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malloc & sysmalloc

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Allocation Order Summary

(No checks are explained in this summary and some case have been omitted for brevity)

  1. __libc_malloc tries to get a chunk from the tcache, if not it calls _int_malloc

  2. _int_malloc :

    1. Tries to generate the arena if there isn't any

    2. If any fast bin chunk of the correct size, use it

      1. Fill tcache with other fast chunks

    3. If any small bin chunk of the correct size, use it

      1. Fill tcache with other chunks of that size

    4. If the requested size isn't for small bins, consolidate fast bin into unsorted bin

    5. Check the unsorted bin, use the first chunk with enough space

      1. If the found chunk is bigger, divide it to return a part and add the reminder back to the unsorted bin

      2. If a chunk is of the same size as the size requested, use to to fill the tcache instead of returning it (until the tcache is full, then return the next one)

      3. For each chunk of smaller size checked, put it in its respective small or large bin

    6. Check the large bin in the index of the requested size

      1. Start looking from the first chunk that is bigger than the requested size, if any is found return it and add the reminders to the small bin

    7. Check the large bins from the next indexes until the end

      1. From the next bigger index check for any chunk, divide the first found chunk to use it for the requested size and add the reminder to the unsorted bin

    8. If nothing is found in the previous bins, get a chunk from the top chunk

    9. If the top chunk wasn't big enough enlarge it with sysmalloc

__libc_malloc

The malloc function actually calls __libc_malloc. This function will check the tcache to see if there is any available chunk of the desired size. If the re is it'll use it and if not it'll check if it's a single thread and in that case it'll call _int_malloc in the main arena, and if not it'll call _int_malloc in arena of the thread.

__libc_malloc code
// From https://github.com/bminor/glibc/blob/master/malloc/malloc.c

#if IS_IN (libc)
void *
__libc_malloc (size_t bytes)
{
  mstate ar_ptr;
  void *victim;

  _Static_assert (PTRDIFF_MAX <= SIZE_MAX / 2,
                  "PTRDIFF_MAX is not more than half of SIZE_MAX");

  if (!__malloc_initialized)
    ptmalloc_init ();
#if USE_TCACHE
  /* int_free also calls request2size, be careful to not pad twice.  */
  size_t tbytes = checked_request2size (bytes);
  if (tbytes == 0)
    {
      __set_errno (ENOMEM);
      return NULL;
    }
  size_t tc_idx = csize2tidx (tbytes);

  MAYBE_INIT_TCACHE ();

  DIAG_PUSH_NEEDS_COMMENT;
  if (tc_idx < mp_.tcache_bins
      && tcache != NULL
      && tcache->counts[tc_idx] > 0)
    {
      victim = tcache_get (tc_idx);
      return tag_new_usable (victim);
    }
  DIAG_POP_NEEDS_COMMENT;
#endif

  if (SINGLE_THREAD_P)
    {
      victim = tag_new_usable (_int_malloc (&main_arena, bytes));
      assert (!victim || chunk_is_mmapped (mem2chunk (victim)) ||
	      &main_arena == arena_for_chunk (mem2chunk (victim)));
      return victim;
    }

  arena_get (ar_ptr, bytes);

  victim = _int_malloc (ar_ptr, bytes);
  /* Retry with another arena only if we were able to find a usable arena
     before.  */
  if (!victim && ar_ptr != NULL)
    {
      LIBC_PROBE (memory_malloc_retry, 1, bytes);
      ar_ptr = arena_get_retry (ar_ptr, bytes);
      victim = _int_malloc (ar_ptr, bytes);
    }

  if (ar_ptr != NULL)
    __libc_lock_unlock (ar_ptr->mutex);

  victim = tag_new_usable (victim);

  assert (!victim || chunk_is_mmapped (mem2chunk (victim)) ||
          ar_ptr == arena_for_chunk (mem2chunk (victim)));
  return victim;
}

Note how it'll always tag the returned pointer with tag_new_usable, from the code:

 void *tag_new_usable (void *ptr)

   Allocate a new random color and use it to color the user region of
   a chunk; this may include data from the subsequent chunk's header
   if tagging is sufficiently fine grained.  Returns PTR suitably
   recolored for accessing the memory there.

_int_malloc

This is the function that allocates memory using the other bins and top chunk.

  • Start

It starts defining some vars and getting the real size the request memory space need to have:

_int_malloc start
// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L3847
static void *
_int_malloc (mstate av, size_t bytes)
{
  INTERNAL_SIZE_T nb;               /* normalized request size */
  unsigned int idx;                 /* associated bin index */
  mbinptr bin;                      /* associated bin */

  mchunkptr victim;                 /* inspected/selected chunk */
  INTERNAL_SIZE_T size;             /* its size */
  int victim_index;                 /* its bin index */

  mchunkptr remainder;              /* remainder from a split */
  unsigned long remainder_size;     /* its size */

  unsigned int block;               /* bit map traverser */
  unsigned int bit;                 /* bit map traverser */
  unsigned int map;                 /* current word of binmap */

  mchunkptr fwd;                    /* misc temp for linking */
  mchunkptr bck;                    /* misc temp for linking */

#if USE_TCACHE
  size_t tcache_unsorted_count;	    /* count of unsorted chunks processed */
#endif

  /*
     Convert request size to internal form by adding SIZE_SZ bytes
     overhead plus possibly more to obtain necessary alignment and/or
     to obtain a size of at least MINSIZE, the smallest allocatable
     size. Also, checked_request2size returns false for request sizes
     that are so large that they wrap around zero when padded and
     aligned.
   */

  nb = checked_request2size (bytes);
  if (nb == 0)
    {
      __set_errno (ENOMEM);
      return NULL;
    }

Arena

In the unlikely event that there aren't usable arenas, it uses sysmalloc to get a chunk from mmap:

_int_malloc not arena
// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L3885C3-L3893C6
/* There are no usable arenas.  Fall back to sysmalloc to get a chunk from
     mmap.  */
  if (__glibc_unlikely (av == NULL))
    {
      void *p = sysmalloc (nb, av);
      if (p != NULL)
	alloc_perturb (p, bytes);
      return p;
    }

Fast Bin

If the needed size is inside the Fast Bins sizes, try to use a chunk from the fast bin. Basically, based on the size, it'll find the fast bin index where valid chunks should be located, and if any, it'll return one of those. Moreover, if tcache is enabled, it'll fill the tcache bin of that size with fast bins.

