How can I get the lua stack trace from a core file using gdb

Question

I have a C++ application (for OS X) that calls lua as a scripting language. I\'m running a large number of these applications (100s) and they can run for a very long time (days

Answer 1

You could check out my Lua GDB helpers. It is a collection of macros that let you inspect the stack and values, and even print backtrace. Essentially what the article referenced by macs contains, in a simple-to-use package.

It provides these macros:

• luastack [L] - lists the values on the current Lua C stack.

• luaprint < value > [verbose] - Pretty-prints a TValue passed as argument. Expects a pointer to a TValue. When verbose is 1, expands tables, metatables and userdata environments.

• luaprinttable < table > - Pretty-prints a Lua Table. Expects a pointer to Table.

• luavalue < index > [L] - Dumps a single value at an index.

• luatraceback [L] - Calls debug.traceback(). Not sure if it will work on core file though...

Answer 2

Based on the comments above, I'd recommend the following article: Lua callstack with C++ debugger. It's giving a good overview about debugging the Lua / C++ combination, especially the section "Inspect Lua data structures" is helpful, when it comes to debugging of core dumps.

Answer 3

This is a small variation to Michael Anderson's GDB script: I had to use this because I was getting Cannot access memory at address 0x656d errors with his script, due to L->base_ci being invalid in my core dump. This starts from the top frame (L->ci) and goes down, in the opposite direction, avoiding the invalid L->base_ci pointer.

set $p = L->ci
while ($p > L->base_ci )
  if ( $p->func->value.gc->cl.c.isC == 1 )
    printf "0x%x   C FUNCTION ", $p
    output $p->func->value.gc->cl.c.f
    printf "\n"
  else
    if ($p->func.tt==6)
      set $proto = $p->func->value.gc->cl.l.p
      set $filename = (char*)(&($proto->source->tsv) + 1)
      set $lineno = $proto->lineinfo[ $p->savedpc - $proto->code -1 ]
      printf "0x%x LUA FUNCTION : %d %s\n", $p, $lineno, $filename
    else
      printf "0x%x LUA BASE\n", $p
    end
  end
  set $p = $p - 1
end

Answer 4

I've created a GDB script to do the stuff in the web page linked to by macs. Its not beautiful, and should probably be properly wrapped into a function etc, but here it is for the curious.

NOTE: It seems that the web page is wrong about the filename for lua functions. In the case where the string comes from luaL_dofile() the filename starts with a @ symbol. If they're called from lua_dostring(). In that case the $filename variable is set to the whole of the string passed to lua_dostring() - and the user is probably only interested in one or two lines of context from that file. I wasn't sure how to fix that up.

set $p = L->base_ci
while ($p <= L->ci )
  if ( $p->func->value.gc->cl.c.isC == 1 )
    printf "0x%x   C FUNCTION", $p
    output $p->func->value.gc->cl.c.f
    printf "\n"
  else
    if ($p->func.tt==6)
      set $proto = $p->func->value.gc->cl.l.p
      set $filename = (char*)(&($proto->source->tsv) + 1)
      set $lineno = $proto->lineinfo[ $p->savedpc - $proto->code -1 ]
      printf "0x%x LUA FUNCTION : %d %s\n", $p, $lineno, $filename
    else
      printf "0x%x LUA BASE\n", $p
    end
  end
  set $p = $p+1
end

This outputs something like:

0x1002b0 LUA BASE
0x1002c8 LUA FUNCTION : 4 @a.lua
0x1002e0 LUA FUNCTION : 3 @b.lua
0x100310   C FUNCTION(lua_CFunction) 0x1fda <crash_function(lua_State*)>

When I debug the crash from this code:

// This is a file designed to crash horribly when run.
// It should generate a core, and it should crash inside some lua functions

#include "lua.h"
#include "lualib.h"
#include "lauxlib.h"

#include <iostream>
#include <signal.h>

int crash_function(lua_State * L)
{
  raise( SIGABRT ); //This should dump core!
  return 0;
}



int main()
{
  lua_State * L = luaL_newstate();
  lua_pushcfunction(L, crash_function);
  lua_setfield(L, LUA_GLOBALSINDEX, "C");

  luaopen_base(L);
  if( 1 == luaL_dofile(L, "a.lua" ))
  {
    std::cout<<"ERROR: "<<lua_tostring(L,-1)<<std::endl;
    return 1;
  }
  if( 1 == luaL_dofile(L, "b.lua" ))
  {
    std::cout<<"ERROR: "<<lua_tostring(L,-1)<<std::endl;
    return 1;
  }

  lua_getfield(L, LUA_GLOBALSINDEX, "A");
  lua_pcall(L, 0, 0, NULL);
}

With a.lua

-- a.lua
-- just calls B, which calls C which should crash
function A()
  B()
end

and b.lua

-- b.lua
function B()
  C()
end