qstrncpy: NUL-terminate even when src is nullptr

The goal of this function is to ensure that dst is _always_
NUL-terminated. The only exception is if there's no space to write
even one NUL byte, of course, but not when src is nullptr but dst
would have space.

Update the docs to the new behavior and make them more precise.

Fix a test that assumed qstrncpy() would not write to dst for
(dst, nullptr, 10).

[ChangeLog][QtCore][qstrncpy()] Now NUL-terminates the target
buffer even when the source pointer is nullptr, provided the
target buffer has space for at least one byte.

Change-Id: I7806d8c71e260f8f02b79af7b6ce94f23599dd69
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 05f913d57d6557d1c540894651cc83a5b1ec7cf7)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 12 insertions, 7 deletions

diff --git a/src/corelib/text/qbytearray.cpp b/src/corelib/text/qbytearray.cpp
index 4cd963588f..e60f114ff3 100644
--- a/src/corelib/text/qbytearray.cpp
+++ b/src/corelib/text/qbytearray.cpp
@@ -146,9 +146,9 @@ char *qstrcpy(char *dst, const char *src)
     A safe \c strncpy() function.
 
     Copies at most \a len bytes from \a src (stopping at \a len or the
-    terminating '\\0' whichever comes first) into \a dst and returns a
-    pointer to \a dst. Guarantees that \a dst is '\\0'-terminated. If
-    \a src or \a dst is \nullptr, returns \nullptr immediately.
+    terminating '\\0' whichever comes first) into \a dst. Guarantees that \a
+    dst is '\\0'-terminated, except when \a dst is \nullptr or \a len is 0. If
+    \a src is \nullptr, returns \nullptr, otherwise returns \a dst.
 
     This function assumes that \a dst is at least \a len characters
     long.
@@ -160,9 +160,11 @@ char *qstrcpy(char *dst, const char *src)
 
 char *qstrncpy(char *dst, const char *src, size_t len)
 {
-    if (!src || !dst)
-        return nullptr;
-    if (len > 0) {
+    if (dst && len > 0) {
+        if (!src) {
+            *dst = '\0';
+            return nullptr;
+        }
 #ifdef Q_CC_MSVC
         strncpy_s(dst, len, src, len - 1);
 #else
@@ -170,7 +172,7 @@ char *qstrncpy(char *dst, const char *src, size_t len)
 #endif
         dst[len-1] = '\0';
     }
-    return dst;
+    return src ? dst : nullptr;
 }
 
 /*! \fn size_t qstrlen(const char *str)
diff --git a/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp b/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp
index fc7247b733..cd750aa268 100644
--- a/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp
+++ b/tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp
@@ -978,7 +978,10 @@ void tst_QByteArray::qstrncpy()
 
     // src == nullptr
     QCOMPARE(::qstrncpy(dst.data(), 0,  0), (char*)0);
+    QCOMPARE(*dst.data(), 'b'); // must not have written to dst
     QCOMPARE(::qstrncpy(dst.data(), 0, 10), (char*)0);
+    QCOMPARE(*dst.data(), '\0'); // must have written to dst
+    *dst.data() = 'b'; // restore
 
     // valid pointers, but len == 0
     QCOMPARE(::qstrncpy(dst.data(), src.data(), 0), dst.data());