_libc_free

void
__libc_free (void *mem)
{mstate ar_ptr;mchunkptr p;                          /* chunk corresponding to mem *//* 判断__free_hook中是否有值，若有值则将其当作函数指针调用 */void (*hook) (void *, const void *)= atomic_forced_read (__free_hook);if (__builtin_expect (hook != NULL, 0)){(*hook)(mem, RETURN_ADDRESS (0));return;}/* 若需要回收的目标指针为NULL，直接返回 */if (mem == 0)                              /* free(0) has no effect */return;/* 获取用户部分指针对应的chunk头的地址 */p = mem2chunk (mem);/* 若malloc_chunk->size字段的M位为1，表示当前chunk是通过mmap映射的 */if (chunk_is_mmapped (p))                       /* release mmapped memory. */{/* see if the dynamic brk/mmap threshold needs adjusting */if (!mp_.no_dyn_threshold&& p->size > mp_.mmap_threshold&& p->size <= DEFAULT_MMAP_THRESHOLD_MAX){mp_.mmap_threshold = chunksize (p);mp_.trim_threshold = 2 * mp_.mmap_threshold;LIBC_PROBE (memory_mallopt_free_dyn_thresholds, 2,mp_.mmap_threshold, mp_.trim_threshold);}munmap_chunk(p);  /* 释放映射的内存 */return;}/* 若chunk不是mmap映射的，则可能位于arena,因此尝试获取当前chunk所属的arena */ar_ptr = arena_for_chunk (p);_int_free (ar_ptr, p, 0);     /* 调用_int_free */
}
libc_hidden_def (__libc_free)

_int_free

函数定义

static void
_int_free(mstate av, mchunkptr p, int have_lock)
{

局部变量

 /* 存储size字段的值 */INTERNAL_SIZE_T size;        /* its size *//* 存储fast bin堆块指针 */mfastbinptr* fb;             /* associated fastbin *//* 下一个堆块指针 */mchunkptr nextchunk;         /* next contiguous chunk *//* 下一个堆块大小 */INTERNAL_SIZE_T nextsize;    /* its size *//* 下一个堆块使用情况 */int nextinuse;               /* true if nextchunk is used *//* 距离上一个堆块距离 */INTERNAL_SIZE_T prevsize;    /* size of previous contiguous chunk *//* 临时，bk指针指向的堆块 */mchunkptr bck;               /* misc temp for linking *//* 临时，fd指针指向的堆块 */mchunkptr fwd;               /* misc temp for linking */const char* errstr = NULL;int locked = 0;

start

 /* 获取目标chunk的size字段的值 */size = chunksize(p);/* Little security check which won't hurt performance: theallocator never wrapps around at the end of the address space.Therefore we can exclude some size values which might appearhere by accident or by "design" from some intruder.  *//* 安全检查，检查chunk指针合法性，是否对齐等，不会影响性能 */if (__builtin_expect((uintptr_t)p > (uintptr_t)-size, 0)|| __builtin_expect(misaligned_chunk(p), 0)){errstr = "free(): invalid pointer";errout:if (!have_lock && locked)(void)mutex_unlock(&av->mutex);malloc_printerr(check_action, errstr, chunk2mem(p), av);return;}/* We know that each chunk is at least MINSIZE bytes in size or amultiple of MALLOC_ALIGNMENT.  *//* 判断size字段是否小于chunk的最小size，以及是否对齐（第四个比特位是否为1(不能为1)） */if (__glibc_unlikely(size < MINSIZE || !aligned_OK(size))){errstr = "free(): invalid size";goto errout;}/* 判断chunk是否处于空闲状态（检查下一个chunk的p位） */check_inuse_chunk(av, p);

