While debugging a memory leak bug during my summer internship, I found out that one of our APIs returned a raw pointer and thus transferred its ownership to the caller (me). In other words, I'm now responsible of delete-ing the pointer after my code finishes. While this is a terrible engineering practice, I grew interested in how memory leaks happen, and how delete[] solves the problem.

After some research & experiments, I wrote this blog post, which hopefully addresses three sets of questions

1. What are memory leaks?
2. How are objects allocated on the heap? How does delete[] know which area of memory to be freed?
3. How can we prevent memory leaks from happening?

While the Stack Overflow question "How does delete[] 'know' the size of the operand array?" sort of answers the second question, I decide to dig deeper into the actual memory area.

Coincidentally, I have worked on a heap exploitation problem with my friend @gzhding in a past CTF event. Thanks to the experience, I learned how to use gdb to dump the heap memory and gain some insight into the problem.

What are Memory Leaks?​

We know that C++ can dynamically allocate memory on the heap. A common example is to use new[] to create an array and delete[] to remove it.

Memory leaks happens when we create an array in the memory (i.e. "allocates" a segment of memory to store the object) and forget to delete it. When the pointer to the memory goes out of scope, the running code becomes ignorant of the allocated memory area. In worst cases, when the leakage happens in a loop, newly allocated memory will continue to pile up without being released, potentially slowing down or crash the computer.

PoC​

Here is a short Proof of Concept (PoC) code. The main() function calls the memory_leak() function, which allocates an array of 26 chars and fills them with capital English alphabets.

void memory_leak() {
    // Always delete pointers created by new to avoid memory leaks!
    char *arr = new char[26];

    for (int i = 0; i < 26; i++) {
        arr[i] = char(65 + i); // 65 is the ascii of 'A'
    }

    // The memory area is not freed!
    // delete[] arr;
}

int main() {
    memory_leak();
    return 0;
}

As the delete[] statement is commented out, when the function memory_leak() returns, the pointer arr goes out of scope and results in the memory area leaked.

A Deeper Look into the Memory​

Let us compile the program with g++ -g3 memory_leak.cpp -o memory_leak (the -g3 flag preserves debug information during compilation) and use gdb to verify the memory leak.

We'll add a breakpoint at the end of function memory_leak() and run the program until it hits the breakpoint.

$ gdb memory_leak

gef➤  b 11
Breakpoint 1 at 0x1179: file memory_leak.cpp, line 11.

gef➤  r
[...]
─────────────────────────────────────────────────────────────── source:memory_leak.cpp+11 ────
      6          arr[i] = char(65 + i); // 65 is the ascii of 'A'
      7      }
      8
      9      // The memory area is not freed!
     10      // delete[] arr;
●→   11  }
     12
     13  int main() {
     14      memory_leak();
     15      return 0;
     16  }
───────────────────────────────────────────────────────────────────────────────── threads ────
[#0] Id 1, Name: "memory_leak", stopped 0x555555555179 in memory_leak (), reason: BREAKPOINT
─────────────────────────────────────────────────────────────────────────────────── trace ────
[#0] 0x555555555179 → memory_leak()
[#1] 0x555555555186 → main()
──────────────────────────────────────────────────────────────────────────────────────────────

gef➤  info locals
arr = 0x55555556aeb0 "ABCDEFGHIJKLMNOPQRSTUVWXYZ"

gef➤  x/8xw 0x55555556aeb0
0x55555556aeb0: 0x44434241      0x48474645      0x4c4b4a49      0x504f4e4d
0x55555556aec0: 0x54535251      0x58575655      0x00005a59      0x00000000

After the program hits the breakpoint, we print out the address of pointer arr and the content of that memory. Note that the memory is stored in little endian order, so 0x44 (D) comes before 0x43 (C), 0x42 (B), and finally 0x41 (A).

Now, let us continue running the program until memory_leak() returns back to main().

gef➤  finish
[...]
─────────────────────────────────────────────────────────────── source:memory_leak.cpp+15 ────
     10      // delete[] arr;
●    11  }
     12
     13  int main() {
     14      memory_leak();
 →   15      return 0;
     16  }
───────────────────────────────────────────────────────────────────────────────── threads ────
[#0] Id 1, Name: "memory_leak", stopped 0x555555555186 in main (), reason: TEMPORARY BREAKPOINT
─────────────────────────────────────────────────────────────────────────────────── trace ────
[#0] 0x555555555186 → main()
──────────────────────────────────────────────────────────────────────────────────────────────

gef➤  info locals
No locals.

gef➤  x/8xw 0x55555556aeb0
0x55555556aeb0: 0x44434241      0x48474645      0x4c4b4a49      0x504f4e4d
0x55555556aec0: 0x54535251      0x58575655      0x00005a59      0x00000000

Now that memory_leak() returns, we have lost the pointer arr pointing to the memory address 0x55555556aeb0. However, as we print out the memory area, the data is still stored in the memory without being released, leading to a memory leak.

