Every time I try to compile a project that includes errno.h in linux, I get issues using the error codes. GCC spits out errors saying the error codes aren't declared and I get the same issue with 3rd party projects that are, of course, known to compile.

I have the Linux headers for my kernel installed. Any suggestions as to what else it could be?

edit:

Snippet at error code: if ((res != 0) && (errno == EEXIST))

Snippet of includes:

    ...
    #include <dirent.h>
    #include <errno.h>
    #include <stdarg.h>
    ...

as is with no ifdef

edit: Well I'm convinced my system's Linux headers are hosed somehow... that, or I'm missing some environment vars.