QProcess Class

Qt 3.0.5

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QProcess Class Reference

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

#include <qprocess.h>

Inherits QObject.

List of all member functions.

Public Members

Public Slots

Signals

Detailed Description

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

You can write to the started program's standard input, and can read the program's standard output and standard error. You can pass command line arguments to the program either in the constructor or with setArguments() or addArgument(). The program's working directory can be set with setWorkingDirectory(). If you need to set up environment variables pass them to the start() or launch() functions (see below). The processExited() signal is emitted if the program exits. The program's exit status is available from exitStatus(), although you could simply call normalExit() to see if the program terminated normally.

There are two different ways to start a process. If you just want to run a program, optionally passing data to its standard input at the beginning, use one of the launch() functions. If you want full control of the program's standard input (especially if you don't know all the data you want to send to standard input at the beginning), use the start() function.

If you use start() you can write to the program's standard input using writeToStdin() and you can close the standard input with closeStdin(). The wroteToStdin() signal is emitted if the data sent to standard input has been written. You can read from the program's standard output using readStdout() or readLineStdout(). These functions return an empty QByteArray if there is no data to read. The readyReadStdout() signal is emitted when there is data available to be read from standard output. Standard error has a set of functions that correspond to the standard output functions, i.e. readStderr(), readLineStderr() and readyReadStderr().

If you use one of the launch() functions the data you pass will be sent to the program's standard input which will be closed once all the data has been written. You should not use writeToStdin() or closeStdin() if you use launch(). If you need to send data to the program's standard input after it has started running use start() instead of launch().

Both start() and launch() can accept a string list of strings each of which has the format, key=value, where the keys are the names of environment variables.

You can test to see if a program is running with isRunning(). The program's process identifier is available from processIdentifier(). If you want to terminate a running program use tryTerminate(), but note that the program may ignore this. If you really want to terminate the program, without it having any chance to clean up, you can use kill().

As an example, suppose we want to start the uic command (a Qt command line tool used with Qt Designer) and perform some operations on the output (the uic outputs the code it generates to standard output by default). Suppose further that we want to run the program on the file "small_dialog.ui" with the command line options "-tr i18n". On the command line we would write: 

    uic -tr i18n small_dialog.ui

A code snippet for this with the QProcess class might look like this: 

    UicManager::UicManager()
    {

        proc = new QProcess( this );

        proc->addArgument( "uic" );
        proc->addArgument( "-tr" );
        proc->addArgument( "i18n" );
        proc->addArgument( "small_dialog.ui" );

        connect( proc, SIGNAL(readyReadStdout()),
                this, SLOT(readFromStdout()) );

        if ( !proc->start() ) {
            // error handling

        }
    }


    void UicManager::readFromStdout()
    {
        // Read and process the data.
        // Bear in mind that the data might be output in chunks.

    }

Although you may need quotes for a file named on the command line (e.g. if it contains spaces) you shouldn't use extra quotes for arguments passed to addArgument() or setArguments().

The readyReadStdout() signal is emitted when there is new data on standard output. This happens asynchronously: you don't know if more data will arrive later. In the above example you could connect the processExited() signal to the slot UicManager::readFromStdout() instead. If you do so, you will be certain that all the data is available when the slot is called. On the other hand, you must wait until the process has finished before doing any processing.

See also QSocket, Input/Output and Networking and Miscellaneous Classes.

Member Type Documentation

QProcess::Communication

This enum type defines the communication channels connected to the process.

  • QProcess::Stdin - Data can be written to the process's standard input.
  • QProcess::Stdout - Data can be read from the process's standard output.
  • QProcess::Stderr - Data can be read from the process's standard error.
  • QProcess::DupStderr - Both the process's standard error output and its standard output are written to its standard output. (Like Unix's dup2().) This means that nothing is sent to the standard error output. This is especially useful if your application requires that the output on standard output and on standard error must be read in the same order that they are produced. This is a flag, so to activate it you must pass Stdout|Stderr|DupStderr, or Stdin|Stdout|Stderr|DupStderr if you want to provide input, to the setCommunication() call.

See also setCommunication() and communication().

