I don't understand why this is not compiling

Hi everyone,

My C is a little rusty, but I figured there’s no time like the present to brush up on it again. I’ve tried programming literally the simplest program I can (see Image), and yet, it does not run.

The error message:

/tmp/ccxkSGWz.o: In function main': main.c:(.text+0x15): undefined reference tofoo’
collect2: error: ld returned 1 exit status

When I move the actual function declaration to the .h file it does just fine. But I know I shouldn’t have to do that. That makes me think there’s a disconnect between my test.c and test.h files?

I suspect it’s not necessarily an Atom problem, since I get the same error when I compile with gcc on the command line.

Any thoughts as to what is going on?