Verification with Valgrind​

Furthermore, we can use automated tools like Valgrind to check for memory leaks.

$ valgrind --leak-check=full ./memory_leak
==382643== Memcheck, a memory error detector
==382643== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==382643== Using Valgrind-3.17.0 and LibVEX; rerun with -h for copyright info
==382643== Command: ./memory_leak
==382643==
==382643==
==382643== HEAP SUMMARY:
==382643==     in use at exit: 26 bytes in 1 blocks
==382643==   total heap usage: 2 allocs, 1 frees, 72,730 bytes allocated
==382643==
==382643== 26 bytes in 1 blocks are definitely lost in loss record 1 of 1
==382643==    at 0x484021F: operator new[](unsigned long) (vg_replace_malloc.c:579)
==382643==    by 0x10914A: memory_leak() (memory_leak.cpp:3)
==382643==    by 0x109185: main (memory_leak.cpp:14)
==382643==
==382643== LEAK SUMMARY:
==382643==    definitely lost: 26 bytes in 1 blocks
==382643==    indirectly lost: 0 bytes in 0 blocks
==382643==      possibly lost: 0 bytes in 0 blocks
==382643==    still reachable: 0 bytes in 0 blocks
==382643==         suppressed: 0 bytes in 0 blocks
==382643==
==382643== For lists of detected and suppressed errors, rerun with: -s
==382643== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

How are objects allocated on the heap?​

To understand better the mechanisms behind memory leaks, we need to know how C++ allocates and frees memories, i.e. how new and delete works. Let us dig deeper into the source code of GNU's libstdc++ implementation (the library used by g++ by default).

How new and delete works​

new[] and delete[] are just wrappers of new and delete​

Interestingly, from the implementation of operator new[] (source), it seems that in stdlibc++, new[] is just an alias for new.

_GLIBCXX_WEAK_DEFINITION void*
operator new[] (std::size_t sz) _GLIBCXX_THROW (std::bad_alloc)
{
  return ::operator new(sz);
}

The same is true for delete[] (source), which is just an alias for delete.

And new & delete are wrappers of malloc and free​

Now let us take a look at the source code of new. It's just a wrapper around C's malloc plus some error handling and returns the malloc-ed raw pointer to the caller.

_GLIBCXX_WEAK_DEFINITION void *
operator new (std::size_t sz) _GLIBCXX_THROW (std::bad_alloc)
{
  void *p;

  /* malloc (0) is unpredictable; avoid it.  */
  if (__builtin_expect (sz == 0, false))
    sz = 1;

  while ((p = malloc (sz)) == 0)
    {
      new_handler handler = std::get_new_handler ();
      if (! handler)
	_GLIBCXX_THROW_OR_ABORT(bad_alloc());
      handler ();
    }

  return p;
}

delete (source) is even simpler, and just calls C's free.

_GLIBCXX_WEAK_DEFINITION void
operator delete(void* ptr) noexcept
{
  std::free(ptr);
}

Hence it seems that we should dive all the way into C standard library's implementation of malloc and free and see what exactly is happening behind the creation and deletion of arrays.

However, instead of covering the full picture of malloc (which could fill another blog post), we'll be mainly focusing on how malloc structures the allocated memory area (answer: using malloc_chunks on the heap) and how free knows which area of memory to be released.

How are malloc_chunks structured?​

The following content comes from the comments in malloc/malloc.c of glibc (source). I've done some minor edits to adapt the format to Markdown (what this website uses).

(The following includes lightly edited explanations by Colin Plumb.)

Chunks of memory are maintained using a `boundary tag' method as described in e.g., Knuth or Standish. (See the paper by Paul Wilson ftp://ftp.cs.utexas.edu/pub/garbage/allocsrv.ps for a survey of such techniques.) Sizes of free chunks are stored both in the front of each chunk and at the end. This makes consolidating fragmented chunks into bigger chunks very fast. The size fields also hold bits representing whether chunks are free or in use.

