[Gc] trouble with gc-7.2d compiling against musl libc

[John Spencer]

i tried to compile the gc on sabotage linux[1] using musl libc[2]

there are multiple issues:

In file included from os_dep.c:44:0:
/usr/include/asm/sigcontext.h:134:8: error: redefinition of 'struct 
_fpstate'
/usr/include/bits/signal.h:6:16: note: originally defined here
/usr/include/asm/sigcontext.h:157:8: error: redefinition of 'struct 
sigcontext'
/usr/include/bits/signal.h:18:8: note: originally defined here

userspace and kernel headers are mixed up. this patch here fixes it:

--- gc-7.2.org/os_dep.c 2013-01-03 17:13:21.878000003 +0000
+++ gc-7.2/os_dep.c     2013-01-04 07:57:17.766000002 +0000
@@ -26,6 +26,7 @@
  #   define __KERNEL__
  #   include <asm/signal.h>
  #   undef __KERNEL__
+# elif 1
  # else
      /* Kernels prior to 2.1.1 defined struct sigcontext_struct instead 
of */
      /* struct sigcontext.  libc6 (glibc2) uses "struct sigcontext" in 
     */


also to the CFLAG -DNO_GETCONTEXT must be passed by hand to get it to 
compile.

but the big issue is that os_dep.c unconditionally uses 
pthread_getattr_np. as the _np suffix suggests, this function is 
non-posix or in other words non-portable, and is missing in musl.
the build succeeds, but later on one gets link errors as soon as the dso 
is used.

i tried to work around it by using this macro:
#define pthread_getattr_np(A, B) (-1)

that makes a valid dso, and a warning is printed that pthread_getattr_np 
failed.

however when GC_init is called a segfault happens
in GC_allochblk_nth

preprocessed code: somewhere in the dark depths of this expanded macro 
GET_HDR the segfault happens.

         for(; 0 != hbp; hbp = hhdr -> hb_next) {
/* GET_HDR(hbp, hhdr);*/
             { register hdr ** _ha;
                 { register bottom_index * bi;
                   { register word hi =
                       (word)(hbp) >> (10 + ((size_t)12));
                          register bottom_index * _bi = 
GC_arrays._top_index[((hi) & ((1 << 11) - 1))];
                          while (_bi -> key != hi && _bi != 
GC_arrays._all_nils)
                                 _bi = _bi -> hash_link;
                                 (bi) = _bi;
                   };
                   (_ha) = &(((bi)->index[((word)(hbp) >> ((size_t)12)) 
& ((1 << 10) - 1)]));
                  }; (hhdr) = *_ha; };


i suspect the fallback code when pthread_getattr_np fails is just buggy.
afaics this code is only there to find the stack address of the main 
thread. in that case i wonder why not simply /proc/pid/maps is used 
instead of using non-portable constructs ?

in any case, i'd welcome advice on how to workaround this issue, so that 
i get a working libgc.so in order to use inkscape.

best regards,
--JS

[1] https://github.com/rofl0r/sabotage
[2] http://www.musl-libc.org