Many application writers used the name dprintf before the dprintf()
     function was introduced in IEEE Std 1003.1 (“POSIX.1”), so a prototype is
     not provided by default in order to avoid compatibility problems.
     Applications that wish to use the dprintf() function described herein
     should either request a strict IEEE Std 1003.1-2008 (“POSIX.1”)
     environment by defining the macro _POSIX_C_SOURCE to the value 200809 or
     greater, or by defining the macro _WITH_DPRINTF, prior to the inclusion
     of <stdio.h>.  For compatibility with GNU libc, defining either
     _BSD_SOURCE or _GNU_SOURCE prior to the inclusion of <stdio.h> will also
     make dprintf() available.