ld: 1 duplicate symbol for architecture x86_64

When you create a new project in Xcode, it automatically gives you a starting file with main() in it. If you created a new file, such as first.c, and then pasted your test code into it, you'll be defining main() twice, and getting that error.

You need to either delete the file (such as main.c, or main.m) that Xcode provides in your new project, or cut and paste your sample code into that file, instead of creating a new one.