An allocated chunk looks like this:

    chunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |             Size of previous chunk, if unallocated (P clear)  |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |             Size of chunk, in bytes                     |A|M|P|
      mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |             User data starts here...                          .
            .                                                               .
            .             (malloc_usable_size() bytes)                      .
            .                                                               |
nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |             (size of chunk, but used for application data)    |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |             Size of next chunk, in bytes                |A|0|1|
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Where "chunk" is the front of the chunk for the purpose of most of the malloc code, but "mem" is the pointer that is returned to the user. "Nextchunk" is the beginning of the next contiguous chunk.

Chunks always begin on even word boundaries, so the mem portion (which is returned to the user) is also on an even word boundary, and thus at least double-word aligned.

Free chunks are stored in circular doubly-linked lists, and look like this:

    chunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |             Size of previous chunk, if unallocated (P clear)  |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    `head:' |             Size of chunk, in bytes                     |A|0|P|
      mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |             Forward pointer to next chunk in list             |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |             Back pointer to previous chunk in list            |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |             Unused space (may be 0 bytes long)                .
            .                                                               .
            .                                                               |
nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    `foot:' |             Size of chunk, in bytes                           |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |             Size of next chunk, in bytes                |A|0|0|
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The P (PREV_INUSE) bit, stored in the unused low-order bit of the chunk size (which is always a multiple of two words), is an in-use bit for the previous chunk. If that bit is clear, then the word before the current chunk size contains the previous chunk size, and can be used to find the front of the previous chunk. The very first chunk allocated always has this bit set, preventing access to non-existent (or non-owned) memory. If prev_inuse is set for any given chunk, then you CANNOT determine the size of the previous chunk, and might even get a memory addressing fault when trying to do so.

[...]

Note that the foot of the current chunk is actually represented as the prev_size of the NEXT chunk. This makes it easier to deal with alignments etc but can be very confusing when trying to extend or adapt this code.

[...]

A verification using PoC​

Now we can use gdb to actually print out the memory area and see how the explanation above applies to our case. Here I used the heap feature introduced by GEF to better visualize the malloc-ed chunk's properties.

gef➤  heap chunk arr
Chunk(addr=0x55555556aeb0, size=0x30, flags=PREV_INUSE)
Chunk size: 48 (0x30)
Usable size: 40 (0x28)
Previous chunk size: 0 (0x0)
PREV_INUSE flag: On
IS_MMAPPED flag: Off
NON_MAIN_ARENA flag: Off

gef➤  x/16xw 0x55555556aeb0-16
0x55555556aea0:	0x00000000	0x00000000	0x00000031	0x00000000
0x55555556aeb0:	0x44434241	0x48474645	0x4c4b4a49	0x504f4e4d
0x55555556aec0:	0x54535251	0x58575655	0x00005a59	0x00000000
0x55555556aed0:	0x00000000	0x00000000	0x0000f131	0x00000000

Note that here the chunk size is 48 bytes, with the usable size (size of actual user memory area) being 40, which is much higher than what we requested (an array of 26 chars, which should be 26 bytes). This is because "chunks always begin on even word boundaries ... and thus at least double-word aligned." (from the blockquote above).

Since the chunk in memory is always double-word aligned, we should minus the address by $2\times8=16$ bytes to obtain the chunk pointer address. Here the first word (represented by 2 32-bit words as in gdb) is filled with 0x00, and will be filled with the area of the previous chunk, if the P flag in unset.

The second word 0x31 (or 0b110001) stores the chunk size and 3 flags. The least significant bit 0b1 indicates that the flag P (PREV_INUSE) is set, so the previous chunk has not been freed. Since all chunks must have size at least of multiples of 8, the 3 least significant bits of the size field is always 0, which is why the 3 LSBs can be used as flags. To calculate the chunk size, we can discard the 3 LSBs and obtain 0b110000 (0x30, or 48) bytes.

So, our "actual" chunk starts at the address 0x55555556aea8 and ends at 0x55555556aec8. The data area starts from 0x55555556aeb0 and ends in 0x55555556aec8. Similarly, the next chunk pointer points to the last word in our chunk's data area (0x55555556aec8).

Then, why does the chunk pointer confusingly points to the last word in the previous chunk? The answer is related to the free design mechanism.

Because when the previous chunk is freed, it'll fill its last word with its size and clear the P flag in its next chunk (ours). Hence, our chunk can use this size "to find the front of the previous chunk" after the previous chunk has been freed.

How does free work?​

By now, you should already have the answer to "how does delete[] know which area of memory to be freed"—because the size of the chunk is stored in its metadata.

However, there are still some intricacies to be solved: why does the chunk pointer points to the last word in the previous chunk? Why do we need the PREV_INUSE (P) flag? To do so, we shall look into how free works.