Member Function Documentation

QProcess::QProcess ( QObject * parent = 0, const char * name = 0 )

Constructs a QProcess object. The parent and name parameters are passed to the QObject constructor.

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

QProcess::QProcess ( const QString & arg0, QObject * parent = 0, const char * name = 0 )

Constructs a QProcess with arg0 as the command to be executed. The parent and name parameters are passed to the QObject constructor.

The process is not started. You must call start() or launch() to start the process.

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

QProcess::QProcess ( const QStringList & args, QObject * parent = 0, const char * name = 0 )

Constructs a QProcess with args as the arguments of the process. The first element in the list is the command to be executed. The other elements in the list are the arguments to this command. The parent and name parameters are passed to the QObject constructor.

The process is not started. You must call start() or launch() to start the process.

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

QProcess::~QProcess ()

Destroys the instance.

If the process is running, it is not terminated! The standard input, standard output and standard error of the process are closed.

You can connect the destroyed() signal to the kill() slot, if you want the process to be terminated automatically when the instance is destroyed.

See also tryTerminate() and kill().

void QProcess::addArgument ( const QString & arg ) [virtual]

Adds arg to the end of the list of arguments.

The first element in the list of arguments is the command to be executed; the following elements are the command's arguments.

See also arguments() and setArguments().

Example: process/process.cpp.

QStringList QProcess::arguments () const

Returns the list of arguments that are set for the process. Arguments can be specified with the constructor or with the functions setArguments() and addArgument().

Note that if you want to iterate over the list, you should iterate over a copy, e.g. 

    QStringList list = myProcess.arguments();
    QStringList::Iterator it = list.begin();
    while( it != list.end() ) {
        myProcessing( *it );
        ++it;
    }

See also setArguments() and addArgument().

bool QProcess::canReadLineStderr () const

Returns TRUE if it's possible to read an entire line of text from standard error at this time; otherwise returns FALSE.

See also readLineStderr() and canReadLineStdout().

bool QProcess::canReadLineStdout () const

Returns TRUE if it's possible to read an entire line of text from standard output at this time; otherwise returns FALSE.

See also readLineStdout() and canReadLineStderr().

void QProcess::clearArguments ()

Clears the list of arguments that are set for the process.

See also setArguments() and addArgument().

void QProcess::closeStdin () [virtual slot]

Closes the process's standard input.

This function also deletes any pending data that has not been written to standard input.

See also wroteToStdin().

int QProcess::communication () const

Returns the communication required with the process, i.e. some combination of the Communication flags.

See also setCommunication().

int QProcess::exitStatus () const

Returns the exit status of the process or 0 if the process is still running. This function returns immediately and does not wait until the process is finished.

If normalExit() is FALSE (e.g. if the program was killed or crashed), this function returns 0, so you should check the return value of normalExit() before relying on this value.

See also normalExit() and processExited().

bool QProcess::isRunning () const

Returns TRUE if the process is running; otherwise returns FALSE.

See also normalExit(), exitStatus() and processExited().

void QProcess::kill () const [slot]

Terminates the process. This is not a safe way to end a process since the process will not be able to do any cleanup. tryTerminate() is safer, but processes can ignore a tryTerminate().

The nice way to end a process and to be sure that it is finished, is to do something like this: 

        process->tryTerminate();
        QTimer::singleShot( 5000, process, SLOT( kill() ) );

This tries to terminate the process the nice way. If the process is still running after 5 seconds, it terminates the process the hard way. The timeout should be chosen depending on the time the process needs to do all its cleanup: use a higher value if the process is likely to do a lot of computation or I/O on cleanup.

The slot returns immediately: it does not wait until the process has finished. When the process terminates, the processExited() signal is emitted.

See also tryTerminate() and processExited().

bool QProcess::launch ( const QByteArray & buf, QStringList * env = 0 ) [virtual]

Runs the process and writes the data buf to the process's standard input. If all the data is written to standard input, standard input is closed. The command is searched for in the path for executable programs; you can also use an absolute path in the command itself.

If env is null, then the process is started with the same environment as the starting process. If env is non-null, then the values in the string list are interpreted as environment setttings of the form key=value and the process is started with these environment settings. For convenience, there is a small exception to this rule under Unix: if env does not contain any settings for the environment variable LD_LIBRARY_PATH, then this variable is inherited from the starting process.

