Re: should "man feature_test_macros" mention _LARGEFILE_SOURCE?

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On 03/19/2014 08:15 PM, Mike Frysinger wrote:
> On Wed 19 Mar 2014 14:37:51 Michael Kerrisk wrote:
>> On 03/19/2014 03:48 AM, Mike Frysinger wrote:
>>> On Tue 18 Mar 2014 15:46:43 Robert P. J. Day wrote:
>>>>   just noticed under /usr/include on my fedora rawhide system the
>>>>
>>>> following:
>>>>
>>>> /usr/include/bits/environments.h:# define __ILP32_OFFBIG_CFLAGS	"-m32
>>>> -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64"
>>>> /usr/include/python3.3m/pyconfig-64.h:#define _LARGEFILE_SOURCE 1
>>>> /usr/include/features.h:   _LARGEFILE_SOURCE	Some more functions for
>>>
>>> correct
>>>
>>>> standard I/O. /usr/include/features.h:#  undef _LARGEFILE_SOURCE
>>>> /usr/include/features.h:#  define _LARGEFILE_SOURCE	1
>>>> /usr/include/features.h:#ifdef _LARGEFILE_SOURCE
>>>> /usr/include/python2.7/pyconfig-64.h:#define _LARGEFILE_SOURCE 1
>>>>
>>>> but there's no mention of "_LARGEFILE_SOURCE" in "man
>>>> feature_test_macros". is that assumed to be implied by
>>>> _LARGEFILE64_SOURCE? it's not clear.
>>>
>>> nope, none of the three LFS flags imply each other, nor should they. 
>>> they're related, but orthogonal by design.
>>>
>>> i could have sworn i saw a description of them in POSIX, but i can't find
>>> it now.  the GNU C library manual covers it though:
>>> https://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.htm
>>> l
>>>
>>> here's a terse summary off the top of my head:
>>>
>>> _LARGEFILE_SOURCE: expose prototypes for new funcs which POSIX got wrong
>>> originally.  e.g. fseek & ftell utilize "long", so fseeko & ftello were
>>> added which use "off_t".
>>
>> Thanks, Mike. I'd forgotten those details.
>>
>> So, as far as I can see, _LARGEFILE_SOURCE is largely of (ancient)
>> historical interest, so I don't think I'll bother adding it to the FTM(7)
>> page.
> 
> would be nice to document it ... i tend to get more confused when something is 
> left out entirely even if it was done on purpose.  especially since it'd give 
> us the opportunity to discourage people from using it and drive people towards 
> the One True Way when it comes to LFS flags.

Fair enough. I added the following:

       _LARGEFILE_SOURCE
              This macro was historically used to expose certain func‐
              tions   (specifically   fseeko(3)  and  ftello(3))  that
              address  limitations  of  earlier  APIs   (feek(3)   and
              ftell(3))  that  use  long  int  for file offsets.  This
              macro is implicitly defined if _XOPEN_SOURCE is  defined
              with a value greater than or equal to 500.  New programs
              should not employ this macro; defining _XOPEN_SOURCE  as
              just  described  or  defining _FILE_OFFSET_BITS with the
              value 64 is the preferred mechanism to achieve the  same
              result.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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