libio is distributed with a complete implementation of the ANSI C stdio facility. It is implemented using streambuf objects. See Wrappers for C stdio.
The stdio package is intended as a replacement for the whatever stdio is in your C library. Since stdio works best when you build libc to contain it, and that may be inconvenient, it is not installed by default.
Extensions beyond ansi:
A stdio FILE is identical to a streambuf. Hence there is no need to worry about synchronizing C and C++ input/output---they are by definition always synchronized.
If you create a new streambuf sub-class (in C++), you can use it as a FILE from C. Thus the system is extensible using the standard streambuf protocol.
You can arbitrarily mix reading and writing, without having to seek in between.
Unbounded ungetc() buffer.