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QProcess Class

The QProcess class is used to start external programs and to communicate with them. More...

Header:	#include <QProcess>
qmake:	QT += core
Inherits:	QIODevice

Note: All functions in this class are reentrant.

Public Types

struct	CreateProcessArguments
typedef	CreateProcessArgumentModifier
enum	ExitStatus { NormalExit, CrashExit }
enum	InputChannelMode { ManagedInputChannel, ForwardedInputChannel }
enum	ProcessChannel { StandardOutput, StandardError }
enum	ProcessChannelMode { SeparateChannels, MergedChannels, ForwardedChannels, ForwardedErrorChannel, ForwardedOutputChannel }
enum	ProcessError { FailedToStart, Crashed, Timedout, WriteError, ReadError, UnknownError }
enum	ProcessState { NotRunning, Starting, Running }

Public Functions

	QProcess(QObject *parent = nullptr)
virtual	~QProcess()
QStringList	arguments() const
void	closeReadChannel(QProcess::ProcessChannel channel)
void	closeWriteChannel()
QProcess::CreateProcessArgumentModifier	createProcessArgumentsModifier() const
QProcess::ProcessError	error() const
int	exitCode() const
QProcess::ExitStatus	exitStatus() const
QProcess::InputChannelMode	inputChannelMode() const
QString	nativeArguments() const
QProcess::ProcessChannelMode	processChannelMode() const
QProcessEnvironment	processEnvironment() const
qint64	processId() const
QString	program() const
QByteArray	readAllStandardError()
QByteArray	readAllStandardOutput()
QProcess::ProcessChannel	readChannel() const
void	setArguments(const QStringList &arguments)
void	setCreateProcessArgumentsModifier(QProcess::CreateProcessArgumentModifier modifier)
void	setInputChannelMode(QProcess::InputChannelMode mode)
void	setNativeArguments(const QString &arguments)
void	setProcessChannelMode(QProcess::ProcessChannelMode mode)
void	setProcessEnvironment(const QProcessEnvironment &environment)
void	setProgram(const QString &program)
void	setReadChannel(QProcess::ProcessChannel channel)
void	setStandardErrorFile(const QString &fileName, QIODevice::OpenMode mode = Truncate)
void	setStandardInputFile(const QString &fileName)
void	setStandardOutputFile(const QString &fileName, QIODevice::OpenMode mode = Truncate)
void	setStandardOutputProcess(QProcess *destination)
void	setWorkingDirectory(const QString &dir)
void	start(const QString &program, const QStringList &arguments, QIODevice::OpenMode mode = ReadWrite)
void	start(QIODevice::OpenMode mode = ReadWrite)
bool	startDetached(qint64 *pid = nullptr)
QProcess::ProcessState	state() const
bool	waitForFinished(int msecs = 30000)
bool	waitForStarted(int msecs = 30000)
QString	workingDirectory() const

Reimplemented Public Functions

virtual bool	atEnd() const override
virtual qint64	bytesAvailable() const override
virtual qint64	bytesToWrite() const override
virtual bool	canReadLine() const override
virtual void	close() override
virtual bool	isSequential() const override
virtual bool	open(QIODevice::OpenMode mode = ReadWrite) override
virtual bool	waitForBytesWritten(int msecs = 30000) override
virtual bool	waitForReadyRead(int msecs = 30000) override

Public Slots

void	kill()
void	terminate()

Signals

void	errorOccurred(QProcess::ProcessError error)
void	finished(int exitCode, QProcess::ExitStatus exitStatus)
void	readyReadStandardError()
void	readyReadStandardOutput()
void	started()
void	stateChanged(QProcess::ProcessState newState)

Static Public Members

int	execute(const QString &program, const QStringList &arguments)
QString	nullDevice()
QStringList	splitCommand(QStringView command)
bool	startDetached(const QString &program, const QStringList &arguments, const QString &workingDirectory = QString(), qint64 *pid = nullptr)
QStringList	systemEnvironment()

Protected Functions

void	setProcessState(QProcess::ProcessState state)
virtual void	setupChildProcess()

Reimplemented Protected Functions

virtual qint64	readData(char *data, qint64 maxlen) override
virtual qint64	writeData(const char *data, qint64 len) override

typedef Q_PID

Macros

QT_NO_PROCESS_COMBINED_ARGUMENT_START

Detailed Description

Running a Process

To start a process, pass the name and command line arguments of the program you want to run as arguments to start(). Arguments are supplied as individual strings in a QStringList.

