[rtems] Tweaks to sys/features.h

[Joel Sherrill]

Ralf Corsepius wrote:
> On Thu, 2008-11-20 at 12:32 -0600, Joel Sherrill wrote:
>   
>> Howland Craig D (Craig) wrote:
>>     
>>> Wouldn't it be better to define _POSIX_SHARED_MEMORY_OBJECTS with value
>>> -1 than to have it not defined?  That way it is known at compile time
>>> that there is no support, rather than needing to wait for runtime for a
>>> sysconf() call.
>>>       
> Likely, but it's what others do in newlib, too.
>   
>   
>> If that's what people are expecting to test for then yes.
>>     
>
> >From SUSv3
> <cite>
> The following symbolic constants, if defined in <unistd.h>, shall have a
> value of -1, 0, or greater, unless otherwise specified below. If these
> are undefined, the fpathconf(), pathconf(), or sysconf() functions can
> be used to determine whether the option is provided for a particular
> invocation of the application.
>
> If a symbolic constant is defined with the value -1, the option is not
> supported. Headers, data types, and function interfaces required only
> for the option need not be supplied. An application that attempts to use
> anything associated only with the option is considered to be requiring
> an extension.
>
> If a symbolic constant is defined with a value greater than zero, the
> option shall always be supported when the application is executed. All
> headers, data types, and functions shall be present and shall operate as
> specified.
>
> If a symbolic constant is defined with the value zero, all headers, data
> types, and functions shall be present. The application can check at
> runtime to see whether the option is supported by calling fpathconf(),
> pathconf(), or sysconf() with the indicated name parameter.
> </cite>
>   

Interesting, then in sysconf(), having something like this
for each _SC_XXX constant would be appropriate:

#if defined(_POSIX_SHARED_MEMORY_OBJECTS)
    case _SC_SHARED_MEMORY_OBJECTS:
      return _POSIX_SHARED_MEMORY_OBJECTS;
#endif

>   
>> And FWIW RTEMS supports sysconf() but doesn't have
>> a lot of cases in its implementation.  What sysconf value
>> covers this one?
>>     
> Cf. above.
>
> However, people are using the defines to detect the ability of a feature
> to "potentially present", while sysconf covers run-time detection.
>
> => 2 different problems.
>
> Ralf
>
>
>   


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