While performing these actions, some security checks are executed in here:

  • If the chunk is misaligned: malloc(): unaligned fastbin chunk detected 2

  • If the forward chunk is misaligned: malloc(): unaligned fastbin chunk detected

  • If the returned chunk has a size that isn't correct because of it's index in the fast bin: malloc(): memory corruption (fast)

  • If any chunk used to fill the tcache is misaligned: malloc(): unaligned fastbin chunk detected 3

_int_malloc fast bin
// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L3895C3-L3967C6
/*
     If the size qualifies as a fastbin, first check corresponding bin.
     This code is safe to execute even if av is not yet initialized, so we
     can try it without checking, which saves some time on this fast path.
   */

#define REMOVE_FB(fb, victim, pp)			\
  do							\
    {							\
      victim = pp;					\
      if (victim == NULL)				\
	break;						\
      pp = REVEAL_PTR (victim->fd);                                     \
      if (__glibc_unlikely (pp != NULL && misaligned_chunk (pp)))       \
	malloc_printerr ("malloc(): unaligned fastbin chunk detected"); \
    }							\
  while ((pp = catomic_compare_and_exchange_val_acq (fb, pp, victim)) \
	 != victim);					\

  if ((unsigned long) (nb) <= (unsigned long) (get_max_fast ()))
    {
      idx = fastbin_index (nb);
      mfastbinptr *fb = &fastbin (av, idx);
      mchunkptr pp;
      victim = *fb;

      if (victim != NULL)
	{
	  if (__glibc_unlikely (misaligned_chunk (victim)))
	    malloc_printerr ("malloc(): unaligned fastbin chunk detected 2");

	  if (SINGLE_THREAD_P)
	    *fb = REVEAL_PTR (victim->fd);
	  else
	    REMOVE_FB (fb, pp, victim);
	  if (__glibc_likely (victim != NULL))
	    {
	      size_t victim_idx = fastbin_index (chunksize (victim));
	      if (__builtin_expect (victim_idx != idx, 0))
		malloc_printerr ("malloc(): memory corruption (fast)");
	      check_remalloced_chunk (av, victim, nb);
#if USE_TCACHE
	      /* While we're here, if we see other chunks of the same size,
		 stash them in the tcache.  */
	      size_t tc_idx = csize2tidx (nb);
	      if (tcache != NULL && tc_idx < mp_.tcache_bins)
		{
		  mchunkptr tc_victim;

		  /* While bin not empty and tcache not full, copy chunks.  */
		  while (tcache->counts[tc_idx] < mp_.tcache_count
			 && (tc_victim = *fb) != NULL)
		    {
		      if (__glibc_unlikely (misaligned_chunk (tc_victim)))
			malloc_printerr ("malloc(): unaligned fastbin chunk detected 3");
		      if (SINGLE_THREAD_P)
			*fb = REVEAL_PTR (tc_victim->fd);
		      else
			{
			  REMOVE_FB (fb, pp, tc_victim);
			  if (__glibc_unlikely (tc_victim == NULL))
			    break;
			}
		      tcache_put (tc_victim, tc_idx);
		    }
		}
#endif
	      void *p = chunk2mem (victim);
	      alloc_perturb (p, bytes);
	      return p;
	    }
	}
    }

Small Bin

As indicated in a comment, small bins hold one size per index, therefore checking if a valid chunk is available is super fast, so after fast bins, small bins are checked.

The first check is to find out if the requested size could be inside a small bin. In that case, get the corresponded index inside the smallbin and see if there is any available chunk.

Then, a security check is performed checking:

  • if victim->bk->fd = victim. To see that both chunks are correctly linked.

In that case, the chunk gets the inuse bit, the doubled linked list is fixed so this chunk disappears from it (as it's going to be used), and the non main arena bit is set if needed.

Finally, fill the tcache index of the requested size with other chunks inside the small bin (if any).

_int_malloc small bin
// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L3895C3-L3967C6

/*
     If a small request, check regular bin.  Since these "smallbins"
     hold one size each, no searching within bins is necessary.
     (For a large request, we need to wait until unsorted chunks are
     processed to find best fit. But for small ones, fits are exact
     anyway, so we can check now, which is faster.)
   */

  if (in_smallbin_range (nb))
    {
      idx = smallbin_index (nb);
      bin = bin_at (av, idx);

      if ((victim = last (bin)) != bin)
        {
          bck = victim->bk;
	  if (__glibc_unlikely (bck->fd != victim))
	    malloc_printerr ("malloc(): smallbin double linked list corrupted");
          set_inuse_bit_at_offset (victim, nb);
          bin->bk = bck;
          bck->fd = bin;

          if (av != &main_arena)
	    set_non_main_arena (victim);
          check_malloced_chunk (av, victim, nb);
#if USE_TCACHE
	  /* While we're here, if we see other chunks of the same size,
	     stash them in the tcache.  */
	  size_t tc_idx = csize2tidx (nb);
	  if (tcache != NULL && tc_idx < mp_.tcache_bins)
	    {
	      mchunkptr tc_victim;

	      /* While bin not empty and tcache not full, copy chunks over.  */
	      while (tcache->counts[tc_idx] < mp_.tcache_count
		     && (tc_victim = last (bin)) != bin)
		{
		  if (tc_victim != 0)
		    {
		      bck = tc_victim->bk;
		      set_inuse_bit_at_offset (tc_victim, nb);
		      if (av != &main_arena)
			set_non_main_arena (tc_victim);
		      bin->bk = bck;
		      bck->fd = bin;

		      tcache_put (tc_victim, tc_idx);
	            }
		}
	    }
#endif
          void *p = chunk2mem (victim);
          alloc_perturb (p, bytes);
          return p;
        }
    }

malloc_consolidate

If it wasn't a small chunk, it's a large chunk, and in this case malloc_consolidate is called to avoid memory fragmentation.

malloc_consolidate call
/*
     If this is a large request, consolidate fastbins before continuing.
     While it might look excessive to kill all fastbins before
     even seeing if there is space available, this avoids
     fragmentation problems normally associated with fastbins.
     Also, in practice, programs tend to have runs of either small or
     large requests, but less often mixtures, so consolidation is not
     invoked all that often in most programs. And the programs that
     it is called frequently in otherwise tend to fragment.
   */

  else
    {
      idx = largebin_index (nb);
      if (atomic_load_relaxed (&av->have_fastchunks))
        malloc_consolidate (av);
    }

The malloc consolidate function basically removes chunks from the fast bin and places them into the unsorted bin. After the next malloc these chunks will be organized in their respective small/fast bins.

Note that if while removing these chunks, if they are found with previous or next chunks that aren't in use they will be unliked and merged before placing the final chunk in the unsorted bin.

