << builder.readAll();

This function was introduced in Qt 4.2.

See also processChannelMode(), ProcessChannelMode, and setReadChannel().

void QProcess::setProcessEnvironment(const QProcessEnvironment &environment)

Sets the environment that QProcess will pass to the child process.

For example, the following code adds the environment variable TMPDIR:

  QProcess process;
  QProcessEnvironment env = QProcessEnvironment::systemEnvironment();
  env.insert("TMPDIR", "C:\\MyApp\\temp"); // Add an environment variable
  process.setProcessEnvironment(env);
  process.start("myapp");

Note how, on Windows, environment variable names are case-insensitive.

This function was introduced in Qt 4.6.

See also processEnvironment(), QProcessEnvironment::systemEnvironment(), and setEnvironment().

[protected] void QProcess::setProcessState(ProcessState state)

Sets the current state of the QProcess to the state specified.

See also state().

void QProcess::setProgram(const QString &program)

Set the program to use when starting the process. This function must be called before start().

This function was introduced in Qt 5.1.

See also start(), setArguments(), and program().

void QProcess::setReadChannel(ProcessChannel channel)

Sets the current read channel of the QProcess to the given channel. The current input channel is used by the functions read(), readAll(), readLine(), and getChar(). It also determines which channel triggers QProcess to emit readyRead().

See also readChannel().

void QProcess::setStandardErrorFile(const QString &fileName, OpenMode mode = Truncate)

Redirects the process' standard error to the file fileName. When the redirection is in place, the standard error read channel is closed: reading from it using read() will always fail, as will readAllStandardError(). The file will be appended to if mode is Append, otherwise, it will be truncated.

See setStandardOutputFile() for more information on how the file is opened.

Note: if setProcessChannelMode() was called with an argument of QProcess::MergedChannels, this function has no effect.

This function was introduced in Qt 4.2.

See also setStandardInputFile(), setStandardOutputFile(), and setStandardOutputProcess().

void QProcess::setStandardInputFile(const QString &fileName)

Redirects the process' standard input to the file indicated by fileName. When an input redirection is in place, the QProcess object will be in read-only mode (calling write() will result in error).

To make the process read EOF right away, pass nullDevice() here. This is cleaner than using closeWriteChannel() before writing any data, because it can be set up prior to starting the process.

If the file fileName does not exist at the moment start() is called or is not readable, starting the process will fail.

Calling setStandardInputFile() after the process has started has no effect.

This function was introduced in Qt 4.2.

See also setStandardOutputFile(), setStandardErrorFile(), and setStandardOutputProcess().

void QProcess::setStandardOutputFile(const QString &fileName, OpenMode mode = Truncate)

Redirects the process' standard output to the file fileName. When the redirection is in place, the standard output read channel is closed: reading from it using read() will always fail, as will readAllStandardOutput().

To discard all standard output from the process, pass nullDevice() here. This is more efficient than simply never reading the standard output, as no QProcess buffers are filled.

If the file fileName doesn't exist at the moment start() is called, it will be created. If it cannot be created, the starting will fail.

If the file exists and mode is QIODevice::Truncate, the file will be truncated. Otherwise (if mode is QIODevice::Append), the file will be appended to.

Calling setStandardOutputFile() after the process has started has no effect.

This function was introduced in Qt 4.2.

See also setStandardInputFile(), setStandardErrorFile(), and setStandardOutputProcess().

void QProcess::setStandardOutputProcess(QProcess *destination)

Pipes the standard output stream of this process to the destination process' standard input.

The following shell command:

  command1 | command2

Can be accomplished with QProcess with the following code:

  QProcess process1;
  QProcess process2;

  process1.setStandardOutputProcess(&process2);

  process1.start("command1");
  process2.start("command2");

This function was introduced in Qt 4.2.

void QProcess::setWorkingDirectory(const QString &dir)

Sets the working directory to dir. QProcess will start the process in this directory. The default behavior is to start the process in the working directory of the calling process.

Note: On QNX, this may cause all application threads to temporarily freeze.

See also workingDirectory() and start().

