Hello Stefan,
On 10/07/2016 05:22 PM, Stefan Tauner wrote:
> Hi,
>
> the current man page indicates the following feature flags for ffs(3):
>
> Since glibc 2.12:
> _POSIX_C_SOURCE\ >=\ 200809L
> || /* Glibc since 2.19: */ _DEFAULT_SOURCE
> || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
>
> So I would expect the following program to compile well without
> warnings:
>
> #define _POSIX_C_SOURCE 200809L
> #include <strings.h>
> int main(void) {
> return ffs(0x4);
> }
>
> However, it does not and warns about the implicit reference of ffs.
> It compiles fine _without_ the POSIX define though (as expected due to
> _DEFAULT_SOURCE).
Confirmed, and thanks for the report!
> According to the standard, ffs(3) is indeed an XSI function:
> http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/strings.h.html
>
> Looking at my local system header from glibc 2.23 seems to confirm
> that glibc handles it that way too:
>
> features.h has:
> #if defined _POSIX_C_SOURCE && (_POSIX_C_SOURCE - 0) >= 200809L
> # define __USE_XOPEN2K8 1
> # undef _ATFILE_SOURCE
> # define _ATFILE_SOURCE 1
> #endif
>
> which is used by strings.h:
> #if defined __USE_MISC || !defined __USE_XOPEN2K8 || defined __USE_XOPEN2K8XSI
> /* Return the position of the first bit set in I, or 0 if none are set.
> The least-significant bit is position 1, the most-significant 32. */
> extern int ffs (int __i) __THROW __attribute__ ((const));
> #endif
>
> Note the ! in the _USE_XOPEN2K8 clause.
Yes, I suspect that I overlooked that '!' when I documented this!
> I am not 100% sure, but I guess that the right fix would be to replace
> the _POSIX_C_SOURCE clause in ffs.3 with _XOPEN_SOURCE >= 700?
I've changed the page to show the corrected details, which I make to be:
ffs():
Since glibc 2.12:
_XOPEN_SOURCE >= 700
|| ! (_POSIX_C_SOURCE >= 200809L)
|| /* Glibc since 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE ||
_SVID_SOURCE
Before glibc 2.12:
none
That of course covers what you found, w.r.t. _XOPEN_SOURCE.
> FWIW, this also makes the test program above work fine on my system.
> I have not checked in detail if the current manpage was correct with
> glibc 2.12 or anything before my 2.23 but I don't think so from looking
> at some relevant changes of strings.h in the glibc repo.
No, the man page has always been wrong :-(. Fixed now, thanks to
your report!
> HTH, thanks for your work Michael and KR,
You're very welcome. Did we meet at KR? (sorry, two conferences
in two weeks; lots of new names...)
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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