For each fast bin chunk a couple of security checks are performed:

  • If the chunk is unaligned trigger: malloc_consolidate(): unaligned fastbin chunk detected

  • If the chunk has a different size that the one it should because of the index it's in: malloc_consolidate(): invalid chunk size

  • If the previous chunk is not in use and the previous chunk has a size different of the one indicated by prev_chunk: corrupted size vs. prev_size in fastbins

malloc_consolidate function
// https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L4810C1-L4905C2

static void malloc_consolidate(mstate av)
{
  mfastbinptr*    fb;                 /* current fastbin being consolidated */
  mfastbinptr*    maxfb;              /* last fastbin (for loop control) */
  mchunkptr       p;                  /* current chunk being consolidated */
  mchunkptr       nextp;              /* next chunk to consolidate */
  mchunkptr       unsorted_bin;       /* bin header */
  mchunkptr       first_unsorted;     /* chunk to link to */

  /* These have same use as in free() */
  mchunkptr       nextchunk;
  INTERNAL_SIZE_T size;
  INTERNAL_SIZE_T nextsize;
  INTERNAL_SIZE_T prevsize;
  int             nextinuse;

  atomic_store_relaxed (&av->have_fastchunks, false);

  unsorted_bin = unsorted_chunks(av);

  /*
    Remove each chunk from fast bin and consolidate it, placing it
    then in unsorted bin. Among other reasons for doing this,
    placing in unsorted bin avoids needing to calculate actual bins
    until malloc is sure that chunks aren't immediately going to be
    reused anyway.
  */

  maxfb = &fastbin (av, NFASTBINS - 1);
  fb = &fastbin (av, 0);
  do {
    p = atomic_exchange_acquire (fb, NULL);
    if (p != 0) {
      do {
	{
	  if (__glibc_unlikely (misaligned_chunk (p)))
	    malloc_printerr ("malloc_consolidate(): "
			     "unaligned fastbin chunk detected");

	  unsigned int idx = fastbin_index (chunksize (p));
	  if ((&fastbin (av, idx)) != fb)
	    malloc_printerr ("malloc_consolidate(): invalid chunk size");
	}

	check_inuse_chunk(av, p);
	nextp = REVEAL_PTR (p->fd);

	/* Slightly streamlined version of consolidation code in free() */
	size = chunksize (p);
	nextchunk = chunk_at_offset(p, size);
	nextsize = chunksize(nextchunk);

	if (!prev_inuse(p)) {
	  prevsize = prev_size (p);
	  size += prevsize;
	  p = chunk_at_offset(p, -((long) prevsize));
	  if (__glibc_unlikely (chunksize(p) != prevsize))
	    malloc_printerr ("corrupted size vs. prev_size in fastbins");
	  unlink_chunk (av, p);
	}

	if (nextchunk != av->top) {
	  nextinuse = inuse_bit_at_offset(nextchunk, nextsize);

	  if (!nextinuse) {
	    size += nextsize;
	    unlink_chunk (av, nextchunk);
	  } else
	    clear_inuse_bit_at_offset(nextchunk, 0);

	  first_unsorted = unsorted_bin->fd;
	  unsorted_bin->fd = p;
	  first_unsorted->bk = p;

	  if (!in_smallbin_range (size)) {
	    p->fd_nextsize = NULL;
	    p->bk_nextsize = NULL;
	  }

	  set_head(p, size | PREV_INUSE);
	  p->bk = unsorted_bin;
	  p->fd = first_unsorted;
	  set_foot(p, size);
	}

	else {
	  size += nextsize;
	  set_head(p, size | PREV_INUSE);
	  av->top = p;
	}

      } while ( (p = nextp) != 0);

    }
  } while (fb++ != maxfb);
}

Unsorted bin

It's time to check the unsorted bin for a potential valid chunk to use.

Start

This starts with a big for look that will be traversing the unsorted bin in the bk direction until it arrives til the end (the arena struct) with while ((victim = unsorted_chunks (av)->bk) != unsorted_chunks (av))

Moreover, some security checks are perform every time a new chunk is considered:

  • If the chunk size is weird (too small or too big): malloc(): invalid size (unsorted)

  • If the next chunk size is weird (too small or too big): malloc(): invalid next size (unsorted)

  • If the previous size indicated by the next chunk differs from the size of the chunk: malloc(): mismatching next->prev_size (unsorted)

  • If not victim->bck->fd == victim or not victim->fd == av (arena): malloc(): unsorted double linked list corrupted

    • As we are always checking the las one, it's fd should be pointing always to the arena struct.

  • If the next chunk isn't indicating that the previous is in use: malloc(): invalid next->prev_inuse (unsorted)

_int_malloc unsorted bin start
/*
     Process recently freed or remaindered chunks, taking one only if
     it is exact fit, or, if this a small request, the chunk is remainder from
     the most recent non-exact fit.  Place other traversed chunks in
     bins.  Note that this step is the only place in any routine where
     chunks are placed in bins.

     The outer loop here is needed because we might not realize until
     near the end of malloc that we should have consolidated, so must
     do so and retry. This happens at most once, and only when we would
     otherwise need to expand memory to service a "small" request.
   */

#if USE_TCACHE
  INTERNAL_SIZE_T tcache_nb = 0;
  size_t tc_idx = csize2tidx (nb);
  if (tcache != NULL && tc_idx < mp_.tcache_bins)
    tcache_nb = nb;
  int return_cached = 0;

  tcache_unsorted_count = 0;
#endif

  for (;; )
    {
      int iters = 0;
      while ((victim = unsorted_chunks (av)->bk) != unsorted_chunks (av))
        {
          bck = victim->bk;
          size = chunksize (victim);
          mchunkptr next = chunk_at_offset (victim, size);

          if (__glibc_unlikely (size <= CHUNK_HDR_SZ)
              || __glibc_unlikely (size > av->system_mem))
            malloc_printerr ("malloc(): invalid size (unsorted)");
          if (__glibc_unlikely (chunksize_nomask (next) < CHUNK_HDR_SZ)
              || __glibc_unlikely (chunksize_nomask (next) > av->system_mem))
            malloc_printerr ("malloc(): invalid next size (unsorted)");
          if (__glibc_unlikely ((prev_size (next) & ~(SIZE_BITS)) != size))
            malloc_printerr ("malloc(): mismatching next->prev_size (unsorted)");
          if (__glibc_unlikely (bck->fd != victim)
              || __glibc_unlikely (victim->fd != unsorted_chunks (av)))
            malloc_printerr ("malloc(): unsorted double linked list corrupted");
          if (__glibc_unlikely (prev_inuse (next)))
            malloc_printerr ("malloc(): invalid next->prev_inuse (unsorted)");

if in_smallbin_range

If the chunk is bigger than the requested size use it, and set the rest of the chunk space into the unsorted list and update the last_remainder with it.

