I have a piece of code in C with header files included. I run it on Mac OS X Maverick with XCode 4.6.2 installed. GCC is also installed. Note that Command Line Tools in XCode are already installed. When I compile it, the error I receive says something like this: add.c:1:19: error: stdio.h: No such file or directory add.c:2:20: error: stdlib.h: No such file or directory add.c:3:20: error: unistd.h: No such file or directory However when I run it on Ubuntu, it compiles without a problem. what to do?