/* We overlay this structure on the user-data portion of a chunk when
the chunk is stored in the per-thread cache. */
typedef struct tcache_entry
{
struct tcache_entry *next;
} tcache_entry;
tcache_entry 用于链接空闲的 chunk 结构体，其中的 next 指针指向下一个大小相同的 chunk。
需要注意的是这里的 next 指向 chunk 的 user data，而 fastbin 的 fd 指向 chunk 开头的地址。
而且，tcache_entry 会复用空闲 chunk 的 user data 部分。
tcache_perthread_struct
/* There is one of these for each thread, which contains the
per-thread cache (hence "tcache_perthread_struct"). Keeping
overall size low is mildly important. Note that COUNTS and ENTRIES
are redundant (we could have just counted the linked list each
time), this is for performance reasons. */
typedef struct tcache_perthread_struct
{
char counts[TCACHE_MAX_BINS];
tcache_entry *entries[TCACHE_MAX_BINS];
} tcache_perthread_struct;
# define TCACHE_MAX_BINS 64
static __thread tcache_perthread_struct *tcache = NULL;
tcache_init(void)
{
mstate ar_ptr;
void *victim = 0;
const size_t bytes = sizeof (tcache_perthread_struct);
if (tcache_shutting_down)
return;
arena_get (ar_ptr, bytes); // 找到可用的 arena
victim = _int_malloc (ar_ptr, bytes); // 申请一个 sizeof(tcache_perthread_struct) 大小的 chunk
if (!victim && ar_ptr != NULL)
{
ar_ptr = arena_get_retry (ar_ptr, bytes);
victim = _int_malloc (ar_ptr, bytes);
}
if (ar_ptr != NULL)
__libc_lock_unlock (ar_ptr->mutex);
/* In a low memory situation, we may not be able to allocate memory
- in which case, we just keep trying later. However, we
typically do this very early, so either there is sufficient
memory, or there isn't enough memory to do non-trivial
allocations anyway. */
if (victim) // 初始化 tcache
{
tcache = (tcache_perthread_struct *) victim;
memset (tcache, 0, sizeof (tcache_perthread_struct));
}
}