_int_malloc unsorted bin in_smallbin_range
// From https://github.com/bminor/glibc/blob/master/malloc/malloc.c#L4090C11-L4124C14

/*
             If a small request, try to use last remainder if it is the
             only chunk in unsorted bin.  This helps promote locality for
             runs of consecutive small requests. This is the only
             exception to best-fit, and applies only when there is
             no exact fit for a small chunk.
           */

          if (in_smallbin_range (nb) &&
              bck == unsorted_chunks (av) &&
              victim == av->last_remainder &&
              (unsigned long) (size) > (unsigned long) (nb + MINSIZE))
            {
              /* split and reattach remainder */
              remainder_size = size - nb;
              remainder = chunk_at_offset (victim, nb);
              unsorted_chunks (av)->bk = unsorted_chunks (av)->fd = remainder;
              av->last_remainder = remainder;
              remainder->bk = remainder->fd = unsorted_chunks (av);
              if (!in_smallbin_range (remainder_size))
                {
                  remainder->fd_nextsize = NULL;
                  remainder->bk_nextsize = NULL;
                }

              set_head (victim, nb | PREV_INUSE |
                        (av != &main_arena ? NON_MAIN_ARENA : 0));
              set_head (remainder, remainder_size | PREV_INUSE);
              set_foot (remainder, remainder_size);

              check_malloced_chunk (av, victim, nb);
              void *p = chunk2mem (victim);
              alloc_perturb (p, bytes);
              return p;
            }

If this was successful, return the chunk ant it's over, if not, continue executing the function...

if equal size

Continue removing the chunk from the bin, in case the requested size is exactly the one of the chunk:

  • If the tcache is not filled, add it to the tcache and continue indicating that there is a tcache chunk that could be used

  • If tcache is full, just use it returning it

_int_malloc unsorted bin equal size
// From https://github.com/bminor/glibc/blob/master/malloc/malloc.c#L4126C11-L4157C14

/* remove from unsorted list */
          unsorted_chunks (av)->bk = bck;
          bck->fd = unsorted_chunks (av);

          /* Take now instead of binning if exact fit */

          if (size == nb)
            {
              set_inuse_bit_at_offset (victim, size);
              if (av != &main_arena)
		set_non_main_arena (victim);
#if USE_TCACHE
	      /* Fill cache first, return to user only if cache fills.
		 We may return one of these chunks later.  */
	      if (tcache_nb > 0
		  && tcache->counts[tc_idx] < mp_.tcache_count)
		{
		  tcache_put (victim, tc_idx);
		  return_cached = 1;
		  continue;
		}
	      else
		{
#endif
              check_malloced_chunk (av, victim, nb);
              void *p = chunk2mem (victim);
              alloc_perturb (p, bytes);
              return p;
#if USE_TCACHE
		}
#endif
            }

If chunk not returned or added to tcache, continue with the code...

place chunk in a bin

Store the checked chunk in the small bin or in the large bin according to the size of the chunk (keeping the large bin properly organized).

There are security checks being performed to make sure both large bin doubled linked list are corrupted:

  • If fwd->bk_nextsize->fd_nextsize != fwd: malloc(): largebin double linked list corrupted (nextsize)

  • If fwd->bk->fd != fwd: malloc(): largebin double linked list corrupted (bk)

_int_malloc place chunk in a bin
/* place chunk in bin */

          if (in_smallbin_range (size))
            {
              victim_index = smallbin_index (size);
              bck = bin_at (av, victim_index);
              fwd = bck->fd;
            }
          else
            {
              victim_index = largebin_index (size);
              bck = bin_at (av, victim_index);
              fwd = bck->fd;

              /* maintain large bins in sorted order */
              if (fwd != bck)
                {
                  /* Or with inuse bit to speed comparisons */
                  size |= PREV_INUSE;
                  /* if smaller than smallest, bypass loop below */
                  assert (chunk_main_arena (bck->bk));
                  if ((unsigned long) (size)
		      < (unsigned long) chunksize_nomask (bck->bk))
                    {
                      fwd = bck;
                      bck = bck->bk;

                      victim->fd_nextsize = fwd->fd;
                      victim->bk_nextsize = fwd->fd->bk_nextsize;
                      fwd->fd->bk_nextsize = victim->bk_nextsize->fd_nextsize = victim;
                    }
                  else
                    {
                      assert (chunk_main_arena (fwd));
                      while ((unsigned long) size < chunksize_nomask (fwd))
                        {
                          fwd = fwd->fd_nextsize;
			  assert (chunk_main_arena (fwd));
                        }

                      if ((unsigned long) size
			  == (unsigned long) chunksize_nomask (fwd))
                        /* Always insert in the second position.  */
                        fwd = fwd->fd;
                      else
                        {
                          victim->fd_nextsize = fwd;
                          victim->bk_nextsize = fwd->bk_nextsize;
                          if (__glibc_unlikely (fwd->bk_nextsize->fd_nextsize != fwd))
                            malloc_printerr ("malloc(): largebin double linked list corrupted (nextsize)");
                          fwd->bk_nextsize = victim;
                          victim->bk_nextsize->fd_nextsize = victim;
                        }
                      bck = fwd->bk;
                      if (bck->fd != fwd)
                        malloc_printerr ("malloc(): largebin double linked list corrupted (bk)");
                    }
                }
              else
                victim->fd_nextsize = victim->bk_nextsize = victim;
            }

          mark_bin (av, victim_index);
          victim->bk = bck;
          victim->fd = fwd;
          fwd->bk = victim;
          bck->fd = victim;

_int_malloc limits

At this point, if some chunk was stored in the tcache that can be used and the limit is reached, just return a tcache chunk.

Moreover, if MAX_ITERS is reached, break from the loop for and get a chunk in a different way (top chunk).

If return_cached was set, just return a chunk from the tcache to avoid larger searches.

_int_malloc limits
// From https://github.com/bminor/glibc/blob/master/malloc/malloc.c#L4227C1-L4250C7

#if USE_TCACHE
      /* If we've processed as many chunks as we're allowed while
	 filling the cache, return one of the cached ones.  */
      ++tcache_unsorted_count;
      if (return_cached
	  && mp_.tcache_unsorted_limit > 0
	  && tcache_unsorted_count > mp_.tcache_unsorted_limit)
	{
	  return tcache_get (tc_idx);
	}
#endif

#define MAX_ITERS       10000
          if (++iters >= MAX_ITERS)
            break;
        }

#if USE_TCACHE
      /* If all the small chunks we found ended up cached, return one now.  */
      if (return_cached)
	{
	  return tcache_get (tc_idx);
	}
#endif

If limits not reached, continue with the code...

Large Bin (by index)

If the request is large (not in small bin) and we haven't yet returned any chunk, get the index of the requested size in the large bin, check if not empty of if the biggest chunk in this bin is bigger than the requested size and in that case find the smallest chunk that can be used for the requested size.

If the reminder space from the finally used chunk can be a new chunk, add it to the unsorted bin and the lsast_reminder is updated.

