diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9902c271d..3813817ec 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -219,6 +219,12 @@ check_include_file(sys/time.h HAVE_SYS_TIME_H)
 # used in qconfig.h
 check_type_size(size_t QT_POINTER_SIZE)
 
+# XSI POSIX.1-2001 functions
+katie_check_function(strerror_r "string.h")
+# POSIX.1-2008 functions
+katie_check_function(realpath "stdlib.h")
+katie_check_function(unsetenv "stdlib.h")
+
 if(KATIE_COMPILER MATCHES "(gcc|clang)")
     set(QT_VISIBILITY_AVAILABLE TRUE)
 endif()
diff --git a/cmake/modules/KatieBuildMacros.cmake b/cmake/modules/KatieBuildMacros.cmake
index 77109641a..6e066268f 100644
--- a/cmake/modules/KatieBuildMacros.cmake
+++ b/cmake/modules/KatieBuildMacros.cmake
@@ -6,6 +6,9 @@ set(KATIE_RCC "rcc")
 set(KATIE_MOC "bootstrap_moc")
 set(KATIE_LRELEASE "lrelease")
 
+include(CheckSymbolExists)
+include(CheckFunctionExists)
+
 # a macro to print a dev warning but only when the build type is Debug
 macro(KATIE_WARNING MESSAGESTR)
     if(CMAKE_BUILD_TYPE STREQUAL "Debug")
@@ -13,6 +16,16 @@ macro(KATIE_WARNING MESSAGESTR)
     endif()
 endmacro()
 
+macro(KATIE_CHECK_FUNCTION FORFUNCTION FROMHEADER)
+    check_symbol_exists("${FORFUNCTION}" "${FROMHEADER}" HAVE_${FORFUNCTION})
+    if(NOT HAVE_${FORFUNCTION})
+        check_function_exists("${FORFUNCTION}" HAVE_${FORFUNCTION})
+    endif()
+    if(NOT HAVE_${FORFUNCTION})
+        message(SEND_ERROR "check_function_exists(${FORFUNCTION}) failed")
+    endif()
+endmacro()
+
 # a macro to write data to file, does nothing if the file exists and its
 # content is the same as the data
 macro(KATIE_WRITE_FILE OUTFILE DATA)
diff --git a/src/core/global/qglobal.cpp b/src/core/global/qglobal.cpp
index 359778440..81a401ba4 100644
--- a/src/core/global/qglobal.cpp
+++ b/src/core/global/qglobal.cpp
@@ -1365,12 +1365,7 @@ QString qt_error_string(int errorCode)
             s = QT_TRANSLATE_NOOP("QIODevice", "No space left on device");
             break;
         default: {
-#if !defined(QT_NO_THREAD) && defined(_POSIX_THREAD_SAFE_FUNCTIONS) && _POSIX_VERSION >= 200112L
-            QByteArray buf(1024, '\0');
-            ret = fromstrerror_helper(strerror_r(errorCode, buf.data(), buf.size()), buf);
-#else
-            ret = QString::fromLocal8Bit(strerror(errorCode));
-#endif
+            ret = fromstrerror_helper(errorCode);
             break;
         }
     }
diff --git a/src/core/io/qfilesystemengine.cpp b/src/core/io/qfilesystemengine.cpp
index ddf6b4d04..e1bfcfe8d 100644
--- a/src/core/io/qfilesystemengine.cpp
+++ b/src/core/io/qfilesystemengine.cpp
@@ -42,55 +42,6 @@
 
