VLC的block_t

2019-11-09 16:53:19

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0 VLC的block_t

1 发现问题

payload 数据内存起始位置是p_buffer, 大小是i_buffer , block本身也有个内存起始位置 p_start ，大小i_size ，令人疑惑。

struct block_t{ block_t *p_next; uint8_t *p_buffer; /**< Payload start */ size_t i_buffer; /**< Payload length */ uint8_t *p_start; /**< Buffer start */ size_t i_size; /**< Buffer total size */ uint32_t i_flags; unsigned i_nb_samples; /* Used for audio */ mtime_t i_pts; mtime_t i_dts; mtime_t i_length; /* Rudimentary support for overloading block (de)allocation. */ block_free_t pf_release;};

2 init构造

构造时发现payload数据内存位置和block缓冲的内存位置初始化为一样的。大小值也是一样的。

void block_Init( block_t *restrict b, void *buf, size_t size ){ /* Fill all fields to their default */ b->p_next = NULL; b->p_buffer = buf; b->i_buffer = size; b->p_start = buf; b->i_size = size; b->i_flags = 0; b->i_nb_samples = 0; b->i_pts = b->i_dts = VLC_TS_INVALID; b->i_length = 0;#ifndef NDEBUG b->pf_release = BlockNoRelease;#endif}

3 实际分配alloc

实际block分配的时候，占用了更多内存, 传递给p_start 和p_buffer的是跳过了block_t 大小（64字节）内存的内存地址；相应的，i_size和i_buffer,也减少了 block_t大小。如果用户要求分配size大小，实际分配的是size+64字节+32（对齐）0+32（头部填充）+32（尾部填充）=size+160（字节），block_t内部拥有的内存大小是size+96字节。

block_t *block_Alloc (size_t size){ /* 2 * BLOCK_PADDING: PRe + post padding */ const size_t alloc = sizeof (block_t)/*64*/ + BLOCK_ALIGN + (2 * BLOCK_PADDING) + size; if (unlikely(alloc <= size)) return NULL; block_t *b = malloc (alloc); if (unlikely(b == NULL)) return NULL; block_Init (b, b + 1, alloc - sizeof (*b)); static_assert ((BLOCK_PADDING % BLOCK_ALIGN) == 0, "BLOCK_PADDING must be a multiple of BLOCK_ALIGN"); b->p_buffer += BLOCK_PADDING + BLOCK_ALIGN - 1; b->p_buffer = (void *)(((uintptr_t)b->p_buffer) & ~(BLOCK_ALIGN - 1)); b->i_buffer = size; b->pf_release = block_generic_Release; return b;}

4 最后还要对齐和调整

参考 [http://blog.csdn.net/fuzhuo233/article/details/8182335] 线性内存分配器的实现 /TODO/

static_assert ((BLOCK_PADDING % BLOCK_ALIGN) == 0, "BLOCK_PADDING must be a multiple of BLOCK_ALIGN"); b->p_buffer += BLOCK_PADDING + BLOCK_ALIGN - 1; b->p_buffer = (void *)(((uintptr_t)b->p_buffer) & ~(BLOCK_ALIGN - 1));

payload的缓冲b->p_buffer 还是要后移然后对齐 payload的缓冲大小还是要等于用户需要的size大小。那这样的话，block的缓冲地址start和size就与payload的区分开了。

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