Alternatively, you can set the program to run with setProgram() and setArguments(), and then call start() or open().

For example, the following code snippet runs the analog clock example in the Fusion style on X11 platforms by passing strings containing "-style" and "fusion" as two items in the list of arguments:

    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 then enters the Starting state, and when the program has started, QProcess enters the Running state and emits started().

QProcess allows you to treat a process as a sequential I/O device. You can write to and read from the process just as you would access a network connection using QTcpSocket. You can then write to the process's standard input by calling write(), and read the standard output by calling read(), readLine(), and getChar(). Because it inherits QIODevice, QProcess can also be used as an input source for QXmlReader, or for generating data to be uploaded using QNetworkAccessManager.

When the process exits, QProcess reenters the NotRunning state (the initial state), and emits finished().

The finished() signal provides the exit code and exit status of the process as arguments, and you can also call exitCode() to obtain the exit code of the last process that finished, and exitStatus() to obtain its exit status. If an error occurs at any point in time, QProcess will emit the errorOccurred() signal. You can also call error() to find the type of error that occurred last, and state() to find the current process state.

Note: QProcess is not supported on VxWorks, iOS, tvOS, watchOS, or the Universal Windows Platform.

Communicating via Channels

Processes have two predefined output channels: The standard output channel (stdout) supplies regular console output, and the standard error channel (stderr) usually supplies the errors that are printed by the process. These channels represent two separate streams of data. You can toggle between them by calling setReadChannel(). QProcess emits readyRead() when data is available on the current read channel. It also emits readyReadStandardOutput() when new standard output data is available, and when new standard error data is available, readyReadStandardError() is emitted. Instead of calling read(), readLine(), or getChar(), you can explicitly read all data from either of the two channels by calling readAllStandardOutput() or readAllStandardError().

The terminology for the channels can be misleading. Be aware that the process's output channels correspond to QProcess's read channels, whereas the process's input channels correspond to QProcess's write channels. This is because what we read using QProcess is the process's output, and what we write becomes the process's input.

QProcess can merge the two output channels, so that standard output and standard error data from the running process both use the standard output channel. Call setProcessChannelMode() with MergedChannels before starting the process to activate this feature. You also have the option of forwarding the output of the running process to the calling, main process, by passing ForwardedChannels as the argument. It is also possible to forward only one of the output channels - typically one would use ForwardedErrorChannel, but ForwardedOutputChannel also exists. Note that using channel forwarding is typically a bad idea in GUI applications - you should present errors graphically instead.

Certain processes need special environment settings in order to operate. You can set environment variables for your process by calling setProcessEnvironment(). To set a working directory, call setWorkingDirectory(). By default, processes are run in the current working directory of the calling process.

The positioning and the screen Z-order of windows belonging to GUI applications started with QProcess are controlled by the underlying windowing system. For Qt 5 applications, the positioning can be specified using the -qwindowgeometry command line option; X11 applications generally accept a -geometry command line option.

Note: On QNX, setting the working directory may cause all application threads, with the exception of the QProcess caller thread, to temporarily freeze during the spawning process, owing to a limitation in the operating system.

Synchronous Process API

QProcess provides a set of functions which allow it to be used without an event loop, by suspending the calling thread until certain signals are emitted:

waitForStarted() blocks until the process has started.
waitForReadyRead() blocks until new data is available for reading on the current read channel.
waitForBytesWritten() blocks until one payload of data has been written to the process.
waitForFinished() blocks until the process has finished.

Calling these functions from the main thread (the thread that calls QApplication::exec()) may cause your user interface to freeze.

The following example runs gzip to compress the string "Qt rocks!", without an event loop:

    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();

Notes for Windows Users

Some Windows commands (for example, dir) are not provided by separate applications, but by the command interpreter itself. If you attempt to use QProcess to execute these commands directly, it won't work. One possible solution is to execute the command interpreter itself (cmd.exe on some Windows systems), and ask the interpreter to execute the desired command.

See also QBuffer, QFile, and QTcpSocket.

Member Type Documentation

typedef QProcess::CreateProcessArgumentModifier

Note: This typedef is only available on desktop Windows.

