    When devices support em(seek operations), classes derived from
hi(streambuf) tt(std::streambuf) should override the members hi(seekoff)
tt(streambuf::seekoff) and hi(seekpos)tt(streambuf::seekpos). The class
ti(IFdSeek), developed in this section, can be used to read information from
devices supporting seek operations. The class tt(IFdSeek) was derived from
tt(IFdStreambuf), so it uses a character buffer of just one character. The
facilities to perform seek operations, which are added to our new class
tt(IFdSeek), ensure that the input buffer is reset when a seek operation is
requested. The class could also be derived from the class
tt(IFdNStreambuf). In that case the arguments to reset the input buffer
must be adapted so that its second and third parameters point beyond the
available input buffer. Let's have a look at the characteristics of
tt(IFdSeek):
    itemization(
    it() As mentioned, tt(IFdSeek) is derived from tt(IFdStreambuf). Like the
latter class, tt(IFdSeek)'s member functions use facilities declared in
tt(unistd.h). So, the header file tthi(unistd.h) must have been read by the
compiler before it can compile the class's members functions.  To reduce the
amount of typing when specifying types and constants from tt(streambuf) and
hi(ios)tt(std::ios), several using-declarations are defined by the class.
These using-declarations refer to types that are defined in the header file
tthi(ios), which must therefore also be included before the compiler can
compile tt(IFdSeek)'s class interface:
        verbinclude(//CLASS examples/fdinseek.h)
    it() The class has a very basic interface. Its (only)
constructor expects the device's i(file descriptor). It has no special tasks
to perform and just calls its i(base class) constructor:
        verbinclude(//CONS examples/fdinseek.h)
    it() The member ti(seek_off) is responsible for performing the actual
seek operations. It calls ti(lseek) to seek a new position in a device whose
file descriptor is known. If seeking succeeds, ti(setg) is called to define
an already empty buffer, so that the base class's tt(underflow) member
refills the buffer at the next input request.
        verbinclude(//SEEKOFF examples/fdinseek.h)
    it() Finally,  the companion function ti(seekpos) is overridden as well:
it is actually defined as a call to tt(seekoff):
        verbinclude(//SEEKPOS examples/fdinseek.h)
    )
    Here is an example of a program using the class tt(IFdSeek). If
this program is given its own source file using input redirection then
seeking is supported (and with the exception of the first line, every other
line is shown twice):
        verbinclude(-a examples/fdinseek.cc)