fast bins部分

    /*If eligible, place chunk on a fastbin so it can be foundand used quickly in malloc.*//* 判断chunk是否小于max_fast（0x80） */if ((unsigned long)(size) <= (unsigned long)(get_max_fast())#if TRIM_FASTBINS/*If TRIM_FASTBINS set, don't place chunksbordering top into fastbins*//* 如果设置了TRIM_FASTBINS，就不能将与top chunk相邻的chunk放入fast bins */&& (chunk_at_offset(p, size) != av->top)
#endif) {/* 检查chunk大小，是否对齐 */if (__builtin_expect(chunk_at_offset(p, size)->size <= 2 * SIZE_SZ, 0)|| __builtin_expect(chunksize(chunk_at_offset(p, size))>= av->system_mem, 0)){/* We might not have a lock at this point and concurrent modificationsof system_mem might have let to a false positive.  Redo the testafter getting the lock.  */if (have_lock|| ({ assert(locked == 0);mutex_lock(&av->mutex);locked = 1;chunk_at_offset(p, size)->size <= 2 * SIZE_SZ|| chunksize(chunk_at_offset(p, size)) >= av->system_mem;})){errstr = "free(): invalid next size (fast)";goto errout;}if (!have_lock){(void)mutex_unlock(&av->mutex);locked = 0;}}/* 特定条件下初始化memory，一般无需关注 */free_perturb(chunk2mem(p), size - 2 * SIZE_SZ);/* 将chunk放入fast bins中 */set_fastchunks(av);unsigned int idx = fastbin_index(size);fb = &fastbin(av, idx);/* Atomically link P to its fastbin: P->FD = *FB; *FB = P;  */mchunkptr old = *fb, old2;unsigned int old_idx = ~0u;do{/* Check that the top of the bin is not the record we are going to add(i.e., double free).  *//* double free检测（检查当前chunk是否为fast bins的最后一个成员） */if (__builtin_expect(old == p, 0)){errstr = "double free or corruption (fasttop)";goto errout;}/* Check that size of fastbin chunk at the top is the same assize of the chunk that we are adding.  We can dereference OLDonly if we have the lock, otherwise it might have already beendeallocated.  See use of OLD_IDX below for the actual check.  */if (have_lock && old != NULL)old_idx = fastbin_index(chunksize(old));p->fd = old2 = old;} while ((old = catomic_compare_and_exchange_val_rel(fb, p, old2)) != old2);if (have_lock && old != NULL && __builtin_expect(old_idx != idx, 0)){errstr = "invalid fastbin entry (free)";goto errout;}}