 QT_BEGIN_NAMESPACE
 
-/*!
-    \internal
-
-    Returns the canonicalized form of \a path (i.e., with all symlinks
-    resolved, and all redundant path elements removed.
-*/
-QString QFileSystemEngine::slowCanonicalized(const QString &path)
-{
-    if (path.isEmpty())
-        return path;
-
-    QFileInfo fi;
-    const QChar slash(QLatin1Char('/'));
-    QString tmpPath = path;
-    int separatorPos = 0;
-    QSet<QString> nonSymlinks;
-    QSet<QString> known;
-
-    known.insert(path);
-    do {
-        separatorPos = tmpPath.indexOf(slash, separatorPos + 1);
-        QString prefix = separatorPos == -1 ? tmpPath : tmpPath.left(separatorPos);
-        if (
-            !nonSymlinks.contains(prefix)) {
-            fi.setFile(prefix);
-            if (fi.isSymLink()) {
-                QString target = fi.readLink();
-                if(QFileInfo(target).isRelative())
-                    target = fi.absolutePath() + slash + target;
-                if (separatorPos != -1) {
-                    if (fi.isDir() && !target.endsWith(slash))
-                        target.append(slash);
-                    target.append(tmpPath.mid(separatorPos));
-                }
-                tmpPath = QDir::cleanPath(target);
-                separatorPos = 0;
-
-                if (known.contains(tmpPath))
-                    return QString();
-                known.insert(tmpPath);
-            } else {
-                nonSymlinks.insert(prefix);
-            }
-        }
-    } while (separatorPos != -1);
-
-    return QDir::cleanPath(tmpPath);
-}
-
 static inline bool _q_checkEntry(QFileSystemEntry &entry, QFileSystemMetaData &data, bool resolvingEntry)
 {
     if (resolvingEntry) {
diff --git a/src/core/io/qfilesystemengine_p.h b/src/core/io/qfilesystemengine_p.h
index 14a60542d..db45ced64 100644
--- a/src/core/io/qfilesystemengine_p.h
+++ b/src/core/io/qfilesystemengine_p.h
@@ -90,8 +90,6 @@ public:
 
     static QAbstractFileEngine *resolveEntryAndCreateLegacyEngine(QFileSystemEntry &entry,
                                                                   QFileSystemMetaData &data);
-private:
-    static QString slowCanonicalized(const QString &path);
 };
 
 QT_END_NAMESPACE
diff --git a/src/core/io/qfilesystemengine_unix.cpp b/src/core/io/qfilesystemengine_unix.cpp
index 96ed1a9ca..b7c91d63b 100644
--- a/src/core/io/qfilesystemengine_unix.cpp
+++ b/src/core/io/qfilesystemengine_unix.cpp
@@ -120,27 +120,12 @@ QFileSystemEntry QFileSystemEngine::canonicalName(const QFileSystemEntry &entry,
     if (entry.isEmpty() || entry.isRoot())
         return entry;
 
-#if _POSIX_VERSION < 200809L
-    // realpath(X,0) is not supported
-    Q_UNUSED(data);
-    return QFileSystemEntry(slowCanonicalized(absoluteName(entry).filePath()));
-#else
-#if _POSIX_VERSION >= 200801L
-    char *ret = realpath(entry.nativeFilePath().constData(), (char*)0);
-#else
-    char *ret = static_cast<char*>(malloc(PATH_MAX + 1));
-    if (realpath(entry.nativeFilePath().constData(), ret) == 0) {
-        const int savedErrno = errno; // errno is checked below, and free() might change it
-        free(ret);
-        errno = savedErrno;
-        ret = 0;
-    }
-#endif
+    char *ret = ::realpath(entry.nativeFilePath().constData(), (char*)0);
     if (ret) {
         data.knownFlagsMask |= QFileSystemMetaData::ExistsAttribute;
         data.entryFlags |= QFileSystemMetaData::ExistsAttribute;
         QString canonicalPath = QDir::cleanPath(QString::fromLocal8Bit(ret));
-        free(ret);
+        ::free(ret);
         return QFileSystemEntry(canonicalPath);
     } else if (errno == ENOENT) { // file doesn't exist
         data.knownFlagsMask |= QFileSystemMetaData::ExistsAttribute;
@@ -148,7 +133,6 @@ QFileSystemEntry QFileSystemEngine::canonicalName(const QFileSystemEntry &entry,
         return QFileSystemEntry();
     }
     return entry;
-#endif
 }
 
 //static
diff --git a/src/core/kernel/qsystemerror.cpp b/src/core/kernel/qsystemerror.cpp
index 59ad0e98e..743a2b886 100644
--- a/src/core/kernel/qsystemerror.cpp
+++ b/src/core/kernel/qsystemerror.cpp
@@ -59,12 +59,7 @@ static QString standardLibraryErrorString(int errorCode)
         s = QT_TRANSLATE_NOOP("QIODevice", "No space left on device");
         break;
     default: {
-#if !defined(QT_NO_THREAD) && defined(_POSIX_THREAD_SAFE_FUNCTIONS) && _POSIX_VERSION >= 200112L
-        QByteArray buf(1024, '\0');
-        ret = fromstrerror_helper(strerror_r(errorCode, buf.data(), buf.size()), buf);
-#else
-        ret = QString::fromLocal8Bit(strerror(errorCode));
-#endif
+        ret = fromstrerror_helper(errorCode);
     break; }
     }
     if (s) {
diff --git a/src/core/qcorecommon_p.h b/src/core/qcorecommon_p.h
index 64916ce1e..a823cfa49 100644
--- a/src/core/qcorecommon_p.h
+++ b/src/core/qcorecommon_p.h
@@ -7,30 +7,31 @@
 #  include <zlib.h>
 #endif
 
