#iiffnnddeeff -- Preprocessing Directive

Include code conditionally

The  preprocessing directive  #iiffnnddeeff  checks whether  _i_d_e_n_t_i_f_i_e_r has  been
defined as  a macro name.  If  _i_d_e_n_t_i_f_i_e_r has _n_o_t been  defined as a macro,
then the  preprocessor includes  _g_r_o_u_p within the  program, up to  the next
#eelliiff,  #eellssee,  or  #eennddiiff  directive.   If  _i_d_e_n_t_i_f_i_e_r has  been  defined,
however, then _g_r_o_u_p is skipped.

An #iiffnnddeeff directive can be followed  by any number of #eelliiff directives, by
one #eellssee directive, and by one #eelliiff directive.

_S_e_e _A_l_s_o
#eelliiff, #eellssee, #eennddiiff, #iiff, #iiffnnddeeff, CC pprreepprroocceessssoorr, ddeeffiinneedd
ANSI Standard, section 6.8.1
