The ISO/ANSI C standard I/O library is just that, and well documented elsewhere, including the ultimate authority, the standard for ISO/ANSI C. A good, readable reference of the C library in general is ``The Standard C Library'' by P.J. Plauger.
Standard I/O compatible with C stdio is provided in many different ways in many different environments. Implementing it is a particulary nasty problem for portable code and minimal operating environments because the design of stream I/O defined by the ISO/ANSI C Standard implies the existence of a compatible native I/O subsystem, and hardware to support it.
uCR has no native I/O system, because most embedded targets have no file system at all. Therefore, uCR does not include stdio support in the core library. However, some targets do have enough I/O power to warrant stream I/O, and even on those targets where such is debatable, there are applications that can make good use of stdio.
However, the uCR environment should not burden with stream I/O those targets and applications that choose not to use it, so uCR imposes no restrictions for the sake of stdio. Furthermore, uCR in no way uses any stdio routines. The stdio support is a completely optional add-on library, that an application may choose to use or not.