Now it's getting really weird. If I add an OpenPanel in the application code and use that to select a file when the application runs, a subsequent call to fopen will successfully open the file! If the OpenPanel cancel button is pressed without selecting that file, or if a different file is selected, then fopen will return NULL.

Additionally, I have observed that fopen fails to read a file in the target directory. To me, this indicates some security feature may be blocking the program from accessing the hard drive, even though I have full disk access enabled for this application.

I enabled full disk access to the application in the security and privacy system settings, but it seems to make no difference.

If I change the current directory using either NSFileManager:changeCurrentDirectoryPath or _wchdir, both functions succeed, but a following call to get the current directory fails. If getcwd is used, it produces errno 1 (operation not permitted), and NFFileManager.currentDicrectoryPath is equal to nil. The Xcode project location is in a subfolder of the user Documents directory.

Nice, I can't edit my answer above to fix the formatting.

Several years later, I have the answer to this. In the main header file I added this definition: #ifdef __APPLE__ #define main cppmain #endif Declare your C++ main function just like normal and the compiler will internally rename it to "cppmain". And then in main.mm I have the real main function: #undef main int main(int argc, const char* args) {     return NSApplicationMain(); } #define main cppmain And in AppDelegate.mm the C++ main function is called with arguments passed to it: ` extern int cppmain(int argc, const char* argv[]); - (void)applicationDidFinishLaunching:(NSNotification )aNotification {     int argc = _NSGetArgc();     char argv = _NSGetArgv();     cppmain(argc, (const char*)argv);     [NSApp terminate:self]; } ` As a result all our existing examples work just fine with the normal main() function declared in C++: https://www.ultraengine.com/learn/CPP/CreateTreeView