[virtual protected] void QProcess::setupChildProcess()

This function is called in the child process context just before the program is executed on Unix or macOS (i.e., after fork(), but before execve()). Reimplement this function to do last minute initialization of the child process. Example:

  class SandboxProcess : public QProcess
  {
      ...
   protected:
       void setupChildProcess();
      ...
  };

  void SandboxProcess::setupChildProcess()
  {
      // Drop all privileges in the child process, and enter
      // a chroot jail.
  #if defined Q_OS_UNIX
      ::setgroups(0, 0);
      ::chroot("/etc/safe");
      ::chdir("/");
      ::setgid(safeGid);
      ::setuid(safeUid);
      ::umask(0);
  #endif
  }

You cannot exit the process (by calling exit(), for instance) from this function. If you need to stop the program before it starts execution, your workaround is to emit finished() and then call exit().

Warning: This function is called by QProcess on Unix and macOS only. On Windows and QNX, it is not called.

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

Starts the given program in a new process, passing the command line arguments in arguments.

The QProcess object will immediately enter the Starting state. If the process starts successfully, QProcess will emit started(); otherwise, errorOccurred() will be emitted.

Note: Processes are started asynchronously, which means the started() and errorOccurred() signals may be delayed. Call waitForStarted() to make sure the process has started (or has failed to start) and those signals have been emitted.

Note: No further splitting of the arguments is performed.

Windows: The arguments are quoted and joined into a command line that is compatible with the CommandLineToArgvW() Windows function. For programs that have different command line quoting requirements, you need to use setNativeArguments(). One notable program that does not follow the CommandLineToArgvW() rules is cmd.exe and, by consequence, all batch scripts.

The OpenMode is set to mode.

If the QProcess object is already running a process, a warning may be printed at the console, and the existing process will continue running unaffected.

See also processId(), started(), waitForStarted(), and setNativeArguments().

void QProcess::start(const QString &command, OpenMode mode = ReadWrite)

This is an overloaded function.

Starts the command command in a new process. The OpenMode is set to mode.

command is a single string of text containing both the program name and its arguments. The arguments are separated by one or more spaces. For example:

  QProcess process;
  process.start("del /s *.txt");
  // same as process.start("del", QStringList() << "/s" << "*.txt");
  ...

Arguments containing spaces must be quoted to be correctly supplied to the new process. For example:

  QProcess process;
  process.start("dir \"My Documents\"");

Literal quotes in the command string are represented by triple quotes. For example:

  QProcess process;
  process.start("dir \"Epic 12\"\"\" Singles\"");

After the command string has been split and unquoted, this function behaves like the overload which takes the arguments as a string list.

You can disable this overload by defining QT_NO_PROCESS_COMBINED_ARGUMENT_START when you compile your applications. This can be useful if you want to ensure that you are not splitting arguments unintentionally, for example. In virtually all cases, using the other overload is the preferred method.

On operating systems where the system API for passing command line arguments to a subprocess natively uses a single string (Windows), one can conceive command lines which cannot be passed via QProcess's portable list-based API. In these rare cases you need to use setProgram() and setNativeArguments() instead of this function.

void QProcess::start(OpenMode mode = ReadWrite)

This is an overloaded function.

Starts the program set by setProgram() with arguments set by setArguments(). The OpenMode is set to mode.

This function was introduced in Qt 5.1.

See also open(), setProgram(), and setArguments().

[static] bool QProcess::startDetached(const QString &program, const QStringList &arguments, const QString &workingDirectory = QString(), qint64 *pid = Q_NULLPTR)

Starts the program program with the arguments arguments in a new process, and detaches from it. Returns true on success; otherwise returns false. If the calling process exits, the detached process will continue to run unaffected.

Argument handling is identical to the respective start() overload.

Unix: The started process will run in its own session and act like a daemon.

The process will be started in the directory workingDirectory. If workingDirectory is empty, the working directory is inherited from the calling process.

Note: On QNX, this may cause all application threads to temporarily freeze.

If the function is successful then *pid is set to the process identifier of the started process.

