diff --git a/src/corelib/text/qbytearrayalgorithms.h b/src/corelib/text/qbytearrayalgorithms.h
index 638af2948b4..a78e6e17099 100644
--- a/src/corelib/text/qbytearrayalgorithms.h
+++ b/src/corelib/text/qbytearrayalgorithms.h
@@ -96,12 +96,10 @@ inline size_t qstrlen(const char *str)
 
 inline size_t qstrnlen(const char *str, size_t maxlen)
 {
-    size_t length = 0;
-    if (str) {
-        while (length < maxlen && *str++)
-            length++;
-    }
-    return length;
+    if (!str)
+        return 0;
+    auto end = static_cast<const char *>(memchr(str, '\0', maxlen));
+    return end ? end - str : maxlen;
 }
 
 // implemented in qbytearray.cpp
diff --git a/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp b/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp
index a4798e77aa4..0f1f00bf6ce 100644
--- a/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp
+++ b/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp
@@ -803,18 +803,18 @@ void tst_QByteArray::qvsnprintf()
 void tst_QByteArray::qstrlen()
 {
     const char *src = "Something about ... \0 a string.";
-    QCOMPARE(::qstrlen((char*)0), (uint)0);
-    QCOMPARE(::qstrlen(src), (uint)20);
+    QCOMPARE(::qstrlen(nullptr), size_t(0));
+    QCOMPARE(::qstrlen(src), size_t(20));
 }
 
 void tst_QByteArray::qstrnlen()
 {
     const char *src = "Something about ... \0 a string.";
-    QCOMPARE(::qstrnlen((char*)0, 1), (uint)0);
-    QCOMPARE(::qstrnlen(src, 31), (uint)20);
-    QCOMPARE(::qstrnlen(src, 19), (uint)19);
-    QCOMPARE(::qstrnlen(src, 21), (uint)20);
-    QCOMPARE(::qstrnlen(src, 20), (uint)20);
+    QCOMPARE(::qstrnlen(nullptr, 1), size_t(0));
+    QCOMPARE(::qstrnlen(src, 31), size_t(20));
+    QCOMPARE(::qstrnlen(src, 19), size_t(19));
+    QCOMPARE(::qstrnlen(src, 21), size_t(20));
+    QCOMPARE(::qstrnlen(src, 20), size_t(20));
 }
 
 void tst_QByteArray::qstrcpy()