On Windows, QProcess uses the Win32 API function CreateProcess to start child processes. While QProcess provides a comfortable way to start processes without worrying about platform details, it is in some cases desirable to fine-tune the parameters that are passed to CreateProcess. This is done by defining a CreateProcessArgumentModifier function and passing it to setCreateProcessArgumentsModifier.

A CreateProcessArgumentModifier function takes one parameter: a pointer to a CreateProcessArguments struct. The members of this struct will be passed to CreateProcess after the CreateProcessArgumentModifier function is called.

The following example demonstrates how to pass custom flags to CreateProcess. When starting a console process B from a console process A, QProcess will reuse the console window of process A for process B by default. In this example, a new console window with a custom color scheme is created for the child process B instead.

    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");

enum QProcess::ExitStatus

This enum describes the different exit statuses of QProcess.

Constant	Value	Description
`QProcess::NormalExit`	`0`	The process exited normally.
`QProcess::CrashExit`	`1`	The process crashed.

See also exitStatus().

enum QProcess::InputChannelMode

This enum describes the process input channel modes of QProcess. Pass one of these values to setInputChannelMode() to set the current write channel mode.

Constant	Value	Description
`QProcess::ManagedInputChannel`	`0`	QProcess manages the input of the running process. This is the default input channel mode of QProcess.
`QProcess::ForwardedInputChannel`	`1`	QProcess forwards the input of the main process onto the running process. The child process reads its standard input from the same source as the main process. Note that the main process must not try to read its standard input while the child process is running.

This enum was introduced or modified in Qt 5.2.

See also setInputChannelMode().

enum QProcess::ProcessChannel

This enum describes the process channels used by the running process. Pass one of these values to setReadChannel() to set the current read channel of QProcess.

Constant	Value	Description
`QProcess::StandardOutput`	`0`	The standard output (stdout) of the running process.
`QProcess::StandardError`	`1`	The standard error (stderr) of the running process.

See also setReadChannel().

enum QProcess::ProcessChannelMode

This enum describes the process output channel modes of QProcess. Pass one of these values to setProcessChannelMode() to set the current read channel mode.

Constant	Value	Description
`QProcess::SeparateChannels`	`0`	QProcess manages the output of the running process, keeping standard output and standard error data in separate internal buffers. You can select the QProcess's current read channel by calling setReadChannel(). This is the default channel mode of QProcess.
`QProcess::MergedChannels`	`1`	QProcess merges the output of the running process into the standard output channel (`stdout`). The standard error channel (`stderr`) will not receive any data. The standard output and standard error data of the running process are interleaved.
`QProcess::ForwardedChannels`	`2`	QProcess forwards the output of the running process onto the main process. Anything the child process writes to its standard output and standard error will be written to the standard output and standard error of the main process.
`QProcess::ForwardedErrorChannel`	`4`	QProcess manages the standard output of the running process, but forwards its standard error onto the main process. This reflects the typical use of command line tools as filters, where the standard output is redirected to another process or a file, while standard error is printed to the console for diagnostic purposes. (This value was introduced in Qt 5.2.)
`QProcess::ForwardedOutputChannel`	`3`	Complementary to ForwardedErrorChannel. (This value was introduced in Qt 5.2.)

Note: Windows intentionally suppresses output from GUI-only applications to inherited consoles. This does not apply to output redirected to files or pipes. To forward the output of GUI-only applications on the console nonetheless, you must use SeparateChannels and do the forwarding yourself by reading the output and writing it to the appropriate output channels.

See also setProcessChannelMode().

enum QProcess::ProcessError

This enum describes the different types of errors that are reported by QProcess.

Constant	Value	Description
`QProcess::FailedToStart`	`0`	The process failed to start. Either the invoked program is missing, or you may have insufficient permissions to invoke the program.
`QProcess::Crashed`	`1`	The process crashed some time after starting successfully.
`QProcess::Timedout`	`2`	The last waitFor...() function timed out. The state of QProcess is unchanged, and you can try calling waitFor...() again.
`QProcess::WriteError`	`4`	An error occurred when attempting to write to the process. For example, the process may not be running, or it may have closed its input channel.
`QProcess::ReadError`	`3`	An error occurred when attempting to read from the process. For example, the process may not be running.
`QProcess::UnknownError`	`5`	An unknown error occurred. This is the default return value of error().

See also error().

enum QProcess::ProcessState

This enum describes the different states of QProcess.

