Examining C/C++ Heap memory statistics in gdb

Question

I'm trying to investigate the state of the C/C++ heap from within gdb on Linux amd64, is there a nice way to do this?

One approach I've tried is to "call mallinfo()" but unfortunately I can't then extract the values I want since gdb doesn't deal with the return value properly.

I'm not easily able to write a function to be compiled into the binary for the process I am attached to, so I can simply implement my own function to extract the values by calling mallinfo() in my own code this way. Is there perhaps a clever trick that will allow me to do this on-the-fly?

Another option could be to locate the heap and traverse the malloc headers / free list; I'd appreciate any pointers to where I could start in finding the location and layout of these.

I've been trying to Google and read around the problem for about 2 hours and I've learnt some fascinating stuff but still not found what I need.

Answer 1

If you can change the code:

#include <malloc.h>
#include <stdio.h>

void dumpMallinfo(void) {
  struct mallinfo m = mallinfo();
  printf("uordblks = %d\nfordblks = %d\n", m.uordblks, m.fordblks);
}

In GDB, you can call dumpMallinfo().

Answer 2

@fd - the RedHat bug had your answer.

The mallinfo function has been deprecated, and won't be updated. A true query stats API is TDB. Today, you have malloc_stats and malloc_info. I can't find any documentation on either one, but here's what they give you.

Is this close enough to what you need?

(gdb) call malloc_stats()
Arena 0:
system bytes     =     135168
in use bytes     =         96
Total (incl. mmap):
system bytes     =     135168
in use bytes     =         96
max mmap regions =          0
max mmap bytes   =          0

(gdb) call malloc_info(0, stdout)
<malloc version="1">
<heap nr="0">
<sizes>
<unsorted from="1228788" to="1229476" total="3917678" count="3221220448"/>
</sizes>
<total type="fast" count="0" size="0"/>
<total type="rest" count="3221220448" size="3917678"/>
<system type="current" size="135168"/>
<system type="max" size="135168"/>
<aspace type="total" size="135168"/>
<aspace type="mprotect" size="135168"/>
</heap>
<total type="fast" count="0" size="0"/>
<total type="rest" count="3221220448" size="3917678"/>
<system type="current" size="135168
/>
<system type="max" size="135168
/>
<aspace type="total" size="135168"/>
<aspace type="mprotect" size="135168"/>
</malloc>