add QProcess::nullDevice()

Change-Id: I15273fa3f3ba323a835350153f2a20404f12420b
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qprocess.cpp

@@ -1317,6 +1317,10 @@ void QProcess::closeWriteChannel()
     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 \a fileName does not exist at the moment start() is
     called or is not readable, starting the process will fail.
 
@@ -1340,6 +1344,10 @@ void QProcess::setStandardInputFile(const QString &fileName)
     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 \a fileName doesn't exist at the moment start() is
     called, it will be created. If it cannot be created, the starting
     will fail.
@@ -2439,6 +2447,25 @@ QStringList QProcess::systemEnvironment()
     \sa QProcess::systemEnvironment()
 */
 
+/*!
+    \since 5.2
+
+    \brief The null device of the operating system.
+
+    The returned file path uses native directory separators.
+
+    \sa QProcess::setStandardInputFile(), QProcess::setStandardOutputFile(),
+        QProcess::setStandardErrorFile()
+*/
+QString QProcess::nullDevice()
+{
+#ifdef Q_OS_WIN
+    return QStringLiteral("\\\\.\\NUL");
+#else
+    return QStringLiteral("/dev/null");
+#endif
+}
+
 /*!
     \typedef Q_PID
     \relates QProcess

src/corelib/io/qprocess.h

@@ -209,6 +209,8 @@ public:
 
     static QStringList systemEnvironment();
 
+    static QString nullDevice();
+
 public Q_SLOTS:
     void terminate();
     void kill();

tests/auto/corelib/io/qprocess/tst_qprocess.cpp

@@ -122,6 +122,7 @@ private slots:
     void setStandardInputFile();
     void setStandardOutputFile_data();
     void setStandardOutputFile();
+    void setStandardOutputFile2();
     void setStandardOutputProcess_data();
     void setStandardOutputProcess();
     void removeFileWhileProcessIsRunning();
@@ -1868,6 +1869,13 @@ void tst_QProcess::setStandardInputFile()
         QByteArray all = process.readAll();
     QCOMPARE(all.size(), int(sizeof data) - 1); // testProcessEcho drops the ending \0
     QVERIFY(all == data);
+
+    QProcess process2;
+    process2.setStandardInputFile(QProcess::nullDevice());
+    process2.start("testProcessEcho/testProcessEcho");
+    QPROCESS_VERIFY(process2, waitForFinished());
+    all = process2.readAll();
+    QCOMPARE(all.size(), 0);
 }
 #endif
 
@@ -1902,6 +1910,23 @@ void tst_QProcess::setStandardOutputFile_data()
                                    << true;
 }
 
+//-----------------------------------------------------------------------------
+#ifndef Q_OS_WINCE
+void tst_QProcess::setStandardOutputFile2()
+{
+    static const char testdata[] = "Test data.";
+
+    QProcess process;
+    process.setStandardOutputFile(QProcess::nullDevice());
+    process.start("testProcessEcho2/testProcessEcho2");
+    process.write(testdata, sizeof testdata);
+    QPROCESS_VERIFY(process,waitForFinished());
+    QVERIFY(!process.bytesAvailable());
+
+    QVERIFY(!QFileInfo(QProcess::nullDevice()).isFile());
+}
+#endif
+
 void tst_QProcess::setStandardOutputFile()
 {
     static const char data[] = "Original data. ";