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I am currently trying to write a tiny Lua extension to handle matrix multiplication. The matrix part of the job was fairly easy and I managed to expose it to Lua without too much effort. But now I face a strange issue, my Lua script randomly segfault with different kind of error.

Here an overview

(lldb) run
Process 44058 launched: '/usr/local/bin/luad' (x86_64)
Process 44058 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x1f8)
    frame #0: 0x000000010001cd19 luad`luaH_getn(t=0x00000001002024f0) at ltable.c:643
   640  */
   641  int luaH_getn (Table *t) {
   642    unsigned int j = t->sizearray;
-> 643    if (j > 0 && ttisnil(&t->array[j - 1])) {
   644      /* there is a boundary in the array part: (binary) search for it */
   645      unsigned int i = 0;
   646      while (j - i > 1) {
Target 0: (luad) stopped.
(lldb) run
Process 44062 launched: '/usr/local/bin/luad' (x86_64)
Process 44062 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x100503bbe)
    frame #0: 0x00007fff513e8e87 libsystem_malloc.dylib`tiny_free_list_add_ptr + 108
libsystem_malloc.dylib`tiny_free_list_add_ptr:
->  0x7fff513e8e87 <+108>: movw   %r11w, -0x2(%rdx,%rax)
    0x7fff513e8e8d <+114>: movw   %r11w, 0x10(%rdx)
    0x7fff513e8e92 <+119>: jmp    0x7fff513e8ea0            ; <+133>
    0x7fff513e8e94 <+121>: testw  %r11w, %r11w
Target 0: (luad) stopped.(lldb) run
Process 45023 launched: '/usr/local/bin/luad' (x86_64)
Process 45023 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
    frame #0: 0x000000010000e79e luad`sweeplist(L=0x0000000101000008, p=0x00000001003059a0, count=51) at lgc.c:740
   737    int white = luaC_white(g);  /* current white */
   738    while (*p != NULL && count-- > 0) {
   739      GCObject *curr = *p;
-> 740      int marked = curr->marked;
   741      if (isdeadm(ow, marked)) {  /* is 'curr' dead? */
   742        *p = curr->next;  /* remove 'curr' from list */
   743        freeobj(L, curr);  /* erase 'curr' */
Target 0: (luad) stopped.

It seem to be linked to memory management and garbage collecting but I have gone by the book. So how can I find my mistake ? It seems that I cannot build a minimal code because ofrandomness. Beside reading all Lua source code I don't see what I can do.

If you are curious you can have a look at my library on github, it have a small amount of code to read.

Any kind of help is welcome.

  • Unrelated. But your `matrix_is_valid` is checking the same condition twice. – Eugene Sh. May 16 '18 at 16:59
  • Thanks for finding that typo –  May 17 '18 at 08:27
  • Hi, I just got the same error with you. Has you solved this segfault? – Yyao Jun 19 '18 at 15:36
  • You can go look at my GitHub to see I still have no clue. I changed the way I use my matrix. Before add was returning a new matrix, now it ask for an object to write the result. And then there no more matrix creation beside on the beginning of the script. I hate this solution because my Lua code is now horrible and not readable. but if you find a way I will be glade to have some direction. –  Jun 20 '18 at 07:12

0 Answers0