lievent源码分析：evbuffer

Posted 2020-08-08 冷冰若水

struct evbuffer定义在evbuffer-internal.h文件中。

evbuffer结构内部保存一个以evbuffer-chain结构为节点的链表，evbuffer内部有两个分别指向首尾节点的指针。

 1 struct evbuffer {
 2     /** The first chain in this buffer‘s linked list of chains. */
 3     struct evbuffer_chain *first;
 4     /** The last chain in this buffer‘s linked list of chains. */
 5     struct evbuffer_chain *last;
 6 
 7     /** Pointer to the next pointer pointing at the ‘last_with_data‘ chain.
 8      *
 9      * To unpack:
10      *
11      * The last_with_data chain is the last chain that has any data in it.
12      * If all chains in the buffer are empty, it is the first chain.
13      * If the buffer has no chains, it is NULL.
14      *
15      * The last_with_datap pointer points at _whatever ‘next‘ pointer_
16      * points at the last_with_datap chain.  If the last_with_data chain
17      * is the first chain, or it is NULL, then the last_with_datap pointer
18      * is &buf->first.
19      */
20     struct evbuffer_chain **last_with_datap;
21 
22     /** Total amount of bytes stored in all chains.*/
23     size_t total_len;
24 
25     /** Number of bytes we have added to the buffer since we last tried to
26      * invoke callbacks. */
27     size_t n_add_for_cb;
28     /** Number of bytes we have removed from the buffer since we last
29      * tried to invoke callbacks. */
30     size_t n_del_for_cb;
31 
32 #ifndef EVENT__DISABLE_THREAD_SUPPORT
33     /** A lock used to mediate access to this buffer. */
34     void *lock;
35 #endif
36     /** True iff we should free the lock field when we free this
37      * evbuffer. */
38     unsigned own_lock : 1;
39     /** True iff we should not allow changes to the front of the buffer
40      * (drains or prepends). */
41     unsigned freeze_start : 1;
42     /** True iff we should not allow changes to the end of the buffer
43      * (appends) */
44     unsigned freeze_end : 1;
45     /** True iff this evbuffer‘s callbacks are not invoked immediately
46      * upon a change in the buffer, but instead are deferred to be invoked
47      * from the event_base‘s loop.    Useful for preventing enormous stack
48      * overflows when we have mutually recursive callbacks, and for
49      * serializing callbacks in a single thread. */
50     unsigned deferred_cbs : 1;
51 #ifdef _WIN32
52     /** True iff this buffer is set up for overlapped IO. */
53     unsigned is_overlapped : 1;
54 #endif
55     /** Zero or more EVBUFFER_FLAG_* bits */
56     ev_uint32_t flags;
57 
58     /** Used to implement deferred callbacks. */
59     struct event_base *cb_queue;
60 
61     /** A reference count on this evbuffer.     When the reference count
62      * reaches 0, the buffer is destroyed.    Manipulated with
63      * evbuffer_incref and evbuffer_decref_and_unlock and
64      * evbuffer_free. */
65     int refcnt;
66 
67     /** A struct event_callback handle to make all of this buffer‘s callbacks
68      * invoked from the event loop. */
69     struct event_callback deferred;
70 
71     /** A doubly-linked-list of callback functions */
72     LIST_HEAD(evbuffer_cb_queue, evbuffer_cb_entry) callbacks;
73 
74     /** The parent bufferevent object this evbuffer belongs to.
75      * NULL if the evbuffer stands alone. */
76     struct bufferevent *parent;
77 };

struct evbuffer_chain：

evbuffer-chain结构内部保存一个表示buffer内容长度的变量以及一个char*的指针指向buffer内容所在的位置。

 1 /** A single item in an evbuffer. */
 2 struct evbuffer_chain {
 3     /** points to next buffer in the chain */
 4     struct evbuffer_chain *next;
 5 
 6     /** total allocation available in the buffer field. */
 7     size_t buffer_len;
 8 
 9     /** unused space at the beginning of buffer or an offset into a
10      * file for sendfile buffers. */
11     ev_misalign_t misalign;
12 
13     /** Offset into buffer + misalign at which to start writing.
14      * In other words, the total number of bytes actually stored
15      * in buffer. */
16     size_t off;
17 
18     /** Set if special handling is required for this chain */
19     unsigned flags;
20 #define EVBUFFER_FILESEGMENT    0x0001  /**< A chain used for a file segment */
21 #define EVBUFFER_SENDFILE    0x0002    /**< a chain used with sendfile */
22 #define EVBUFFER_REFERENCE    0x0004    /**< a chain with a mem reference */
23 #define EVBUFFER_IMMUTABLE    0x0008    /**< read-only chain */
24     /** a chain that mustn‘t be reallocated or freed, or have its contents
25      * memmoved, until the chain is un-pinned. */
26 #define EVBUFFER_MEM_PINNED_R    0x0010
27 #define EVBUFFER_MEM_PINNED_W    0x0020
28 #define EVBUFFER_MEM_PINNED_ANY (EVBUFFER_MEM_PINNED_R|EVBUFFER_MEM_PINNED_W)
29     /** a chain that should be freed, but can‘t be freed until it is
30      * un-pinned. */
31 #define EVBUFFER_DANGLING    0x0040
32     /** a chain that is a referenced copy of another chain */
33 #define EVBUFFER_MULTICAST    0x0080
34 
35     /** number of references to this chain */
36     int refcnt;
37 
38     /** Usually points to the read-write memory belonging to this
39      * buffer allocated as part of the evbuffer_chain allocation.
40      * For mmap, this can be a read-only buffer and
41      * EVBUFFER_IMMUTABLE will be set in flags.  For sendfile, it
42      * may point to NULL.
43      */
44     unsigned char *buffer;
45 };
46 
47 /** callback for a reference chain; lets us know what to do with it when
48  * we‘re done with it. Lives at the end of an evbuffer_chain with the
49  * EVBUFFER_REFERENCE flag set */
50 struct evbuffer_chain_reference {
51     evbuffer_ref_cleanup_cb cleanupfn;
52     void *extra;
53 };