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Re: Bogus ifdef guard in sys/null.h

Bernd Ernesti <> wrote:

> On Sat, Jul 17, 2010 at 12:16:13PM +0300, Stathis Kamperis wrote:
>> in ,
>> the _SYS_NULL_H guard is bogus, since the whole header defines NULL
>> only. And there's already a guard for NULL. The code as it is, breaks
>> the following snippet:
>> % cat t.c
>> #include <stddef.h>
>> #undef NULL
>> #include <unistd.h>
> Doing that between two includes is the fault of the one who wrote that code.

That was my initial reaction too, but C99 says:

       7.1.3  Reserved identifiers

       [#1]  Each header declares or defines all identifiers listed
       in its associated  subclause,  and  optionally  declares  or
       defines  identifiers listed in its associated future library
       directions  subclause  and  identifiers  which  are   always
       reserved  either  for  any  use  or  for  use  as file scope

         -- All identifiers  that  begin  with  an  underscore  and
            either  an  uppercase  letter or another underscore are
            always reserved for any use.


         -- Each macro name in  any  of  the  following  subclauses
            (including  the  future library directions) is reserved
            for use as specified if any of its  associated  headers
            is  included;  unless  explicitly stated otherwise (see


       [#3]  If  the  program  removes  (with  #undef)  any   macro
       definition of an identifier in the first group listed above,
       the behavior is undefined.

I read [#3] to mean that you shouldn't #undef only "identifiers that
begin with an underscore and either an uppercase letter or another
underscore" (the first item in the list in [#1]), and that it doesn't
cover all items from the list.


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