First off, I don't have the appliance, I had to place cs50.h in the same directory as my source code. As you will see from my code I did provide the correct header for that situation.
The error in the title is what Xcode spits out.
When I write the code in note pad, and make it from the terminal, I get this error:
cc cipher.c -o cipher Undefined symbols for architecture x86_64: "_main", referenced from: implicit entry/start for main executable ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation) make: *** [cipher] Error 1
What am I missing?