Re: why is there no __MUSL__ macro?

[Rich Felker <dalias@libc.org>]

On Fri, Sep 12, 2014 at 09:35:22AM +0200, Jörg Krause wrote:
> 
> On 09/11/2014 04:47 PM, Natanael Copa wrote:
> >On Thu, 11 Sep 2014 13:00:09 +0200
> >Jörg Krause <jkrause@posteo.de> wrote:
> >
> >>Hi,
> >>
> >>I am trying to add support for the musl toolchain to FFmpeg.
> >>
> >>FFmpeg needs support for library features defined in POSIX.1-2001 with
> >>XSI extension and the standards below. Currently configure probes the
> >>host and target libc by checking for defined macros like __GLIBC__ and
> >>__UCLIBC__. In case of glibc and uclibc it sets -D_XOPEN_SOURCE=600
> >>properly.
> >>
> >>After this it checks for some combinations of hardware and the probed
> >>libc to set some more compile options, if necessary.
> >>
> >>I know that musl does not have a macro __MUSL__ and I have read the
> >>explanation. However, I don't understand what's meant by "[..] it's a
> >>bug to assume a certain implementation has particular properties rather
> >>than testing." and how does it affect the way FFmpeg probes for the libc.
> >>
> >>What could be a solution which supports musl?
> >>
> >>Many thanks!
> >>Jörg
> >This is what we do on alpine linux:
> >http://git.alpinelinux.org/cgit/aports/tree/main/ffmpeg/fix-defines.patch
> >
> >--- ffmpeg-1.2.2.orig/libavutil/error.c
> >+++ ffmpeg-1.2.2/libavutil/error.c
> >@@ -17,6 +17,7 @@
> >   */
> >  #undef _GNU_SOURCE
> >+#define _XOPEN_SOURCE 600
> >  #include "avutil.h"
> >  #include "avstring.h"
> >  #include "common.h"
> >
> 
> Hi Natanal,
> 
> I had a look to alpine already. I submitted this patch to FFmpeg,
> but building FFmpeg with my configuration libavutils/error.c is not
> the only file which needs a feature test macro. The people of FFmpeg
> did not like the idea to have a lot of test macros in there source
> so I stopped with this solution and looked for a way to adopt the
> musl toolchain to their configure file.

It might work to add -D_DEFAULT_SOURCE (note: not available until
recent musl git) or -D_BSD_SOURCE to the global CFLAGS.

What's happening, I think, is that because -std=c99 or similar is in
the CFLAGS, default feature profile is getting suppressed, and
_GNU_SOURCE is the only thing bringing back the usual features (in
addition to lots of _GNU_SOURCE-only stuff, and strerror_r breakage).
Having at least one more feature test macro defined globally would
prevent this.

Note also that modern glibc supports _DEFAULT_SOURCE, with the same
semantics as musl: keeping the default stuff exposed even when a
-std=* option would otherwise suppress it.

Rich