+.Sh COMPATIBILITY
+Many application writers used the name
+.Va dprintf
+before the
+.Fn dprintf
+function was introduced in
+.St -p1003.1 ,
+so a prototype is not provided by default in order to avoid
+compatibility problems.
+Applications that wish to use the
+.Fn dprintf
+function described herein should either request a strict
+.St -p1003.1-2008
+environment by defining the macro
+.Dv _POSIX_C_SOURCE
+to the value 200809 or greater, or by defining the macro
+.Dv _WITH_DPRINTF ,
+prior to the inclusion of
+.In stdio.h .
+For compatibility with GNU libc, defining either
+.Dv _BSD_SOURCE
+or
+.Dv _GNU_SOURCE
+prior to the inclusion of
+.In stdio.h
+will also make
+.Fn dprintf
+available.
+.Pp
+The conversion formats
+.Cm \&%D , \&%O ,
+and
+.Cm %U
+are not standard and
+are provided only for backward compatibility.
+The effect of padding the
+.Cm %p
+format with zeros (either by the
+.Cm 0
+flag or by specifying a precision), and the benign effect (i.e., none)
+of the
+.Cm #
+flag on
+.Cm %n
+and
+.Cm %p
+conversions, as well as other
+nonsensical combinations such as
+.Cm %Ld ,
+are not standard; such combinations
+should be avoided.