This page is a dictionary of the macros used in Pd's C code.
__APPLE__:: for only Apple Mac OS X
__gnu_linux__:: for GNU/Linux only
__linux__:: for GNU/Linux only
_MSC_VER:: for only Microsoft's compiler
MSW:: for Windows, Pd-only, deprecated, use _WIN32 or _MSC_VER
NT:: for Windows, Pd-only, deprecated, use _WIN32 or _MSC_VER
PD:: for selecting code between Pd and Max/MSP APIs
UNISTD:: Pd-only, usually means UNIX-like platforms (i.e. not Win32). Deprecated, use #ifndef _WIN32
UNIX:: Pd-only, usually means for GNU/Linux and historically IRIX. Deprecated, use __linux__, __APPLE__, or #ifndef _WIN32 to isolate platforms.
_WIN32:: for the Win32 API (e.g. MSVC and MinGW, but not Cygwin)