Returns TRUE if the process could be started; otherwise returns FALSE.

Note that you should not use the slots writeToStdin() and closeStdin() on processes started with launch(), since the result is not well-defined. If you need these slots, use start() instead.

The process may or may not read the buf data sent to its standard input.

You can call this function even when a process that was started with this instance is still running. Be aware that if you do this the standard input of the process that was launched first will be closed, with any pending data being deleted, and the process will be left to run out of your control. Similarly, if the process could not be started the standard input will be closed and the pending data deleted. (On operating systems that have zombie processes, Qt will also wait() on the old process.)

The object emits the signal launchFinished() when this function call is finished. If the start was successful, this signal is emitted after all the data has been written to standard input. If the start failed, then this signal is emitted immediately.

See also start() and launchFinished().

bool QProcess::launch ( const QString & buf, QStringList * env = 0 ) [virtual]

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

The data buf is written to standard input with writeToStdin() using the QString::local8Bit() representation of the strings.

void QProcess::launchFinished () [signal]

This signal is emitted when the process was started with launch(). If the start was successful, this signal is emitted after all the data has been written to standard input. If the start failed, then this signal is emitted immediately.

This signal is especially useful if you want to know when you can safely delete the QProcess object when you are not intrested in reading from standard output or standard error.

See also launch() and QObject::deleteLater().

bool QProcess::normalExit () const

Returns TRUE if the process has exited normally; otherwise returns FALSE. This implies that this function returns FALSE if the process is still running.

See also isRunning(), exitStatus() and processExited().

void QProcess::processExited () [signal]

This signal is emitted when the process has exited.

See also isRunning(), normalExit(), exitStatus(), start() and launch().

Example: process/process.cpp.

PID QProcess::processIdentifier ()

Returns platform dependent information about the process. This can be used together with platform specific system calls.

Under Unix the return value is the PID of the process, or -1 if no process is belongs to this object.

Under Windows it is a pointer to the PROCESS_INFORMATION struct, or 0 if no process is belongs to this object.

Use of this function's return value is likely to be non-portable.

QString QProcess::readLineStderr () [virtual]

Reads a line of text from standard error, excluding any trailing newline or carriage return characters and returns it. Returns QString::null if canReadLineStderr() returns FALSE.

See also canReadLineStderr(), readyReadStderr(), readStderr() and readLineStdout().

QString QProcess::readLineStdout () [virtual]

Reads a line of text from standard output, excluding any trailing newline or carriage return characters, and returns it. Returns QString::null if canReadLineStdout() returns FALSE.

See also canReadLineStdout(), readyReadStdout(), readStdout() and readLineStderr().

QByteArray QProcess::readStderr () [virtual]

Reads the data that the process has written to standard error. When new data is written to standard error, the class emits the signal readyReadStderr().

If there is no data to read, this function returns a QByteArray of size 0: it does not wait until there is something to read.

See also readyReadStderr(), readLineStderr(), readStdout() and writeToStdin().

QByteArray QProcess::readStdout () [virtual]

Reads the data that the process has written to standard output. When new data is written to standard output, the class emits the signal readyReadStdout().

If there is no data to read, this function returns a QByteArray of size 0: it does not wait until there is something to read.

See also readyReadStdout(), readLineStdout(), readStderr() and writeToStdin().

Example: process/process.cpp.

void QProcess::readyReadStderr () [signal]

This signal is emitted when the process has written data to standard error. You can read the data with readStderr().

Note that this signal is only emitted when there is new data and not when there is old, but unread data. In the slot connected to this signal, you should always read everything that is available at that moment to make sure that you don't lose any data.

See also readStderr(), readLineStderr() and readyReadStdout().

void QProcess::readyReadStdout () [signal]

This signal is emitted when the process has written data to standard output. You can read the data with readStdout().

Note that this signal is only emitted when there is new data and not when there is old, but unread data. In the slot connected to this signal, you should always read everything that is available at that moment to make sure that you don't lose any data.

See also readStdout(), readLineStdout() and readyReadStderr().

