The role of ifdef and ifndef

Question

define one 0  ifdef one printf  quot one is defined  quot     ifndef one printf  quot one is not defined  quot     In this what is the role of  ifdef and  ifndef  and what s the output

User · Answer

The code looks strange because the printf are not in any function blocks

User · Answer

if one  means that if   define one  has been written   if one  is executed otherwise   ifndef one  is executed   This is just the C Pre-Processor  CPP  Directive equivalent of the if  then  else branch statements in the C language   i e  if   define one  then printf  one evaluates to a truth     else printf  one is not defined     so if there was no  define one statement then the else branch of the statement would be executed

User · Answer

Someone should mention that in the question there is a little trap   ifdef will only check if the following symbol has been defined via  define or by command line  but its value  its substitution in fact  is irrelevant  You could even write   define one   precompilers accept that   But if you use  if it s another thing     define one 0  if one     printf  one evaluates to a truth      endif  if  one     printf  one does not evaluate to truth      endif   will give one does not evaluate to truth  The keyword defined allows to get the desired behaviour    if defined one     is therefore equivalent to  ifdef  The advantage of the  if construct is to allow a better handling of code paths  try to do something like that with the old  ifdef  ifndef pair    if defined ORA PROC     defined   GNUC   amp  amp    GNUC VERSION  gt  300

User · Answer

Text inside an ifdef endif or ifndef endif pair will be left in or removed by the pre-processor depending on the condition  ifdef means  if the following is defined  while ifndef means  if the following is not defined    So    define one 0  ifdef one     printf  one is defined      endif  ifndef one     printf  one is not defined      endif   is equivalent to   printf  one is defined       since one is defined so the ifdef is true and the ifndef is false  It doesn t matter what it s defined as  A similar  better in my opinion  piece of code to that would be    define one 0  ifdef one     printf  one is defined      else     printf  one is not defined      endif   since that specifies the intent more clearly in this particular situation   In your particular case  the text after the ifdef is not removed since one is defined  The text after the ifndef is removed for the same reason  There will need to be two closing endif lines at some point and the first will cause lines to start being included again  as follows         define one 0  ---  ifdef one      printf  one is defined            Everything in here is included     -  ifndef one      printf  one is not defined        Everything in here is excluded            -  endif                                        Everything in here is included again   ---  endif

[c-preprocessor] The role of #ifdef and #ifndef

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