unsorted bins部分

    /*Consolidate other non-mmapped chunks as they arrive.*//* 如果chunk是通过mmap映射的（检查M位）不是则执行这一块代码chunk会进入unsorted bins */else if (!chunk_is_mmapped(p)) {if (!have_lock) {(void)mutex_lock(&av->mutex);locked = 1;}/* 定位下一个chunk位置 */nextchunk = chunk_at_offset(p, size);/* Lightweight tests: check whether the block is already thetop block.  *//* 如果申请释放的堆块为top chunk，触发异常 */if (__glibc_unlikely(p == av->top)){errstr = "double free or corruption (top)";goto errout;}/* Or whether the next chunk is beyond the boundaries of the arena.  *//* 检查下一个堆块地址是否超过top chunk地址，超过则触发异常 */if (__builtin_expect(contiguous(av)&& (char*)nextchunk>= ((char*)av->top + chunksize(av->top)), 0)){errstr = "double free or corruption (out)";goto errout;}/* Or whether the block is actually not marked used.  *//* 若当前堆块已经处于空闲状态，触发异常 */if (__glibc_unlikely(!prev_inuse(nextchunk))){errstr = "double free or corruption (!prev)";goto errout;}/* 检查size位大小是否合法与对齐情况 */nextsize = chunksize(nextchunk);if (__builtin_expect(nextchunk->size <= 2 * SIZE_SZ, 0)|| __builtin_expect(nextsize >= av->system_mem, 0)){errstr = "free(): invalid next size (normal)";goto errout;}/* 特定条件下初始化chunk数据 */free_perturb(chunk2mem(p), size - 2 * SIZE_SZ);/* consolidate backward *//* 若下一个chunk的p位也为1的话，则进行Unlink，向上合并 */if (!prev_inuse(p)) {prevsize = p->prev_size;size += prevsize;p = chunk_at_offset(p, -((long)prevsize));unlink(av, p, bck, fwd);}/* 如果下一个chunk不是top chunk的话 */if (nextchunk != av->top) {/* get and clear inuse bit *//* 获取下一个chunk p位的值 */nextinuse = inuse_bit_at_offset(nextchunk, nextsize);/* consolidate forward *//* 如果下一个chunk也处于空闲状态的话，unlink，向下合并 */if (!nextinuse) {unlink(av, nextchunk, bck, fwd);size += nextsize;}else    /* 否则将下一个chunk的p位置为0，表示当前chunk已经被释放 */clear_inuse_bit_at_offset(nextchunk, 0);/*Place the chunk in unsorted chunk list. Chunks arenot placed into regular bins until after they havebeen given one chance to be used in malloc.*//* 得到arena中bins指针 */bck = unsorted_chunks(av);fwd = bck->fd;    /* 指向最后一个进入unsorted bins的指针 *//* 如果最后一个堆块的bk不是指向bins的话，触发异常 */if (__glibc_unlikely(fwd->bk != bck)){errstr = "free(): corrupted unsorted chunks";goto errout;}/* 将当前chunk挂入unsorted bins */p->fd = fwd;p->bk = bck;if (!in_smallbin_range(size)) /* 若chunk属于Large bins范围，将fd_nextsize和bk_nextsize置为NULL */{p->fd_nextsize = NULL;p->bk_nextsize = NULL;}bck->fd = p;fwd->bk = p;/* 设置标志位 */set_head(p, size | PREV_INUSE);set_foot(p, size);check_free_chunk(av, p);  /* 包含各种检测 */}/*If the chunk borders the current high end of memory,consolidate into top*//* 如果当前chunk是堆中的唯一一个chunk，且位于堆地址顶部，则与top chunk合并 */else {size += nextsize;set_head(p, size | PREV_INUSE);av->top = p;check_chunk(av, p);}/*If freeing a large space, consolidate possibly-surroundingchunks. Then, if the total unused topmost memory exceeds trimthreshold, ask malloc_trim to reduce top.Unless max_fast is 0, we don't know if there are fastbinsbordering top, so we cannot tell for sure whether thresholdhas been reached unless fastbins are consolidated.  But wedon't want to consolidate on each free.  As a compromise,consolidation is performed if FASTBIN_CONSOLIDATION_THRESHOLDis reached.*//* 如果size字段的值大于65536，则合并堆中所有空闲的fast bin */if ((unsigned long)(size) >= FASTBIN_CONSOLIDATION_THRESHOLD) {if (have_fastchunks(av))malloc_consolidate(av);/* 如果当前arena为main_arena，且top chunk大小超过阈值，则切割arena */if (av == &main_arena) {#ifndef MORECORE_CANNOT_TRIMif ((unsigned long)(chunksize(av->top)) >=(unsigned long)(mp_.trim_threshold))systrim(mp_.top_pad, av);
#endif}else {/* Always try heap_trim(), even if the top chunk is notlarge, because the corresponding heap might go away.  *//* 不论如何，尝试切割堆 */heap_info* heap = heap_for_ptr(top(av));assert(heap->ar_ptr == av);heap_trim(heap, mp_.top_pad);}}/* 解除互斥锁 */if (!have_lock) {assert(locked);(void)mutex_unlock(&av->mutex);}}

mmap部分

    /*If the chunk was allocated via mmap, release via munmap().*//* 不符合以上任何一种情况，直接解除内存映射 */else {munmap_chunk(p);}
}

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glibc-2.23学习笔记（二）—— free部分源码分析

glibc-2.23学习笔记（二）—— free部分源码分析

_libc_free

_int_free

函数定义

局部变量

start

fast bins部分

unsorted bins部分

mmap部分

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