Constant	Value	Description
`QProcess::NotRunning`	`0`	The process is not running.
`QProcess::Starting`	`1`	The process is starting, but the program has not yet been invoked.
`QProcess::Running`	`2`	The process is running and is ready for reading and writing.

See also state().

Member Function Documentation

QProcess::QProcess(QObject **parent* = nullptr)

Constructs a QProcess object with the given parent.

`[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.

`[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 signal by using the function pointer syntax, Qt provides a convenient helper for obtaining the function pointer as shown in this example:

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

See also exitStatus().

`[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().

`[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().

`[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.

`[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.

`[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]` 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().

`[override virtual]` bool QProcess::atEnd() const

Reimplements: QIODevice::atEnd() const.

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

`[override virtual]` qint64 QProcess::bytesAvailable() const

Reimplements: QIODevice::bytesAvailable() const.

`[override virtual]` qint64 QProcess::bytesToWrite() const

Reimplements: QIODevice::bytesToWrite() const.

`[override virtual]` bool QProcess::canReadLine() const

Reimplements: QIODevice::canReadLine() const.

This function operates on the current read channel.

See also readChannel() and setReadChannel().

`[override virtual]` void QProcess::close()

Reimplements: 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(QProcess::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().

QProcess::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.

QProcess::ProcessError QProcess::error() const

Returns the type of error that occurred last.

See also state().

`[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().

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.

QProcess::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.

`[override virtual]` bool QProcess::isSequential() const

Reimplements: QIODevice::isSequential() const.

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.

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

Reimplements: QIODevice::open(QIODevice::OpenMode mode).

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.

Returns true if the program has been started.

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

QProcess::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().

QProcess::ProcessChannel QProcess::readChannel() const

Returns the current read channel of the QProcess.

See also setReadChannel().

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

Reimplements: QIODevice::readData(char *data, qint64 maxSize).

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(QProcess::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.

void QProcess::setInputChannelMode(QProcess::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(QProcess::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.

`[protected]` void QProcess::setProcessState(QProcess::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(QProcess::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, QIODevice::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, QIODevice::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() override;
    ...
};

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.

`[static]` QStringList QProcess::splitCommand(QStringView command)

Splits the string command into a list of tokens, and returns the list.

Tokens with spaces can be surrounded by double quotes; three consecutive double quotes represent the quote character itself.

This function was introduced in Qt 5.15.

void QProcess::start(const QString &program, const QStringList &arguments, QIODevice::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(QIODevice::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().

bool QProcess::startDetached(qint64 **pid* = nullptr)

Starts the program set by setProgram() with arguments set by setArguments() 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.

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

The process will be started in the directory set by setWorkingDirectory(). 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. Note that the child process may exit and the PID may become invalid without notice. Furthermore, after the child process exits, the same PID may be recycled and used by a completely different process. User code should be careful when using this variable, especially if one intends to forcibly terminate the process by operating system means.

Only the following property setters are supported by startDetached():

All other properties of the QProcess object are ignored.

Note: The called process inherits the console window of the calling process. To suppress console output, redirect standard/error output to QProcess::nullDevice().

This function was introduced in Qt 5.10.

See also start(), startDetached(const QString &program, const QStringList &arguments, const QString &workingDirectory, qint64 *pid), and startDetached(const QString &command).

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

This function overloads startDetached().

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.

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

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

See also start().

QProcess::ProcessState QProcess::state() const

Returns the current state of the process.

See also stateChanged() and error().

`[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.

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

Reimplements: QIODevice::waitForBytesWritten(int msecs).

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().

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

Reimplements: QIODevice::waitForReadyRead(int msecs).

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().

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

Reimplements: QIODevice::writeData(const char *data, qint64 maxSize).

Related Non-Members

typedef Q_PID

Typedef for the identifiers used to represent processes on the underlying platform. On Unix, this corresponds to qint64; on Windows, it corresponds to _PROCESS_INFORMATION*.

See also QProcess::pid().

Macro Documentation

QT_NO_PROCESS_COMBINED_ARGUMENT_START

Disables the QProcess::start() overload taking a single string. In most cases where it is used, the user intends for the first argument to be treated atomically as per the other overload.

This function was introduced in Qt 5.6.