See also start().

[static] bool QProcess::startDetached(const QString &command)

This is an overloaded function.

Starts the command command in a new process, and detaches from it. Returns true on success; otherwise returns false.

Argument handling is identical to the respective start() overload.

After the command string has been split and unquoted, this function behaves like the overload which takes the arguments as a string list.

See also start(const QString &command, OpenMode mode).

[signal] void QProcess::started()

This signal is emitted by QProcess when the process has started, and state() returns Running.

Note: This is a private signal. It can be used in signal connections but cannot be emitted by the user.

QProcess::ProcessState QProcess::state() const

Returns the current state of the process.

See also stateChanged() and error().

[signal] void QProcess::stateChanged(QProcess::ProcessState newState)

This signal is emitted whenever the state of QProcess changes. The newState argument is the state QProcess changed to.

Note: This is a private signal. It can be used in signal connections but cannot be emitted by the user.

[static] QStringList QProcess::systemEnvironment()

Returns the environment of the calling process as a list of key=value pairs. Example:

  QStringList environment = QProcess::systemEnvironment();
  // environment = {"PATH=/usr/bin:/usr/local/bin",
  //                "USER=greg", "HOME=/home/greg"}

This function does not cache the system environment. Therefore, it's possible to obtain an updated version of the environment if low-level C library functions like setenv or putenv have been called.

However, note that repeated calls to this function will recreate the list of environment variables, which is a non-trivial operation.

Note: For new code, it is recommended to use QProcessEnvironment::systemEnvironment()

This function was introduced in Qt 4.1.

See also QProcessEnvironment::systemEnvironment() and setProcessEnvironment().

[slot] void QProcess::terminate()

Attempts to terminate the process.

The process may not exit as a result of calling this function (it is given the chance to prompt the user for any unsaved files, etc).

On Windows, terminate() posts a WM_CLOSE message to all top-level windows of the process and then to the main thread of the process itself. On Unix and macOS the SIGTERM signal is sent.

Console applications on Windows that do not run an event loop, or whose event loop does not handle the WM_CLOSE message, can only be terminated by calling kill().

See also kill().

[virtual] bool QProcess::waitForBytesWritten(int msecs = 30000)

Reimplemented from QIODevice::waitForBytesWritten().

bool QProcess::waitForFinished(int msecs = 30000)

Blocks until the process has finished and the finished() signal has been emitted, or until msecs milliseconds have passed.

Returns true if the process finished; otherwise returns false (if the operation timed out, if an error occurred, or if this QProcess is already finished).

This function can operate without an event loop. It is useful when writing non-GUI applications and when performing I/O operations in a non-GUI thread.

Warning: Calling this function from the main (GUI) thread might cause your user interface to freeze.

If msecs is -1, this function will not time out.

See also finished(), waitForStarted(), waitForReadyRead(), and waitForBytesWritten().

[virtual] bool QProcess::waitForReadyRead(int msecs = 30000)

Reimplemented from QIODevice::waitForReadyRead().

bool QProcess::waitForStarted(int msecs = 30000)

Blocks until the process has started and the started() signal has been emitted, or until msecs milliseconds have passed.

Returns true if the process was started successfully; otherwise returns false (if the operation timed out or if an error occurred).

This function can operate without an event loop. It is useful when writing non-GUI applications and when performing I/O operations in a non-GUI thread.

Warning: Calling this function from the main (GUI) thread might cause your user interface to freeze.

If msecs is -1, this function will not time out.

Note: On some UNIX operating systems, this function may return true but the process may later report a QProcess::FailedToStart error.

See also started(), waitForReadyRead(), waitForBytesWritten(), and waitForFinished().

QString QProcess::workingDirectory() const

If QProcess has been assigned a working directory, this function returns the working directory that the QProcess will enter before the program has started. Otherwise, (i.e., no directory has been assigned,) an empty string is returned, and QProcess will use the application's current working directory instead.

See also setWorkingDirectory().

[virtual protected] qint64 QProcess::writeData(const char *data, qint64 len)

Reimplemented from QIODevice::writeData().