Tuesday, June 11, 2013

How are sbrk_base and main_arena related in gdb?

Inside the malloc.c code, there is the use of sbrk_base. How is it related to the main heap (declared as main_arena)?

From the glib v2.15+ code:

/* A contiguous main_arena is consistent with sbrk_base.  */
  if (av == &main_arena && contiguous(av))
    assert((char*)mp_.sbrk_base + av->system_mem ==
       (char*)av->top + chunksize(av->top));

You can gdb to a process and try to verify. You'll need to have the libc6-dev and libc6-dbg packages to get the correct debugging symbols for libc:

(gdb) print (char *)main_arena->top + (main_arena->top->size & ~(0x4 | 0x2 | 0x1))
$1 = 0xc17000
(gdb) print (char *)mp_.sbrk_base + main_arena->system_mem
$2 = 0xc17000

Therefore, the sbrk_base does not really applied for other heap arenas (see http://siddhesh.in/journal/2012/10/24/malloc-per-thread-arenas-in-glibc/ for more context)

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