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

© The Qt Company Ltd
Licensed under the GNU Free Documentation License, Version 1.3.
https://doc.qt.io/qt-5.15/qprocess.html

QProcess Class

Public Types

Public Functions

Reimplemented Public Functions

Public Slots

Signals

Static Public Members

Protected Functions

Reimplemented Protected Functions

Related Non-Members

Macros

Detailed Description

Running a Process

Communicating via Channels

Synchronous Process API

Notes for Windows Users

Member Type Documentation

typedef QProcess::CreateProcessArgumentModifier

enum QProcess::ExitStatus

enum QProcess::InputChannelMode

enum QProcess::ProcessChannel

enum QProcess::ProcessChannelMode

enum QProcess::ProcessError

enum QProcess::ProcessState

Member Function Documentation

QProcess::QProcess(QObject *parent = nullptr)

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

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

[slot] void QProcess::kill()

[signal] void QProcess::readyReadStandardError()

[signal] void QProcess::readyReadStandardOutput()

[signal] void QProcess::started()

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

[slot] void QProcess::terminate()

[virtual] QProcess::~QProcess()

QStringList QProcess::arguments() const

[override virtual] bool QProcess::atEnd() const

[override virtual] qint64 QProcess::bytesAvailable() const

[override virtual] qint64 QProcess::bytesToWrite() const

[override virtual] bool QProcess::canReadLine() const

[override virtual] void QProcess::close()

void QProcess::closeReadChannel(QProcess::ProcessChannel channel)

void QProcess::closeWriteChannel()

QProcess::CreateProcessArgumentModifier QProcess::createProcessArgumentsModifier() const

QProcess::ProcessError QProcess::error() const

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

int QProcess::exitCode() const

QProcess::ExitStatus QProcess::exitStatus() const

QProcess::InputChannelMode QProcess::inputChannelMode() const

[override virtual] bool QProcess::isSequential() const

QString QProcess::nativeArguments() const

[static] QString QProcess::nullDevice()

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

QProcess::ProcessChannelMode QProcess::processChannelMode() const

QProcessEnvironment QProcess::processEnvironment() const

qint64 QProcess::processId() const

QString QProcess::program() const

QByteArray QProcess::readAllStandardError()

QByteArray QProcess::readAllStandardOutput()

QProcess::ProcessChannel QProcess::readChannel() const

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

void QProcess::setArguments(const QStringList &arguments)

void QProcess::setCreateProcessArgumentsModifier(QProcess::CreateProcessArgumentModifier modifier)

void QProcess::setInputChannelMode(QProcess::InputChannelMode mode)

void QProcess::setNativeArguments(const QString &arguments)

void QProcess::setProcessChannelMode(QProcess::ProcessChannelMode mode)

void QProcess::setProcessEnvironment(const QProcessEnvironment &environment)

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

void QProcess::setProgram(const QString &program)

void QProcess::setReadChannel(QProcess::ProcessChannel channel)

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

void QProcess::setStandardInputFile(const QString &fileName)

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

void QProcess::setStandardOutputProcess(QProcess *destination)

void QProcess::setWorkingDirectory(const QString &dir)

[virtual protected] void QProcess::setupChildProcess()

[static] QStringList QProcess::splitCommand(QStringView command)

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

void QProcess::start(QIODevice::OpenMode mode = ReadWrite)

bool QProcess::startDetached(qint64 *pid = nullptr)

QProcess::QProcess(QObject **parent* = nullptr)

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

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

`[slot]` void QProcess::kill()

`[signal]` void QProcess::readyReadStandardError()

`[signal]` void QProcess::readyReadStandardOutput()

`[signal]` void QProcess::started()

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

`[slot]` void QProcess::terminate()

`[virtual]` QProcess::~QProcess()

`[override virtual]` bool QProcess::atEnd() const

`[override virtual]` qint64 QProcess::bytesAvailable() const

`[override virtual]` qint64 QProcess::bytesToWrite() const

`[override virtual]` bool QProcess::canReadLine() const

`[override virtual]` void QProcess::close()

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

`[override virtual]` bool QProcess::isSequential() const

`[static]` QString QProcess::nullDevice()

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

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

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

void QProcess::setStandardOutputProcess(QProcess **destination*)

`[virtual protected]` void QProcess::setupChildProcess()

`[static]` QStringList QProcess::splitCommand(QStringView command)

bool QProcess::startDetached(qint64 **pid* = nullptr)

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

`[static]` QStringList QProcess::systemEnvironment()

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

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

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