A security check is made when adding the reminder to the unsorted bin:

  • bck->fd-> bk != bck: malloc(): corrupted unsorted chunks

_int_malloc Large bin (by index)
// From https://github.com/bminor/glibc/blob/master/malloc/malloc.c#L4252C7-L4317C10

/*
         If a large request, scan through the chunks of current bin in
         sorted order to find smallest that fits.  Use the skip list for this.
       */

      if (!in_smallbin_range (nb))
        {
          bin = bin_at (av, idx);

          /* skip scan if empty or largest chunk is too small */
          if ((victim = first (bin)) != bin
	      && (unsigned long) chunksize_nomask (victim)
	        >= (unsigned long) (nb))
            {
              victim = victim->bk_nextsize;
              while (((unsigned long) (size = chunksize (victim)) <
                      (unsigned long) (nb)))
                victim = victim->bk_nextsize;

              /* Avoid removing the first entry for a size so that the skip
                 list does not have to be rerouted.  */
              if (victim != last (bin)
		  && chunksize_nomask (victim)
		    == chunksize_nomask (victim->fd))
                victim = victim->fd;

              remainder_size = size - nb;
              unlink_chunk (av, victim);

              /* Exhaust */
              if (remainder_size < MINSIZE)
                {
                  set_inuse_bit_at_offset (victim, size);
                  if (av != &main_arena)
		    set_non_main_arena (victim);
                }
              /* Split */
              else
                {
                  remainder = chunk_at_offset (victim, nb);
                  /* We cannot assume the unsorted list is empty and therefore
                     have to perform a complete insert here.  */
                  bck = unsorted_chunks (av);
                  fwd = bck->fd;
		  if (__glibc_unlikely (fwd->bk != bck))
		    malloc_printerr ("malloc(): corrupted unsorted chunks");
                  last_re->bk = bck;
                  remainder->fd = fwd;
                  bck->fd = remainder;
                  fwd->bk = remainder;
                  if (!in_smallbin_range (remainder_size))
                    {
                      remainder->fd_nextsize = NULL;
                      remainder->bk_nextsize = NULL;
                    }
                  set_head (victim, nb | PREV_INUSE |
                            (av != &main_arena ? NON_MAIN_ARENA : 0));
                  set_head (remainder, remainder_size | PREV_INUSE);
                  set_foot (remainder, remainder_size);
                }
              check_malloced_chunk (av, victim, nb);
              void *p = chunk2mem (victim);
              alloc_perturb (p, bytes);
              return p;
            }
        }

If a chunk isn't found suitable for this, continue

Large Bin (next bigger)

If in the exact large bin there wasn't any chunk that could be used, start looping through all the next large bin (starting y the immediately larger) until one is found (if any).

The reminder of the split chunk is added in the unsorted bin, last_reminder is updated and the same security check is performed:

  • bck->fd-> bk != bck: malloc(): corrupted unsorted chunks2

_int_malloc Large bin (next bigger)
// From https://github.com/bminor/glibc/blob/master/malloc/malloc.c#L4319C7-L4425C10

/*
         Search for a chunk by scanning bins, starting with next largest
         bin. This search is strictly by best-fit; i.e., the smallest
         (with ties going to approximately the least recently used) chunk
         that fits is selected.

         The bitmap avoids needing to check that most blocks are nonempty.
         The particular case of skipping all bins during warm-up phases
         when no chunks have been returned yet is faster than it might look.
       */

      ++idx;
      bin = bin_at (av, idx);
      block = idx2block (idx);
      map = av->binmap[block];
      bit = idx2bit (idx);

      for (;; )
        {
          /* Skip rest of block if there are no more set bits in this block.  */
          if (bit > map || bit == 0)
            {
              do
                {
                  if (++block >= BINMAPSIZE) /* out of bins */
                    goto use_top;
                }
              while ((map = av->binmap[block]) == 0);

              bin = bin_at (av, (block << BINMAPSHIFT));
              bit = 1;
            }

          /* Advance to bin with set bit. There must be one. */
          while ((bit & map) == 0)
            {
              bin = next_bin (bin);
              bit <<= 1;
              assert (bit != 0);
            }

          /* Inspect the bin. It is likely to be non-empty */
          victim = last (bin);

          /*  If a false alarm (empty bin), clear the bit. */
          if (victim == bin)
            {
              av->binmap[block] = map &= ~bit; /* Write through */
              bin = next_bin (bin);
              bit <<= 1;
            }

          else
            {
              size = chunksize (victim);

              /*  We know the first chunk in this bin is big enough to use. */
              assert ((unsigned long) (size) >= (unsigned long) (nb));

              remainder_size = size - nb;

              /* unlink */
              unlink_chunk (av, victim);

              /* Exhaust */
              if (remainder_size < MINSIZE)
                {
                  set_inuse_bit_at_offset (victim, size);
                  if (av != &main_arena)
		    set_non_main_arena (victim);
                }

              /* Split */
              else
                {
                  remainder = chunk_at_offset (victim, nb);

                  /* We cannot assume the unsorted list is empty and therefore
                     have to perform a complete insert here.  */
                  bck = unsorted_chunks (av);
                  fwd = bck->fd;
		  if (__glibc_unlikely (fwd->bk != bck))
		    malloc_printerr ("malloc(): corrupted unsorted chunks 2");
                  remainder->bk = bck;
                  remainder->fd = fwd;
                  bck->fd = remainder;
                  fwd->bk = remainder;

                  /* advertise as last remainder */
                  if (in_smallbin_range (nb))
                    av->last_remainder = remainder;
                  if (!in_smallbin_range (remainder_size))
                    {
                      remainder->fd_nextsize = NULL;
                      remainder->bk_nextsize = NULL;
                    }
                  set_head (victim, nb | PREV_INUSE |
                            (av != &main_arena ? NON_MAIN_ARENA : 0));
                  set_head (remainder, remainder_size | PREV_INUSE);
                  set_foot (remainder, remainder_size);
                }
              check_malloced_chunk (av, victim, nb);
              void *p = chunk2mem (victim);
              alloc_perturb (p, bytes);
              return p;
            }
        }

Top Chunk

At this point, it's time to get a new chunk from the Top chunk (if big enough).

It starts with a security check making sure that the size of the chunk size is not too big (corrupted):

  • chunksize(av->top) > av->system_mem: malloc(): corrupted top size

Then, it'll use the top chunk space if it's large enough to create a chunk of the requested size. If not, if there are fast chunks, consolidate them and try again. Finally, if not enough space use sysmalloc to allocate enough size.

_int_malloc Top chunk
use_top:
      /*
         If large enough, split off the chunk bordering the end of memory
         (held in av->top). Note that this is in accord with the best-fit
         search rule.  In effect, av->top is treated as larger (and thus
         less well fitting) than any other available chunk since it can
         be extended to be as large as necessary (up to system
         limitations).

