Doc: fix null pointer passing to fprintf

When these docs were written, the context.file and context.function
pointers were never null. But in commit d78fb442d750b33afe2e41f31588ec94
(Qt 5.4), we made the logging pass null pointers in release builds. The
C standard does not say that passing null pointers is permitted. In
fact, it says "If no l length modifier is present, the argument shall be
a pointer to the initial element of an array of character type." and
that's pretty explicit that it needs to point to the initial element of
a string.

Fixes: QTBUG-72478
Change-Id: I4ac1156702324f0fb814fffd156f624ffefa1445
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>

1 files changed, 7 insertions, 5 deletions

diff --git a/src/corelib/doc/snippets/code/src_corelib_global_qglobal.cpp b/src/corelib/doc/snippets/code/src_corelib_global_qglobal.cpp
index 0248640369..03904659f5 100644
--- a/src/corelib/doc/snippets/code/src_corelib_global_qglobal.cpp
+++ b/src/corelib/doc/snippets/code/src_corelib_global_qglobal.cpp
@@ -279,21 +279,23 @@ const TInputType &myMin(const TInputType &value1, const TInputType &value2)
 void myMessageOutput(QtMsgType type, const QMessageLogContext &context, const QString &msg)
 {
     QByteArray localMsg = msg.toLocal8Bit();
+    const char *file = context.file ? context.file : "";
+    const char *function = context.function ? context.function : "";
     switch (type) {
     case QtDebugMsg:
-        fprintf(stderr, "Debug: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
+        fprintf(stderr, "Debug: %s (%s:%u, %s)\n", localMsg.constData(), file, context.line, function);
         break;
     case QtInfoMsg:
-        fprintf(stderr, "Info: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
+        fprintf(stderr, "Info: %s (%s:%u, %s)\n", localMsg.constData(), file, context.line, function);
         break;
     case QtWarningMsg:
-        fprintf(stderr, "Warning: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
+        fprintf(stderr, "Warning: %s (%s:%u, %s)\n", localMsg.constData(), file, context.line, function);
         break;
     case QtCriticalMsg:
-        fprintf(stderr, "Critical: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
+        fprintf(stderr, "Critical: %s (%s:%u, %s)\n", localMsg.constData(), file, context.line, function);
         break;
     case QtFatalMsg:
-        fprintf(stderr, "Fatal: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
+        fprintf(stderr, "Fatal: %s (%s:%u, %s)\n", localMsg.constData(), file, context.line, function);
         break;
     }
 }