3273  static C_char *utf_index(C_word s, C_word i) 
  3274  {
  3275      C_word bv = C_block_item(s, 0);
  3276      if(i == 0) {
  3277          C_set_block_item(s, 2, C_fix(0));
  3278          C_set_block_item(s, 3, C_fix(0));                
  3279          return C_c_string(bv);
  3280      } else if(C_header_size(bv) - 1 == C_unfix(C_block_item(s, 1))) {
  3281          /* len == codepoints */
  3282          C_set_block_item(s, 2, C_fix(i));
  3283          C_set_block_item(s, 3, C_fix(i));        
  3284          return C_c_string(bv) + i;
  3285      }
  3286      return utf_index1(s, i);
  3287  }

(gdb) bt 30
#0  0x00007ffff711b000 in __GI___poll (fds=0x55555562af90, nfds=1, timeout=-1) at ../sysdeps/unix/sysv/linux/poll.c:29
#1  0x00007ffff7723e43 in C_ready_fds_timeout (to=0, tm=0) at scheduler.c:85
#2  0x00007ffff7723f59 in stub524 (C_buf=30, C_a0=6, C_a1=1) at scheduler.c:173
#3  0x00007ffff7735909 in f_2029 (c=2, av=0x7ffffff45610) at scheduler.c:2665
#4  0x00007ffff77357e5 in f_2020 (c=2, av=0x7ffffff45610) at scheduler.c:2647
#5  0x00007ffff7730487 in f_1741 (t0=140737487593184, t1=140737487593552, t2=14) at scheduler.c:1955
#6  0x00007ffff7732549 in f_1837 (c=2, av=0x7ffffff457a0) at scheduler.c:2230
#7  0x00007ffff77327bc in f_1849 (t0=140737487592560, t1=140737487592480, t2=14) at scheduler.c:2252
#8  0x00007ffff7732952 in f_1859 (c=2, av=0x7ffffff459c0) at scheduler.c:2264
#9  0x00007ffff77311b1 in f_1752 (t0=140737487592432, t1=140737487592192, t2=140737323511216) at scheduler.c:2000
#10 0x00007ffff7732780 in f_1849 (t0=140737487592560, t1=140737487592480, t2=140737487595888) at scheduler.c:2245
#11 0x00007ffff7730d18 in f_1741 (t0=140737487593184, t1=140737487593552, t2=140737487595936) at scheduler.c:1970
#12 0x00007ffff7732baa in f_1874 (c=2, av=0x7ffffff46010) at scheduler.c:2285
#13 0x00007ffff73da252 in f_9241 (c=3, av=0x7ffffff46010) at library.c:66080
#14 0x00007ffff7724e4e in f_1008 (c=2, av=0x7ffffff46200) at scheduler.c:742
#15 0x00007ffff7724970 in f_1004 (t0=140737487594096, t1=140737487595264) at scheduler.c:686
#16 0x00007ffff7742a28 in f_999 (c=2, av=0x7ffffff46620) at scheduler.c:4255
#17 0x00007ffff77427d9 in f_968 (c=2, av=0x7ffffff46620) at scheduler.c:4236
#18 0x00007ffff738d250 in f_29662 (c=3, av=0x7ffffff46620) at library.c:53421
#19 0x00007ffff793a8b3 in C_call_cc (c=3, av=0x7ffffff46670) at runtime.c:7671
#20 0x00007ffff738cf82 in f_29656 (c=2, av=0x7ffffff46bd0) at library.c:53396
#21 0x00007ffff75478f1 in f_2578 (c=2, av=0x7ffffff46bd0) at tcp.c:5030
#22 0x00007ffff7734606 in f_1957 (c=2, av=0x7ffffff46bd0) at scheduler.c:2558
#23 0x00007ffff7734938 in f_1972 (t0=140737487596800, t1=140737487596736, t2=14) at scheduler.c:2584
#24 0x00007ffff7733fa9 in f_1953 (c=5, av=0x7ffffff46df0) at scheduler.c:2536
#25 0x00007ffff7547711 in f_2575 (c=2, av=0x7ffffff47020) at tcp.c:5011
#26 0x00007ffff75474ae in f_2571 (c=2, av=0x7ffffff47020) at tcp.c:4984
#27 0x00007ffff75471f7 in f_2566 (t0=140737487597728, t1=6) at tcp.c:4950
#28 0x00007ffff7546f79 in f_2563 (c=2, av=0x7ffffff47550) at tcp.c:4930
#29 0x00007ffff731b678 in f_21330 (c=2, av=0x7ffffff47550) at library.c:33920
(More stack frames follow...)