ddeeffiinneedd -- Preprocessor Operator

Perform an action if a macro is defined

The preprocessor  directive ddeeffiinneedd determines whether  a symbol is defined
to the #iiff preprocessor directive.  For example,

    #if defined(SYMBOL)

or

    #if defined SYMBOL

is equivalent to

    #ifdef SYMBOL

except that it can be used in more complex expressions, such as

    #if defined FOO && defined BAR && FOO==10

ddeeffiinneedd is recognized only in lines beginning with #iiff or #eelliiff.

_S_e_e _A_l_s_o
#eelliiff, #iiff, #iiffddeeff, ccpppp, CC pprreepprroocceessssoorr
ANSI Standard, section 6.8.1

_N_o_t_e_s
Note that ddeeffiinneedd is  a preprocessor operator, not a preprocessor directive
or a  C keyword.  The  difference lies in  the fact that you  could write a
function called ddeeffiinneedd() without any complaint from the C compiler; and if
ddeeffiinneedd does not appear within  an #iiff or #eelliiff directive, the preprocessor
ignores it.
