MinGW at its core is just a cross compiler / toolchain, so it doesn't attempt to make Windows look like POSIX. It provides header files & linker stubs for the Win32 APIs. So you are developing against Win32 APIs, rather than POSIX.
If you want a Windows based runtime environment that strives to completely simulate POSIX APIs, then Cygwin is likely a more suitable choice. If you are happy to work against the Win32 APIs more directly, then GNULib provides wrappers around a number of Win32 APIs to give you a degree of POSIX compatability often good enough for many apps. Finally there are higher level APIs like GLib, QT, NSPR which attempt to provide a platform agnostic APIs.