         We require that av->top always exists (i.e., has size >=
         MINSIZE) after initialization, so if it would otherwise be
         exhausted by current request, it is replenished. (The main
         reason for ensuring it exists is that we may need MINSIZE space
         to put in fenceposts in sysmalloc.)
       */

      victim = av->top;
      size = chunksize (victim);

      if (__glibc_unlikely (size > av->system_mem))
        malloc_printerr ("malloc(): corrupted top size");

      if ((unsigned long) (size) >= (unsigned long) (nb + MINSIZE))
        {
          remainder_size = size - nb;
          remainder = chunk_at_offset (victim, nb);
          av->top = remainder;
          set_head (victim, nb | PREV_INUSE |
                    (av != &main_arena ? NON_MAIN_ARENA : 0));
          set_head (remainder, remainder_size | PREV_INUSE);

          check_malloced_chunk (av, victim, nb);
          void *p = chunk2mem (victim);
          alloc_perturb (p, bytes);
          return p;
        }

      /* When we are using atomic ops to free fast chunks we can get
         here for all block sizes.  */
      else if (atomic_load_relaxed (&av->have_fastchunks))
        {
          malloc_consolidate (av);
          /* restore original bin index */
          if (in_smallbin_range (nb))
            idx = smallbin_index (nb);
          else
            idx = largebin_index (nb);
        }

      /*
         Otherwise, relay to handle system-dependent cases
       */
      else
        {
          void *p = sysmalloc (nb, av);
          if (p != NULL)
            alloc_perturb (p, bytes);
          return p;
        }
    }
}

sysmalloc

sysmalloc start

If arena is null or the requested size is too big (and there are mmaps left permitted) use sysmalloc_mmap to allocate space and return it.

sysmalloc start
// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L2531

/*
   sysmalloc handles malloc cases requiring more memory from the system.
   On entry, it is assumed that av->top does not have enough
   space to service request for nb bytes, thus requiring that av->top
   be extended or replaced.
 */
 
 static void *
sysmalloc (INTERNAL_SIZE_T nb, mstate av)
{
  mchunkptr old_top;              /* incoming value of av->top */
  INTERNAL_SIZE_T old_size;       /* its size */
  char *old_end;                  /* its end address */

  long size;                      /* arg to first MORECORE or mmap call */
  char *brk;                      /* return value from MORECORE */

  long correction;                /* arg to 2nd MORECORE call */
  char *snd_brk;                  /* 2nd return val */

  INTERNAL_SIZE_T front_misalign; /* unusable bytes at front of new space */
  INTERNAL_SIZE_T end_misalign;   /* partial page left at end of new space */
  char *aligned_brk;              /* aligned offset into brk */

  mchunkptr p;                    /* the allocated/returned chunk */
  mchunkptr remainder;            /* remainder from allocation */
  unsigned long remainder_size;   /* its size */


  size_t pagesize = GLRO (dl_pagesize);
  bool tried_mmap = false;


  /*
     If have mmap, and the request size meets the mmap threshold, and
     the system supports mmap, and there are few enough currently
     allocated mmapped regions, try to directly map this request
     rather than expanding top.
   */

  if (av == NULL
      || ((unsigned long) (nb) >= (unsigned long) (mp_.mmap_threshold)
	  && (mp_.n_mmaps < mp_.n_mmaps_max)))
    {
      char *mm;
      if (mp_.hp_pagesize > 0 && nb >= mp_.hp_pagesize)
	{
	  /* There is no need to issue the THP madvise call if Huge Pages are
	     used directly.  */
	  mm = sysmalloc_mmap (nb, mp_.hp_pagesize, mp_.hp_flags, av);
	  if (mm != MAP_FAILED)
	    return mm;
	}
      mm = sysmalloc_mmap (nb, pagesize, 0, av);
      if (mm != MAP_FAILED)
	return mm;
      tried_mmap = true;
    }

  /* There are no usable arenas and mmap also failed.  */
  if (av == NULL)
    return 0;

sysmalloc checks

It starts by getting old top chunk information and checking that some of the following condations are true:

  • The old heap size is 0 (new heap)

  • The size of the previous heap is greater and MINSIZE and the old Top is in use

  • The heap is aligned to page size (0x1000 so the lower 12 bits need to be 0)

Then it also checks that:

  • The old size hasn't enough space to create a chunk for the requested size

sysmalloc checks
/* Record incoming configuration of top */

  old_top = av->top;
  old_size = chunksize (old_top);
  old_end = (char *) (chunk_at_offset (old_top, old_size));

  brk = snd_brk = (char *) (MORECORE_FAILURE);

  /*
     If not the first time through, we require old_size to be
     at least MINSIZE and to have prev_inuse set.
   */

  assert ((old_top == initial_top (av) && old_size == 0) ||
          ((unsigned long) (old_size) >= MINSIZE &&
           prev_inuse (old_top) &&
           ((unsigned long) old_end & (pagesize - 1)) == 0));

  /* Precondition: not enough current space to satisfy nb request */
  assert ((unsigned long) (old_size) < (unsigned long) (nb + MINSIZE));

sysmalloc not main arena

It'll first try to extend the previous heap for this heap. If not possible try to allocate a new heap and update the pointers to be able to use it. Finally if that didn't work, try calling sysmalloc_mmap.

sysmalloc not main arena
if (av != &main_arena)
    {
      heap_info *old_heap, *heap;
      size_t old_heap_size;

      /* First try to extend the current heap. */
      old_heap = heap_for_ptr (old_top);
      old_heap_size = old_heap->size;
      if ((long) (MINSIZE + nb - old_size) > 0
          && grow_heap (old_heap, MINSIZE + nb - old_size) == 0)
        {
          av->system_mem += old_heap->size - old_heap_size;
          set_head (old_top, (((char *) old_heap + old_heap->size) - (char *) old_top)
                    | PREV_INUSE);
        }
      else if ((heap = new_heap (nb + (MINSIZE + sizeof (*heap)), mp_.top_pad)))
        {
          /* Use a newly allocated heap.  */
          heap->ar_ptr = av;
          heap->prev = old_heap;
          av->system_mem += heap->size;
          /* Set up the new top.  */
          top (av) = chunk_at_offset (heap, sizeof (*heap));
          set_head (top (av), (heap->size - sizeof (*heap)) | PREV_INUSE);

          /* Setup fencepost and free the old top chunk with a multiple of
             MALLOC_ALIGNMENT in size. */
          /* The fencepost takes at least MINSIZE bytes, because it might
             become the top chunk again later.  Note that a footer is set
             up, too, although the chunk is marked in use. */
          old_size = (old_size - MINSIZE) & ~MALLOC_ALIGN_MASK;
          set_head (chunk_at_offset (old_top, old_size + CHUNK_HDR_SZ),
		    0 | PREV_INUSE);
          if (old_size >= MINSIZE)
            {
              set_head (chunk_at_offset (old_top, old_size),
			CHUNK_HDR_SZ | PREV_INUSE);
              set_foot (chunk_at_offset (old_top, old_size), CHUNK_HDR_SZ);
              set_head (old_top, old_size | PREV_INUSE | NON_MAIN_ARENA);
              _int_free (av, old_top, 1);
            }
          else
            {
              set_head (old_top, (old_size + CHUNK_HDR_SZ) | PREV_INUSE);
              set_foot (old_top, (old_size + CHUNK_HDR_SZ));
            }
        }
      else if (!tried_mmap)
	{
	  /* We can at least try to use to mmap memory.  If new_heap fails
	     it is unlikely that trying to allocate huge pages will
	     succeed.  */
	  char *mm = sysmalloc_mmap (nb, pagesize, 0, av);
	  if (mm != MAP_FAILED)
	    return mm;
	}
    }

sysmalloc main arena

It starts calculating the amount of memory needed. It'll start by requesting contiguous memory so in this case it'll be possible to use the old memory not used. Also some align operations are performed.

