    When deriving a class (e.g., ti(IFdStreambuf)) from tt(streambuf) using a
buffer of one character, at least its member
 hi(underflow)
    tt(streambuf::underflow) should be overridden, as this member eventually
receives all requests for input. The member
 hi(setg)
    tt(streambuf::setg) is used to inform the tt(streambuf) base class of the
size and location of the input buffer, so that it is able to set up its input
buffer pointers accordingly. This ensures that hi(eback)
tt(streambuf::eback), hi(gptr) tt(streambuf::gptr), and hi(egptr)
tt(streambuf::egptr) return correct values.

    The class tt(IFdStreambuf) is designed like this:
    itemization(
    it() Its member functions use low-level functions operating on file
descriptors. Therefore, in addition to tt(streambuf), the tthi(unistd.h)
header file must have been read by the compiler before its member functions
can be compiled.
    it() Like most classes designed for input operations, this class is derived
from hi(streambuf)tt(std::streambuf) as well.
    it() The class defines two data members, one of them a fixed-sized one
character buffer. The data members are defined as ti(protected) data members
so that derived classes (e.g., see section ref(IFDSEEK)) can access them. Here
is the full class interface:
        verbinclude(//CLASS examples/ifdbuf.h)
    it() The constructor initializes the buffer. However, the initialization
sets ti(gptr)'s return value equal to ti(egptr)'s return value. This
implies that the buffer is empty so tt(underflow) is immediately called
to fill the buffer:
        verbinclude(//CONS examples/ifdbuf.h)
    it() Finally tt(underflow) is overridden. The buffer is refilled by
reading from the file descriptor. If this fails (for whatever reason),
endOfFile() is returned. More sophisticated implementations could act more
intelligently here, of course. If the buffer could be refilled, ti(setg) is
called to set up tt(streambuf)'s buffer pointers correctly:
        verbinclude(//UFLOW examples/ifdbuf.h)
    )
    The following tt(main) function shows how tt(IFdStreambuf) can be used:
        verbinclude(//MAIN examples/ifdbuf.cc)
