My question is about glibc.
What is the name of the bk member an abbreviation for within malloc_chunk?
struct malloc_chunk {
INTERNAL_SIZE_T mchunk_prev_size; /* Size of previous chunk (if free). */
INTERNAL_SIZE_T mchunk_size; /* Size in bytes, including overhead. */
struct malloc_chunk* fd; /* double links -- used only if free. */
struct malloc_chunk* bk;
/* Only used for large blocks: pointer to next larger size. */
struct malloc_chunk* fd_nextsize; /* double links -- used only if free. */
struct malloc_chunk* bk_nextsize;
};
Refer to: https://code.woboq.org/userspace/glibc/malloc/malloc.c.html#malloc_chunk
The fd and bk members are the forward and backward pointers in the doubly-linked list.
In other words, fd is the next pointer and bk is the previous pointer, as illustrated in the following three-element list:
forward links -->
_(fd)_ _(fd)_
/ \ / \ _(fd)-> NULL
/ V / V /
head ---> item1 item2 item3
/ ^ / ^ /
NULL <-(bk)_/ \_(bk)_/ \_(bk)_/
<-- backward links
Note that this particular example has a head and tail, with end element pointing to NULL. That's probably not the case for the circular list maintained by glibc (since it's a circular list), it's just used to show one method of implementation.
In fact, the link you provide in your question states this in no uncertain terms, starting fifty-odd lines beyond the structure definition (slightly paraphrased):
Free chunks are stored in circular
doubly-linked lists, and look
like this:
+------------------------------------------+
| Size of previous chunk, if unallocated | <- mchunk_prev_size
+------------------------------------------+
| Size of chunk, in bytes (and some flags) | <- mchunk_size
+------------------------------------------+
| Forward pointer to next chunk in list | <- fd
+------------------------------------------+
| Back pointer to previous chunk in list | <- bk
+------------------------------------------+
: <other stuff> :