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Re: Problems with default _POSIX_C_SOURCE in features.h ??
On 11/04/2019 22:40, Adhemerval Zanella wrote:
On 10/04/2019 08:21, Adhemerval Zanella wrote:
On 09/04/2019 16:00, Chris Hall wrote:
On 09/04/2019 18:42, Adhemerval Zanella wrote:
...
The main problem in my view is there is no guarantee from glibc for which
_POSIX_C_SOURCE _DEFAULT_SOURCE will define in the future for possible new
_POSIX_C_SOURCE values. I think it is worth a discussion and I will raise
this question on libc-alpha.
My guess is that it will select the latest and greatest.
Will anyone commit to it setting _POSIX_C_SOURCE=200809L forever ??
Which leads to the problem that to enable XSI with _DEFAULT_SOURCE, you
(currently) have to know which -D_XOPEN_SOURCE=xxx to use, and that
depends on which version of the library is being compiled for.
As I said, there was not need to change it from original default of POSIX1.2008,
and I asked on libc-alpha which would be the idea for this flag [1].
[1] https://sourceware.org/ml/libc-alpha/2019-04/msg00239.html
It seems the current consensus is indeed that _DEFAULT_SOURCE would map to
latest available POSIX1 standard [1].
[1] https://sourceware.org/ml/libc-alpha/2019-04/msg00244.html
OK. If I want -D_DEFAULT_SOURCE plus the corresponding latest available
XSI (on whatever version of library my users are compiling against),
then I observe:
1) -D_DEFAULT_SOURCE -D_XOPEN_SOURCE=-1UL will do the trick
-- ugly, though.
2) -D_DEFAULT_SOURCE -D_XOPEN_SOURCE=700 does the job now...
...but will be broken in the future.
3) a new _DEFAULT_XOPEN_SOURCE might be cleaner ?
It is possible that my worry is a symptom of ignorance.
I have a large network application which is required to compile
for more or less any UN*X. I develop and test on Linux. I
would like the minimum of surprise when compiling and testing on
other systems.
Now, FreeBSD has caused me some trouble in the past. ...
...
So, I figured that on Linux, with glibc, I could:
-D_XOPEN_SOURCE=600 -D_BSD_SOURCE
and that might approximate to FreeBSD, and other UN*X which more
or less conform to POSIX.1-2001 or are certified UNIX 03.
But late model glibc won't let me do that
...
Perhaps I am, in any case, wasting my time ? Perhaps the reality
is that I cannot avoid being caught out occasionally by differences
between systems... and I should just be grateful it's only
occasionally.
We can't really guarantee compatibility for _BSD_SOURCE in particular
(specially because we don't aim to be fully BSD compatible). But we
usually treat _POSIC_C_SOURCE an _XOPEN_SOURCE deviations from
standards as bugs (if you find some please open a bug report).
That much is clear. As I say, my hope was to minimise surprises when
testing on BSD-like systems.
...>> Of course, glibc encourages the use of _GNU_SOURCE, which selects
the latest and greatest _XOPEN_SOURCE. But, it also enables all
the GNU libc specials, which I really don't want. It also seems
to select C11 things, even if the compiler is not
__STDC_VERSION__ 201112L, which I cannot say I like the look
of, either.
_GNU_SOURCE definitely does not enable C11 symbols. It relies solely on
__STDC_VERSION__ value:
include/features.h:
190 #ifdef _GNU_SOURCE
191 #undef _ISOC95_SOURCE
192 #define _ISOC95_SOURCE 1
193 #undef _ISOC99_SOURCE
194 #define _ISOC99_SOURCE 1
195 #undef _ISOC11_SOURCE
196 #define _ISOC11_SOURCE 1
Forcing C99 for POSIX-2008 makes some sense. But C11 should be skipped
if __STRICT_ANSI__ ?
...> And I am not aware of any bug related to this. Are you sure we
you are seeing it or are you using a compiler version that sets
__STDC_VERSION__ as default?
The effect of __USE_ISOC11 is small (8 appearances outside <features.h>).
I'd like a way to select the latest and greatest _XOPEN_SOURCE
and/or _POSIX_C_SOURCE, while not selecting any GNU extensions.
Perhaps, say, _XOPEN_SOURCE_LATEST and/or _POSIX_SOURCE_LATEST ?
or _XOPEN_LATEST_SOURCE and/or _POSIX_LATEST_SOURCE ?
[I guess I could require users to install glibc, and surrender
to the _GNU_SOURCE... but that doesn't feel right.]
Suggestions, anyone ?
AFAIK it should be exactly as POSIX recommends:
-D_POSIX_C_SOURCE=value and/or -D_XOPEN_SOURCE=value.
I should have said that I'd like a way to *automatically* select the
latest and greatest _XOPEN_SOURCE/POSIX_C_SOURCE -- like _GNU_SOURCE does.
For a given version of glibc, past and future, what should my configure
script do to determine the value to use ? [_GNU_SOURCE appears to
select the latest and greatest, but that's not explicitly stated in the
documentation.]
Or should I use -1UL ?
If I simply set -D_XOPEN_SOURCE=700, then I will need to update the
application when the next version becomes available.
Perhaps nobody really needs an automatic way to select the latest
available ? [Except, as noted above, to enable XSI stuff with
-D_DEFAULT_SOURCE.]
Thanks,
Chris