Example: process/process.cpp.

void QProcess::setArguments ( const QStringList & args ) [virtual]

Sets args as the arguments for the process. The first element in the list is the command to be executed. The other elements in the list are the arguments to the command. Any previous arguments are deleted.

QProcess does perform argument substitutions; for example, if you specify "*" or "$DISPLAY", these values are passed to the process literally. If you want to have the same behavior as the shell provides, you must do the substitutions yourself; i.e. instead of specifying a "*" you must specify the list of all the filenames in the current directory, and instead of "$DISPLAY" you must specify the value of the environment variable DISPLAY.

Note for Windows users. The standard Windows shells, e.g. command.com and cmd.exe, do not perform file globbing, i.e. they do not convert a "*" on the command line into a list of files in the current directory. For this reason most Windows applications implement their own file globbing, and as a result of this, specifying an argument of "*" for a Windows application is likely to result in the application performing a file glob and ending up with a list of filenames.

See also arguments() and addArgument().

void QProcess::setCommunication ( int commFlags )

Sets commFlags as the communication required with the process.

commFlags is a bitwise OR of the flags defined by the Communication enum.

The default is Stdin|Stdout|Stderr.

See also communication().

void QProcess::setWorkingDirectory ( const QDir & dir ) [virtual]

Sets dir as the working directory for processes. This does not affect running processes; only processes that are started afterwards are affected.

Setting the working directory is especially useful for processes that try to access files with relative paths.

See also workingDirectory() and start().

bool QProcess::start ( QStringList * env = 0 ) [virtual]

Tries to run a process for the command and arguments that were specified with setArguments(), addArgument() or that were specified in the constructor. The command is searched for in the path for executable programs; you can also use an absolute path in the command itself.

If env is null, then the process is started with the same environment as the starting process. If env is non-null, then the values in the stringlist are interpreted as environment setttings of the form key=value and the process is started in these environment settings. For convenience, there is a small exception to this rule: under Unix, if env does not contain any settings for the environment variable LD_LIBRARY_PATH, then this variable is inherited from the starting process; under Windows the same applies for the environment variable PATH.

Returns TRUE if the process could be started; otherwise returns FALSE.

You can write data to the process's standard input with writeToStdin(). You can close standard input with closeStdin() and you can terminate the process with tryTerminate(), or with kill().

You can call this function even if you've used this instance to create a another process which is still running. In such cases, QProcess closes the old process's standard input and deletes pending data, i.e., you lose all control over the old process, but the old process is not terminated. This applies also if the process could not be started. (On operating systems that have zombie processes, Qt will also wait() on the old process.)

See also launch() and closeStdin().

Example: process/process.cpp.

void QProcess::tryTerminate () const [slot]

Asks the process to terminate. Processes can ignore this if they wish. If you want to be certain that the process really terminates, you can use kill() instead.

The slot returns immediately: it does not wait until the process has finished. When the process terminates, the processExited() signal is emitted.

See also kill() and processExited().

QDir QProcess::workingDirectory () const

Returns the working directory that was set with setWorkingDirectory(), or the current directory if none has been explicitly set.

See also setWorkingDirectory() and QDir::current().

void QProcess::writeToStdin ( const QByteArray & buf ) [virtual slot]

Writes the data buf to the process's standard input. The process may or may not read this data.

This function returns immediately; the QProcess class might write the data at a later point (you must enter the event loop for this to occur). When all the data is written to the process, the signal wroteToStdin() is emitted. This does not mean that the process actually read the data, since this class only detects when it was able to write the data to the operating system.

See also wroteToStdin(), closeStdin(), readStdout() and readStderr().

void QProcess::writeToStdin ( const QString & buf ) [virtual slot]

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

The string buf is handled as text using the QString::local8Bit() representation.

void QProcess::wroteToStdin () [signal]

This signal is emitted if the data sent to standard input (via writeToStdin()) was actually written to the process. This does not imply that the process really read the data, since this class only detects when it was able to write the data to the operating system. But it is now safe to close standard input without losing pending data.

See also writeToStdin() and closeStdin().

This file is part of the Qt toolkit. Copyright © 1995-2002 Trolltech. All Rights Reserved.

Copyright © 2002 TrolltechTrademarks
Qt version 3.0.5

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