sysmalloc main arena
// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L2665C1-L2713C10

  else     /* av == main_arena */


    { /* Request enough space for nb + pad + overhead */
      size = nb + mp_.top_pad + MINSIZE;

      /*
         If contiguous, we can subtract out existing space that we hope to
         combine with new space. We add it back later only if
         we don't actually get contiguous space.
       */

      if (contiguous (av))
        size -= old_size;

      /*
         Round to a multiple of page size or huge page size.
         If MORECORE is not contiguous, this ensures that we only call it
         with whole-page arguments.  And if MORECORE is contiguous and
         this is not first time through, this preserves page-alignment of
         previous calls. Otherwise, we correct to page-align below.
       */

#ifdef MADV_HUGEPAGE
      /* Defined in brk.c.  */
      extern void *__curbrk;
      if (__glibc_unlikely (mp_.thp_pagesize != 0))
	{
	  uintptr_t top = ALIGN_UP ((uintptr_t) __curbrk + size,
				    mp_.thp_pagesize);
	  size = top - (uintptr_t) __curbrk;
	}
      else
#endif
	size = ALIGN_UP (size, GLRO(dl_pagesize));

      /*
         Don't try to call MORECORE if argument is so big as to appear
         negative. Note that since mmap takes size_t arg, it may succeed
         below even if we cannot call MORECORE.
       */

      if (size > 0)
        {
          brk = (char *) (MORECORE (size));
	  if (brk != (char *) (MORECORE_FAILURE))
	    madvise_thp (brk, size);
          LIBC_PROBE (memory_sbrk_more, 2, brk, size);
        }

sysmalloc main arena previous error 1

If the previous returned MORECORE_FAILURE, try agin to allocate memory using sysmalloc_mmap_fallback

sysmalloc main arena previous error 1
// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L2715C7-L2740C10

if (brk == (char *) (MORECORE_FAILURE))
        {
          /*
             If have mmap, try using it as a backup when MORECORE fails or
             cannot be used. This is worth doing on systems that have "holes" in
             address space, so sbrk cannot extend to give contiguous space, but
             space is available elsewhere.  Note that we ignore mmap max count
             and threshold limits, since the space will not be used as a
             segregated mmap region.
           */

	  char *mbrk = MAP_FAILED;
	  if (mp_.hp_pagesize > 0)
	    mbrk = sysmalloc_mmap_fallback (&size, nb, old_size,
					    mp_.hp_pagesize, mp_.hp_pagesize,
					    mp_.hp_flags, av);
	  if (mbrk == MAP_FAILED)
	    mbrk = sysmalloc_mmap_fallback (&size, nb, old_size, MMAP_AS_MORECORE_SIZE,
					    pagesize, 0, av);
	  if (mbrk != MAP_FAILED)
	    {
	      /* We do not need, and cannot use, another sbrk call to find end */
	      brk = mbrk;
	      snd_brk = brk + size;
	    }
        }

sysmalloc main arena continue

If the previous didn't return MORECORE_FAILURE, if it worked create some alignments:

sysmalloc main arena previous error 2
// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L2742

if (brk != (char *) (MORECORE_FAILURE))
        {
          if (mp_.sbrk_base == 0)
            mp_.sbrk_base = brk;
          av->system_mem += size;

          /*
             If MORECORE extends previous space, we can likewise extend top size.
           */

          if (brk == old_end && snd_brk == (char *) (MORECORE_FAILURE))
            set_head (old_top, (size + old_size) | PREV_INUSE);

          else if (contiguous (av) && old_size && brk < old_end)
	    /* Oops!  Someone else killed our space..  Can't touch anything.  */
	    malloc_printerr ("break adjusted to free malloc space");

          /*
             Otherwise, make adjustments:

           * If the first time through or noncontiguous, we need to call sbrk
              just to find out where the end of memory lies.

           * We need to ensure that all returned chunks from malloc will meet
              MALLOC_ALIGNMENT

           * If there was an intervening foreign sbrk, we need to adjust sbrk
              request size to account for fact that we will not be able to
              combine new space with existing space in old_top.

           * Almost all systems internally allocate whole pages at a time, in
              which case we might as well use the whole last page of request.
              So we allocate enough more memory to hit a page boundary now,
              which in turn causes future contiguous calls to page-align.
           */

          else
            {
              front_misalign = 0;
              end_misalign = 0;
              correction = 0;
              aligned_brk = brk;

              /* handle contiguous cases */
              if (contiguous (av))
                {
                  /* Count foreign sbrk as system_mem.  */
                  if (old_size)
                    av->system_mem += brk - old_end;

                  /* Guarantee alignment of first new chunk made from this space */

                  front_misalign = (INTERNAL_SIZE_T) chunk2mem (brk) & MALLOC_ALIGN_MASK;
                  if (front_misalign > 0)
                    {
                      /*
                         Skip over some bytes to arrive at an aligned position.
                         We don't need to specially mark these wasted front bytes.
                         They will never be accessed anyway because
                         prev_inuse of av->top (and any chunk created from its start)
                         is always true after initialization.
                       */

                      correction = MALLOC_ALIGNMENT - front_misalign;
                      aligned_brk += correction;
                    }

                  /*
                     If this isn't adjacent to existing space, then we will not
                     be able to merge with old_top space, so must add to 2nd request.
                   */

                  correction += old_size;

                  /* Extend the end address to hit a page boundary */
                  end_misalign = (INTERNAL_SIZE_T) (brk + size + correction);
                  correction += (ALIGN_UP (end_misalign, pagesize)) - end_misalign;

                  assert (correction >= 0);
                  snd_brk = (char *) (MORECORE (correction));

                  /*
                     If can't allocate correction, try to at least find out current
                     brk.  It might be enough to proceed without failing.

