Issue #14816 has been updated by akamch (Anatoly Kamchatnov).


> I guess this is not our fault?

Not entirely. Most likely it's nobody's fault but you can always blame autoconf :) Alpine's author thinks that "the configure script does not detect isnan/isinf as macros, call ruby devs". Autoconf's doc says
~~~
isinf
isnan
    The C99 standard says that isinf and isnan are macros. On some systems just macros are available (e.g., HP-UX and Solaris 10), on some systems both macros and functions (e.g., glibc 2.3.2), and on some systems only functions (e.g., IRIX 6 and Solaris 9). In some cases these functions are declared in nonstandard headers like <sunmath.h> and defined in non-default libraries like -lm or -lsunmath.

    The C99 isinf and isnan macros work correctly with long double arguments, but pre-C99 systems that use functions typically assume double arguments. On such a system, isinf incorrectly returns true for a finite long double argument that is outside the range of double.

    The best workaround for these issues is to use gnulib modules isinf and isnan (see Gnulib). But a lighter weight solution involves code like the following.

              #include <math.h>
              
              #ifndef isnan
              # define isnan(x) \
                  (sizeof (x) == sizeof (long double) ? isnan_ld (x) \
                   : sizeof (x) == sizeof (double) ? isnan_d (x) \
                   : isnan_f (x))
              static inline int isnan_f  (float       x) { return x != x; }
              static inline int isnan_d  (double      x) { return x != x; }
              static inline int isnan_ld (long double x) { return x != x; }
              #endif
              
              #ifndef isinf
              # define isinf(x) \
                  (sizeof (x) == sizeof (long double) ? isinf_ld (x) \
                   : sizeof (x) == sizeof (double) ? isinf_d (x) \
                   : isinf_f (x))
              static inline int isinf_f  (float       x)
              { return !isnan (x) && isnan (x - x); }
              static inline int isinf_d  (double      x)
              { return !isnan (x) && isnan (x - x); }
              static inline int isinf_ld (long double x)
              { return !isnan (x) && isnan (x - x); }
              #endif

~~~
https://www.gnu.org/software/autoconf/manual/autoconf.html#Function-Portability

Looks like musl is not unlike HP-UX and Solaris 10 in that regard. The question is where one should fix this: some ruby routines around extconf.rb, gem in question, anywhere else? That I don't know, but the first option still seems plausible to me.

----------------------------------------
Bug #14816: Extension build failure on a system with musl libc
https://bugs.ruby-lang.org/issues/14816#change-72375

* Author: akamch (Anatoly Kamchatnov)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.0preview2 (2018-05-31 trunk 63539) [x86_64-linux]
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN
----------------------------------------
Some extensions fail to build on a Linux with musl (Void Linux). Build of unf_ext is an example. 
isinf() and isnan() are defined as macros in musl: https://git.musl-libc.org/cgit/musl/tree/include/math.h
https://github.com/gliderlabs/docker-alpine/issues/261 "Cannot build native extensions for unf_ext gem" seems to be a related issue.

~~~
$ gem install unf_ext                                                                              
Building native extensions. This could take a while...
ERROR:  Error installing unf_ext:
        ERROR: Failed to build gem native extension.

    current directory: /home/rev/.gem/ruby/2.6.0/gems/unf_ext-0.0.7.5/ext/unf_ext
/home/rev/.rbenv/versions/2.6.0-preview2/bin/ruby -r ./siteconf20180603-29655-wgzu56.rb extconf.rb
checking for -lstdc++... yes
creating Makefile

current directory: /home/rev/.gem/ruby/2.6.0/gems/unf_ext-0.0.7.5/ext/unf_ext
make "DESTDIR=" clean

current directory: /home/rev/.gem/ruby/2.6.0/gems/unf_ext-0.0.7.5/ext/unf_ext
make "DESTDIR="
compiling unf.cc
cc1plus: warning: command line option '-Wimplicit-int' is valid for C/ObjC but not for C++
cc1plus: warning: command line option '-Wdeclaration-after-statement' is valid for C/ObjC but not for C++
cc1plus: warning: command line option '-Wimplicit-function-declaration' is valid for C/ObjC but not for C++
In file included from /home/rev/.rbenv/versions/2.6.0-preview2/include/ruby-2.6.0/ruby/defines.h:153:0,
                 from /home/rev/.rbenv/versions/2.6.0-preview2/include/ruby-2.6.0/ruby/ruby.h:29,
                 from /home/rev/.rbenv/versions/2.6.0-preview2/include/ruby-2.6.0/ruby.h:33,
                 from unf.cc:3:
/home/rev/.rbenv/versions/2.6.0-preview2/include/ruby-2.6.0/ruby/missing.h:172:29: error: 'int isinf(double)' conflicts with a previous declaration
 RUBY_EXTERN int isinf(double);
                             ^
In file included from /usr/include/c++/7.3/math.h:36:0,
                 from /home/rev/.rbenv/versions/2.6.0-preview2/include/ruby-2.6.0/ruby/missing.h:23,
                 from /home/rev/.rbenv/versions/2.6.0-preview2/include/ruby-2.6.0/ruby/defines.h:153,
                 from /home/rev/.rbenv/versions/2.6.0-preview2/include/ruby-2.6.0/ruby/ruby.h:29,
                 from /home/rev/.rbenv/versions/2.6.0-preview2/include/ruby-2.6.0/ruby.h:33,
                 from unf.cc:3:
/usr/include/c++/7.3/cmath:599:3: note: previous declaration 'constexpr bool std::isinf(double)'
   isinf(double __x)
   ^~~~~
In file included from /home/rev/.rbenv/versions/2.6.0-preview2/include/ruby-2.6.0/ruby/defines.h:153:0,
                 from /home/rev/.rbenv/versions/2.6.0-preview2/include/ruby-2.6.0/ruby/ruby.h:29,
                 from /home/rev/.rbenv/versions/2.6.0-preview2/include/ruby-2.6.0/ruby.h:33,
                 from unf.cc:3:
/home/rev/.rbenv/versions/2.6.0-preview2/include/ruby-2.6.0/ruby/missing.h:179:29: error: 'int isnan(double)' conflicts with a previous declaration
 RUBY_EXTERN int isnan(double);
                             ^
In file included from /usr/include/c++/7.3/math.h:36:0,
                 from /home/rev/.rbenv/versions/2.6.0-preview2/include/ruby-2.6.0/ruby/missing.h:23,
                 from /home/rev/.rbenv/versions/2.6.0-preview2/include/ruby-2.6.0/ruby/defines.h:153,
                 from /home/rev/.rbenv/versions/2.6.0-preview2/include/ruby-2.6.0/ruby/ruby.h:29,
                 from /home/rev/.rbenv/versions/2.6.0-preview2/include/ruby-2.6.0/ruby.h:33,
                 from unf.cc:3:
/usr/include/c++/7.3/cmath:626:3: note: previous declaration 'constexpr bool std::isnan(double)'
   isnan(double __x)
   ^~~~~
cc1plus: warning: unrecognized command line option '-Wno-cast-function-type'
cc1plus: warning: unrecognized command line option '-Wno-self-assign'
cc1plus: warning: unrecognized command line option '-Wno-constant-logical-operand'
cc1plus: warning: unrecognized command line option '-Wno-parentheses-equality'
make: *** [Makefile:211: unf.o] Error 1

make failed, exit code 2

~~~



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