チャンクの格納効率を向上させるために、各チャンクは1つのリンクリストにだけではなく、複数のタイプが存在します。これらはビンであり、5種類のビンがあります：62 small bins、63 large bins、1 unsorted bin、10 fast bins、およびスレッドごとに64個のtcache binsがあります。
以下のコードでは、max binsとchunks per index、**tcache_entry構造体が作成されて二重解放を避けるために使用されており、また、各スレッドがビンの各インデックスのアドレスを格納するために使用するtcache_perthread_struct**という構造体が見られます。
// From https://github.com/bminor/glibc/blob/a07e000e82cb71238259e674529c37c12dc7d423/malloc/malloc.c#L1711/*FastbinsAn array of lists holding recently freed small chunks. Fastbinsare not doubly linked. It is faster to single-link them, andsince chunks are never removed from the middles of these lists,double linking is not necessary. Also, unlike regular bins, theyare not even processed in FIFO order (they use faster LIFO) sinceordering doesn't much matter in the transient contexts in whichfastbins are normally used.Chunks in fastbins keep their inuse bit set, so they cannotbe consolidated with other free chunks. malloc_consolidatereleases all chunks in fastbins and consolidates them withother free chunks.*/typedefstruct malloc_chunk *mfastbinptr;#definefastbin(ar_ptr, idx) ((ar_ptr)->fastbinsY[idx])/* offset 2 to use otherwise unindexable first 2 bins */#definefastbin_index(sz) \((((unsignedint) (sz)) >> (SIZE_SZ ==8?4:3)) -2)/* The maximum fastbin request size we support */#defineMAX_FAST_SIZE (80* SIZE_SZ /4)#defineNFASTBINS (fastbin_index (request2size (MAX_FAST_SIZE)) +1)
// From https://github.com/bminor/glibc/blob/a07e000e82cb71238259e674529c37c12dc7d423/malloc/malloc.c#L1711/*TopThe top-most available chunk (i.e., the one bordering the end ofavailable memory) is treated specially. It is never included inany bin, is used only if no other chunk is available, and isreleased back to the system if it is very large (seeM_TRIM_THRESHOLD). Because top initiallypoints to its own bin with initial zero size, thus forcingextension on the first malloc request, we avoid having any specialcode in malloc to check whether it even exists yet. But we stillneed to do so when getting memory from system, so we makeinitial_top treat the bin as a legal but unusable chunk during theinterval between initialization and the first call tosysmalloc. (This is somewhat delicate, since it relies onthe 2 preceding words to be zero during this interval as well.)*//* Conveniently, the unsorted bin can be used as dummy top on first call */#defineinitial_top(M) (unsorted_chunks (M))
基本的に、これは現在利用可能なヒープ全体を含むチャンクです。mallocが実行されると、使用可能なフリーチャンクがない場合、このトップチャンクは必要なスペースを提供するためにサイズを減らします。
Top Chunkへのポインタはmalloc_state構造体に格納されています。
