iOS: Wrap user's main by renaming symbol and relying on weak linking

This approach is similar to the earlier apprach of defining main=qt_main
when building the user's sources, but uses the linker to rename the
symbol instead, which is less fragile than using the preprocessor.

To keep the hybrid usecase unaffected by our wrapper logic we declare
both our main wrapper and a fallback qt_main as weak symbols, which
ensures that when the user's application links in our plugin the
real main/qt_main provided by the user is preferred over our weak
symbols.

Change-Id: Ic76f3ba8932430c4b13a1d3a40b8ed2322fe5eea
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>

1 files changed, 14 insertions, 3 deletions

diff --git a/src/plugins/platforms/ios/qioseventdispatcher.mm b/src/plugins/platforms/ios/qioseventdispatcher.mm
index 9991a59d9e..3dd9c7ad9f 100644
--- a/src/plugins/platforms/ios/qioseventdispatcher.mm
+++ b/src/plugins/platforms/ios/qioseventdispatcher.mm
@@ -213,7 +213,7 @@ static int infoPlistValue(NSString* key, int defaultValue)
     return value ? [value intValue] : defaultValue;
 }
 
-extern "C" int qtmn(int argc, char *argv[])
+extern "C" int __attribute__((weak)) main(int argc, char *argv[])
 {
     @autoreleasepool {
         size_t defaultStackSize = 512 * kBytesPerKiloByte; // Same as secondary threads
@@ -248,7 +248,18 @@ enum SetJumpResult
     kJumpedFromUserMainTrampoline,
 };
 
-extern "C" int main(int argc, char *argv[]);
+// We define qt_main so that user_main_trampoline() will not cause
+// missing symbols in the case of hybrid applications that don't
+// user our main wrapper. Since the symbol is weak, it will not
+// get used or cause a clash in the normal Qt application usecase,
+// where we rename main to qt_main.
+extern "C" int __attribute__((weak)) qt_main(int argc, char *argv[])
+{
+    Q_UNUSED(argc);
+    Q_UNUSED(argv);
+
+    Q_UNREACHABLE();
+}
 
 static void __attribute__((noinline, noreturn)) user_main_trampoline()
 {
@@ -261,7 +272,7 @@ static void __attribute__((noinline, noreturn)) user_main_trampoline()
         strcpy(argv[i], [arg cStringUsingEncoding:[NSString defaultCStringEncoding]]);
     }
 
-    int exitCode = main(argc, argv);
+    int exitCode = qt_main(argc, argv);
     delete[] argv;
 
     qEventDispatcherDebug() << "Returned from main with exit code " << exitCode;