Long story short, free works like the following. When it is called (source), the user would pass a pointer to the memory area to it, free would then call mem2chunk (source) to convert the pointer to point to the chunk header. Then, if the chunk is allocated by mmap indicated by the M flag, free calls munmap (man 3p | man 2) to release the chunk; if not, it passes the chunk pointer to _int_free (source) for the actual freeing process.

However, free-ing a memory chunk "does not actually return it to the operating system for other applications to use. The free() call marks a chunk of memory as 'free to be reused' by the application, but from the operating system's point of view, the memory still 'belongs' to the application" (source) heap, meaning that the heap manager is still responsible of keeping track of the memory and reusing them when necessary.

That is why we use a circular linked list, with each chunk storing pointers to the previous and after, to organize the free-d chunks. Moreover, the size of the chunk is stored at the end of its memory area, i.e. the chunk pointer of the next chunk. Hence, the next chunk can use the size to access this free-d chunk and its header. When the next chunk is also free-d, we can use this property to coalesce the two chunks.

Of course, the actual free-ing is much more complicated and the chunks will be put into different bins for efficient reallocation. You can read the official glibc wiki, this more detailed blog post, or the source code for _int_free for more information.

How can we prevent memory leaks?​

It's probably time to come back to where we started: now that we've known what memory leaks are, and how they happened, how do we effectively prevent them?

1. Always delete (delete[]) objects created with new (new[]).
   • The simplest thing to do, if you insist using new.
2. Avoid calling new and delete explicitly
   • Explanation
   • TL;DR: Use a resource handle instead of naked pointers, which can be leaked.
   • Solution: use a smart pointer like unique_ptr or shared_ptr.
3. Never transfer ownership by a raw pointer (T*) or reference (T&)
   • Explanation
   • TL;DR: it is unclear who should delete the pointer
   • Instead: return the object itself, or use a smart pointer

In general, requiring the programmer to manually free the resources is error-prone. You should consider managing resources automatically using resource handles and RAII (Resource Acquisition is Initialization).

References & Further Readings​

• Stroustrup, Bjarne and Sutter, Herb. "C++ Core Guidelines". Updated Jun 17, 2021. Accessed Aug 08, 2021.
• glibc wiki. "MallocInternals". Updated May 20, 2019. Accessed Aug 08, 2021.
• Azeria Labs. "Heap Exploitation Part 2: Understanding the Glibc Heap Implementation". Accessed Aug 08, 2021.
• CTF Wiki. "堆相关数据结构" (in Chinese). Accessed Aug 10, 2021.
• glibc Contributors. glibc v2.34 source code. Aug 2, 2021. Accessed Aug 8, 2021.
• gcc Contributors. gcc v11.2.0 source code. Jul 28, 2021. Accessed Aug 8, 2021.
• StackOverflow. "How does delete[] know the size of the operand array?"

BTW, while Googling memory leaks, I found a wiki page "Dealing With Memory Leaks" from Daya Bay Reactor Neutrino Experiment project hosted under Brookhaven National Lab's domain. I didn't know that Daya Bay Reactor also hosted a multinational research project 😂.

This is not the first personal website I've ever made. I coded my very first personal website in 2014 using static HTML and CSS (that was the time when most websites were still using HTML 4.01 and you had to decide whether you want a "strict" version of HTML), after a bit of self-learning with W3Schools. In fact, I wasn't even learning from the true W3Schools, but an imitation of it.

That was my first step, before I got into building more complicated (and dynamic!) websites using PHP, with the help from lecture videos of Harvard's open course CS75. Shortly afterwards, I hosted my first dynamic website on the free hosting space RedHat OpenShift by 2016 (though it served more as a proxy server for me to get around the Internet censorship—that would be the topic of some other day). Sadly, all these codes were lost during the years (I didn't even know how to use git at that time).

Since then, I have hosted different sorts of personal websites: portfolios, WordPress blogs, and many lightweight alternatives like typecho or hugo. But to be honest I was more interested in the process of trying out new frameworks or toolsets, instead of actually settling down and writing something. And what use are websites of if not for sharing information?

In fact, that's what motivates me to still build blogs and websites to this day, in the year of 2021, when most people are turning to centralized social media platforms like WeChat, Instagram, Twitter ..., when small websites are almost extinct from public sight. I became so fed up by the frequent warnings of "sensitive words" that force me to water down and censor my comments so I finally decided to start (again) a website of my own to at least have a place to myself where I can actually write something undisturbed. And that's why, out of the innumerable possibilities, you are reading this article right now.