Re: [Qemu-devel] Fix build break during configuration on musl-libc based Linux systems.

[Paolo Bonzini <pbonzini@redhat.com>]

On 16/02/2017 18:23, Laszlo Ersek wrote:
> On 02/16/17 17:58, Paolo Bonzini wrote:
>>
>>
>> On 16/02/2017 17:30, Chad Joan wrote:
>>> Hello,
>>>
>>> This is a one-line patch to the configure script that will allow QEMU to be
>>> built on musl-libc based Linux systems.  This problem is only noticeable
>>> when QEMU is built with --enable-curses.
>>>
>>> Detailed reading material if you want to know where this came from:
>>> https://bugs.gentoo.org/show_bug.cgi?id=609364
>>
>> Hi,
>>
>> can you explain exactly which function is missing without
>> -D_XOPEN_SOURCE=500?  If it is curses' wide-char functions, why does it
>> fail with musl but not with glibc?  Is _XOPEN_SOURCE always defined by
>> glibc if you have _D_GNU_SOURCE?
> 
> It is not necessarily auto-defined, but the effect is "as if".

Ok, so the bug is that glibc _does_ auto-define _XOPEN_SOURCE:

#ifdef _GNU_SOURCE
# undef  _ISOC95_SOURCE
# define _ISOC95_SOURCE 1
# undef  _ISOC99_SOURCE
# define _ISOC99_SOURCE 1
# undef  _ISOC11_SOURCE
# define _ISOC11_SOURCE 1
# undef  _POSIX_SOURCE
# define _POSIX_SOURCE  1
# undef  _POSIX_C_SOURCE
# define _POSIX_C_SOURCE        200809L
# undef  _XOPEN_SOURCE
# define _XOPEN_SOURCE  700
# undef  _XOPEN_SOURCE_EXTENDED
# define _XOPEN_SOURCE_EXTENDED 1
# undef  _LARGEFILE64_SOURCE
# define _LARGEFILE64_SOURCE    1
# undef  _DEFAULT_SOURCE
# define _DEFAULT_SOURCE        1
# undef  _ATFILE_SOURCE
# define _ATFILE_SOURCE 1
#endif

and ncursesw looks for _XOPEN_SOURCE exclusively:

#ifndef NCURSES_WIDECHAR
#if defined(_XOPEN_SOURCE_EXTENDED) || (defined(_XOPEN_SOURCE) && (_XOPEN_SOURCE - 0 >= 500))
#define NCURSES_WIDECHAR 1
#else
#define NCURSES_WIDECHAR 0
#endif
#endif /* NCURSES_WIDECHAR */


So I do think that if musl wants to support _GNU_SOURCE, it should also auto-define;
however it doesn't, see https://git.musl-libc.org/cgit/musl/tree/include/features.h.

Alternatively, ncurses should be patched to look at _GNU_SOURCE in addition to
_XOPEN_SOURCE and _XOPEN_SOURCE_EXTENDED.

In any case, it's not a QEMU bug.  Other packages likely are going to have the same
issue, and fixing all of them is the wrong course of action.

Paolo