fopen and paths in Xcode

I'm using Xcode for a C programming class, and while most of it is over my head, a lot of the features are really helpful. I'm a little puzzled about fopen, though. When I try this

fp = fopen("inventory.data", "r");

fp (which is a FILE*) is 0 during debugging. However, if I use the full path (/Volumes...) it works. I'm required to upload and compile my code on the school's servers, so using those paths is inconvenient. I've put the inventory.data file in my project folder along with the source files, and have added it under the source section in Xcode. Is there some way I can reference the file like this?