What exactly does an if 0 endif block do

Question

In C C    What happens to code placed between an  if 0  endif block    if 0    Code goes here   endif   Does the code simply get skipped and therefore does not get executed

User · Answer

It permanently comments out that code so the compiler will never compile it.

The coder can later change the #ifdef to have that code compile in the program if he wants to.

It's exactly like the code doesn't exist.

User · Answer

Not quite  int main void        if 0      the apostrophe   causes a warning     endif    return 0      It shows  t c 4 19  warning  missing terminating   character  with gcc 4 2 4

User · Answer

Lines beginning with a   are preprocessor directives    if 0        endif blocks do not make it to the compiler and will generate no machine code    You can demonstrate what happens with the preprocessor with a source file ifdef cxx    if 0 This code will not be compiled  else int i   0   endif   Running gcc -E ifdef cxx will show you what gets compiled   You may choose to use this mechanism to prevent a block of code being compiled during the development cycle  but you would probably not want to check it in to your source control as it just adds cruft to your code and reduces readability  If it s a historical piece of code that has been commented out  then it should be removed  source control contains the history  right   Also  the answer may be the same for both C and C   but there is no language called C C   and it s not a good habit to refer to such a language

User · Answer

When the preprocessor sees  if it checks whether the next token has a non-zero value   If it does  it keeps the code around for the compiler   If it doesn t  it gets rid of that code so the compiler never sees it   If someone says  if 0 they are effectively commenting out the code so it will never be compiled   You can think of this the same as if they had put           around it   It s not quite the same  but it has the same effect   If you want to understand what happened in detail  you can often look   Many compilers will allow you to see the files after the preprocessor has run   For example  on Visual C   the switch  P command will execute the preprocessor and put the results in a  i file

User · Answer

It s identical to commenting out the block  except with one important difference  Nesting is not a problem  Consider this code   foo    bar x  y      x must not be NULL    baz      If I want to comment it out  I might try      foo    bar x  y      x must not be NULL    baz         Bzzt  Syntax error  Why  Because block comments do not nest  and so  as you can see from SO s syntax highlighting  the    after the word  NULL  terminates the comment  making the baz call not commented out  and the    after baz a syntax error  On the other hand    if 0 foo    bar x  y      x must not be NULL    baz     endif   Works to comment out the entire thing  And the  if 0s will nest with each other  like so    if 0 pre foo     if 0 foo    bar x  y      x must not be NULL    baz     endif quux     endif   Although of course this can get a bit confusing and become a maintenance headache if not commented properly

User · Answer

Not only does it not get executed  it doesn t even get compiled    if is a preprocessor command  which gets evaluated before the actual compilation step  The code inside that block doesn t appear in the compiled binary   It s often used for temporarily removing segments of code with the intention of turning them back on later

User · Answer

It is a cheap way to comment out  but I suspect that it could have debugging potential  For example  let s suppose you have a build that output values to a file  You might not want that in a final version so you can use the  if 0     endif   Also  I suspect a better way of doing it for debug purpose would be to do    ifdef DEBUG    output to file  endif   You can do something like that and it might make more sense and all you have to do is define DEBUG to see the results

User · Answer

I d like to add on for the  else case    if 0       Code here will NOT be complied      else       Code will be compiled      endif    if 1       Code will be complied      else       Code will NOT be compiled      endif

[c-preprocessor] What exactly does an #if 0 ..... #endif block do?

Examples related to c-preprocessor