                     Note that if second sbrk did NOT fail, we assume that space
                     is contiguous with first sbrk. This is a safe assumption unless
                     program is multithreaded but doesn't use locks and a foreign sbrk
                     occurred between our first and second calls.
                   */

                  if (snd_brk == (char *) (MORECORE_FAILURE))
                    {
                      correction = 0;
                      snd_brk = (char *) (MORECORE (0));
                    }
		  else
		    madvise_thp (snd_brk, correction);
                }

              /* handle non-contiguous cases */
              else
                {
                  if (MALLOC_ALIGNMENT == CHUNK_HDR_SZ)
                    /* MORECORE/mmap must correctly align */
                    assert (((unsigned long) chunk2mem (brk) & MALLOC_ALIGN_MASK) == 0);
                  else
                    {
                      front_misalign = (INTERNAL_SIZE_T) chunk2mem (brk) & MALLOC_ALIGN_MASK;
                      if (front_misalign > 0)
                        {
                          /*
                             Skip over some bytes to arrive at an aligned position.
                             We don't need to specially mark these wasted front bytes.
                             They will never be accessed anyway because
                             prev_inuse of av->top (and any chunk created from its start)
                             is always true after initialization.
                           */

                          aligned_brk += MALLOC_ALIGNMENT - front_misalign;
                        }
                    }

                  /* Find out current end of memory */
                  if (snd_brk == (char *) (MORECORE_FAILURE))
                    {
                      snd_brk = (char *) (MORECORE (0));
                    }
                }

              /* Adjust top based on results of second sbrk */
              if (snd_brk != (char *) (MORECORE_FAILURE))
                {
                  av->top = (mchunkptr) aligned_brk;
                  set_head (av->top, (snd_brk - aligned_brk + correction) | PREV_INUSE);
                  av->system_mem += correction;

                  /*
                     If not the first time through, we either have a
                     gap due to foreign sbrk or a non-contiguous region.  Insert a
                     double fencepost at old_top to prevent consolidation with space
                     we don't own. These fenceposts are artificial chunks that are
                     marked as inuse and are in any case too small to use.  We need
                     two to make sizes and alignments work out.
                   */

                  if (old_size != 0)
                    {
                      /*
                         Shrink old_top to insert fenceposts, keeping size a
                         multiple of MALLOC_ALIGNMENT. We know there is at least
                         enough space in old_top to do this.
                       */
                      old_size = (old_size - 2 * CHUNK_HDR_SZ) & ~MALLOC_ALIGN_MASK;
                      set_head (old_top, old_size | PREV_INUSE);

                      /*
                         Note that the following assignments completely overwrite
                         old_top when old_size was previously MINSIZE.  This is
                         intentional. We need the fencepost, even if old_top otherwise gets
                         lost.
                       */
		      set_head (chunk_at_offset (old_top, old_size),
				CHUNK_HDR_SZ | PREV_INUSE);
		      set_head (chunk_at_offset (old_top,
						 old_size + CHUNK_HDR_SZ),
				CHUNK_HDR_SZ | PREV_INUSE);

                      /* If possible, release the rest. */
                      if (old_size >= MINSIZE)
                        {
                          _int_free (av, old_top, 1);
                        }
                    }
                }
            }
        }
    } /* if (av !=  &main_arena) */

sysmalloc finale

Finish the allocation updating the arena information

// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L2921C3-L2943C12

if ((unsigned long) av->system_mem > (unsigned long) (av->max_system_mem))
    av->max_system_mem = av->system_mem;
  check_malloc_state (av);

  /* finally, do the allocation */
  p = av->top;
  size = chunksize (p);

  /* check that one of the above allocation paths succeeded */
  if ((unsigned long) (size) >= (unsigned long) (nb + MINSIZE))
    {
      remainder_size = size - nb;
      remainder = chunk_at_offset (p, nb);
      av->top = remainder;
      set_head (p, nb | PREV_INUSE | (av != &main_arena ? NON_MAIN_ARENA : 0));
      set_head (remainder, remainder_size | PREV_INUSE);
      check_malloced_chunk (av, p, nb);
      return chunk2mem (p);
    }

  /* catch all failure paths */
  __set_errno (ENOMEM);
  return 0;

sysmalloc_mmap

sysmalloc_mmap code
// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L2392C1-L2481C2

static void *
sysmalloc_mmap (INTERNAL_SIZE_T nb, size_t pagesize, int extra_flags, mstate av)
{
  long int size;

  /*
    Round up size to nearest page.  For mmapped chunks, the overhead is one
    SIZE_SZ unit larger than for normal chunks, because there is no
    following chunk whose prev_size field could be used.

    See the front_misalign handling below, for glibc there is no need for
    further alignments unless we have have high alignment.
   */
  if (MALLOC_ALIGNMENT == CHUNK_HDR_SZ)
    size = ALIGN_UP (nb + SIZE_SZ, pagesize);
  else
    size = ALIGN_UP (nb + SIZE_SZ + MALLOC_ALIGN_MASK, pagesize);

  /* Don't try if size wraps around 0.  */
  if ((unsigned long) (size) <= (unsigned long) (nb))
    return MAP_FAILED;

  char *mm = (char *) MMAP (0, size,
			    mtag_mmap_flags | PROT_READ | PROT_WRITE,
			    extra_flags);
  if (mm == MAP_FAILED)
    return mm;

#ifdef MAP_HUGETLB
  if (!(extra_flags & MAP_HUGETLB))
    madvise_thp (mm, size);
#endif

  __set_vma_name (mm, size, " glibc: malloc");

  /*
    The offset to the start of the mmapped region is stored in the prev_size
    field of the chunk.  This allows us to adjust returned start address to
    meet alignment requirements here and in memalign(), and still be able to
    compute proper address argument for later munmap in free() and realloc().
   */

  INTERNAL_SIZE_T front_misalign; /* unusable bytes at front of new space */

  if (MALLOC_ALIGNMENT == CHUNK_HDR_SZ)
    {
      /* For glibc, chunk2mem increases the address by CHUNK_HDR_SZ and
	 MALLOC_ALIGN_MASK is CHUNK_HDR_SZ-1.  Each mmap'ed area is page
	 aligned and therefore definitely MALLOC_ALIGN_MASK-aligned.  */
      assert (((INTERNAL_SIZE_T) chunk2mem (mm) & MALLOC_ALIGN_MASK) == 0);
      front_misalign = 0;
    }
  else
    front_misalign = (INTERNAL_SIZE_T) chunk2mem (mm) & MALLOC_ALIGN_MASK;

  mchunkptr p;                    /* the allocated/returned chunk */

  if (front_misalign > 0)
    {
      ptrdiff_t correction = MALLOC_ALIGNMENT - front_misalign;
      p = (mchunkptr) (mm + correction);
      set_prev_size (p, correction);
      set_head (p, (size - correction) | IS_MMAPPED);
    }
  else
    {
      p = (mchunkptr) mm;
      set_prev_size (p, 0);
      set_head (p, size | IS_MMAPPED);
    }

  /* update statistics */
  int new = atomic_fetch_add_relaxed (&mp_.n_mmaps, 1) + 1;
  atomic_max (&mp_.max_n_mmaps, new);

  unsigned long sum;
  sum = atomic_fetch_add_relaxed (&mp_.mmapped_mem, size) + size;
  atomic_max (&mp_.max_mmapped_mem, sum);

  check_chunk (av, p);

  return chunk2mem (p);
}

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