+#include <string.h>
+
 QT_BEGIN_NAMESPACE
 
 // enough space to hold BOM, each char as surrogate pair and terminator
 #define QMAXUSTRLEN(X) 4 + (X * 2) + 2
 
-#if !defined(QT_NO_THREAD) && defined(_POSIX_THREAD_SAFE_FUNCTIONS) && _POSIX_VERSION >= 200112L
 // There are two incompatible versions of strerror_r:
 // a) the XSI/POSIX.1 version, which returns an int,
 //    indicating success or not
 // b) the GNU version, which returns a char*, which may or may not
 //    be the beginning of the buffer we used
 // The GNU libc manpage for strerror_r says you should use the the XSI
-// version in portable code. However, it's impossible to do that if
-// _GNU_SOURCE is defined so we use C++ overloading to decide what to do
-// depending on the return type
-static inline QString fromstrerror_helper(int, const QByteArray &buf)
+// version in portable code.
+static inline QString fromstrerror_helper(const int errorcode)
 {
-    return QString::fromLocal8Bit(buf.constData());
-}
-static inline QString fromstrerror_helper(const char *str, const QByteArray &)
-{
-    return QString::fromLocal8Bit(str);
-}
+#if !defined(QT_NO_THREAD) && defined(_POSIX_THREAD_SAFE_FUNCTIONS)
+    char errbuf[1024];
+    ::memset(errbuf, '\0', sizeof(errbuf));
+    ::strerror_r(errorcode, errbuf, sizeof(errbuf));
+    return QString::fromLocal8Bit(errbuf, sizeof(errbuf));
+#else
+    return QString::fromLocal8Bit(::strerror(errorcode));
 #endif
+}
 
 static inline void report_error(int code, const char *where, const char *what)
 {
diff --git a/src/gui/kernel/qapplication_x11.cpp b/src/gui/kernel/qapplication_x11.cpp
index 8eb560bae..7b2b60901 100644
--- a/src/gui/kernel/qapplication_x11.cpp
+++ b/src/gui/kernel/qapplication_x11.cpp
@@ -95,10 +95,6 @@
 #  include <errno.h>
 #endif
 
-#if _POSIX_VERSION+0 < 200112L && !defined(Q_OS_BSD4)
-# define QT_NO_UNSETENV
-#endif
-
 QT_BEGIN_NAMESPACE
 
 //#define X_NOT_BROKEN
@@ -1624,13 +1620,7 @@ void qt_init(QApplicationPrivate *priv, int,
 
         qt_x11Data->startupId = getenv("DESKTOP_STARTUP_ID");
         if (qt_x11Data->startupId) {
-#ifndef QT_NO_UNSETENV
-            unsetenv("DESKTOP_STARTUP_ID");
-#else
-            // it's a small memory leak, however we won't crash if Qt is
-            // unloaded and someones tries to use the envoriment.
-            putenv(strdup("DESKTOP_STARTUP_ID="));
-#endif
+            ::unsetenv("DESKTOP_STARTUP_ID");
         }
    } else {
         // read some non-GUI settings when not using the X server...
diff --git a/tests/auto/qdir/tst_qdir.cpp b/tests/auto/qdir/tst_qdir.cpp
index 9183c024b..309c4e440 100644
--- a/tests/auto/qdir/tst_qdir.cpp
+++ b/tests/auto/qdir/tst_qdir.cpp
@@ -1081,9 +1081,7 @@ void tst_QDir::homePath()
         QVERIFY(!strHome.endsWith('/'));
 
     QByteArray envHome = qgetenv("HOME");
-#if !defined(_WRS_KERNEL) // unsetenv is not available on VxWorks DKM mode
-    unsetenv("HOME");
-#endif
+    ::unsetenv("HOME");
     QCOMPARE(QDir::homePath(), QDir::rootPath());
     qputenv("HOME", envHome);
 #endif