Evbuffer是用来充当缓存网络IO的缓存功能

创建销毁一个Evbuffer

struct evbuffer *evbuffer_new(void);
void evbuffer_free(struct evbuffer *buf);

Evbuffer和线程安全

int evbuffer_enable_locking(struct evbuffer *buf, void *lock);
void evbuffer_lock(struct evbuffer *buf);
void evbuffer_unlock(struct evbuffer *buf);

默认情况下，Evbuffer不是线程安全的。如果想让Evbuffer变得线程安全，可以用evbuffer_enable_locking，如果lock为NULL，则自动分配一个新锁。

size_t evbuffer_get_length(const struct evbuffer *buf);

返回存储的字节数

size_t evbuffer_get_contiguous_space(const struct evbuffer *buf);

evbuffer里的字节可能分布在内存中不同的块上，这个函数返回当前存储的第一个连续块上的字节数。

添加数据到evbuffer

int evbuffer_add(struct evbuffer *buf, const void *data, size_t datlen);

这个函数添加data里datlen字节的数据到buf的末尾。

int evbuffer_add_printf(struct evbuffer *buf, const char *fmt, ...)
int evbuffer_add_vprintf(struct evbuffer *buf, const char *fmt, va_list ap);

int evbuffer_expand(struct evbuffer *buf, size_t datlen);

这个函数将buf的在内存中的最后一个块转换为一个新块，让buf可以包含datlen大小的字节。

移动data从一个evbuffer到另一个

int evbuffer_add_buffer(struct evbuffer *dst, struct evbuffer *src);
int evbuffer_remove_buffer(struct evbuffer *src, struct evbuffer *dst,
    size_t datlen);

移动data到另一个evbuffer的首部

int evbuffer_prepend(struct evbuffer *buf, const void *data, size_t size);
int evbuffer_prepend_buffer(struct evbuffer *dst, struct evbuffer* src);

对evbuffer内部进行重新布局

unsigned char *evbuffer_pullup(struct evbuffer *buf, ev_ssize_t size);

这个函数让buf里的前size字节复制或者移动确保他们是连续的线化且在同一个块上。如果size是负数，那这个函数线化整个buf，如果size比buffer还大，那么返回NULL。用很大的size值调用这个函数会执行的很慢。因为会有复制操作

#include <event2/buffer.h>
#include <event2/util.h>

#include <string.h>

int parse_socks4(struct evbuffer *buf, ev_uint16_t *port, ev_uint32_t *addr)
{
    /* Let's parse the start of a SOCKS4 request!  The format is easy:
     * 1 byte of version, 1 byte of command, 2 bytes destport, 4 bytes of
     * destip. */
    unsigned char *mem;

    mem = evbuffer_pullup(buf, 8);

    if (mem == NULL) {
        /* Not enough data in the buffer */
        return 0;
    } else if (mem[0] != 4 || mem[1] != 1) {
        /* Unrecognized protocol or command */
        return -1;
    } else {
        memcpy(port, mem+2, 2);
        memcpy(addr, mem+4, 4);
        *port = ntohs(*port);
        *addr = ntohl(*addr);
        /* Actually remove the data from the buffer now that we know we
           like it. */
        evbuffer_drain(buf, 8);
        return 1;
    }
}

从一个evbuffer移动数据

int evbuffer_drain(struct evbuffer *buf, size_t len);
int evbuffer_remove(struct evbuffer *buf, void *data, size_t datlen);

从一个evbuffer中复制数据

ev_ssize_t evbuffer_copyout(struct evbuffer *buf, void *data, size_t datlen);
ev_ssize_t evbuffer_copyout_from(struct evbuffer *buf,
     const struct evbuffer_ptr *pos,
     void *data_out, size_t datlen);

例子：

#include <event2/buffer.h>
#include <event2/util.h>
#include <stdlib.h>
#include <stdlib.h>

int get_record(struct evbuffer *buf, size_t *size_out, char **record_out)
{
    /* Let's assume that we're speaking some protocol where records
       contain a 4-byte size field in network order, followed by that
       number of bytes.  We will return 1 and set the 'out' fields if we
       have a whole record, return 0 if the record isn't here yet, and
       -1 on error.  */
    size_t buffer_len = evbuffer_get_length(buf);
    ev_uint32_t record_len;
    char *record;

    if (buffer_len < 4)
       return 0; /* The size field hasn't arrived. */

   /* We use evbuffer_copyout here so that the size field will stay on
       the buffer for now. */
    evbuffer_copyout(buf, &record_len, 4);
    /* Convert len_buf into host order. */
    record_len = ntohl(record_len);
    if (buffer_len < record_len + 4)
        return 0; /* The record hasn't arrived */

    /* Okay, _now_ we can remove the record. */
    record = malloc(record_len);
    if (record == NULL)
        return -1;

    evbuffer_drain(buf, 4);
    evbuffer_remove(buf, record, record_len);

    *record_out = record;
    *size_out = record_len;
    return 1;
}

在一个evbuffer里搜索

struct evbuffer_ptr {
        ev_ssize_t pos;
        struct {
                /* internal fields */
        } _internal;
};
struct evbuffer_ptr evbuffer_search(struct evbuffer *buffer,
    const char *what, size_t len, const struct evbuffer_ptr *start);
struct evbuffer_ptr evbuffer_search_range(struct evbuffer *buffer,
    const char *what, size_t len, const struct evbuffer_ptr *start,
    const struct evbuffer_ptr *end);
struct evbuffer_ptr evbuffer_search_eol(struct evbuffer *buffer,
    struct evbuffer_ptr *start, size_t *eol_len_out,
    enum evbuffer_eol_style eol_style);

例子：

#include <event2/buffer.h>
#include <string.h>

/* Count the total occurrences of 'str' in 'buf'. */
int count_instances(struct evbuffer *buf, const char *str)
{
    size_t len = strlen(str);
    int total = 0;
    struct evbuffer_ptr p;

    if (!len)
        /* Don't try to count the occurrences of a 0-length string. */
        return -1;

    evbuffer_ptr_set(buf, &p, 0, EVBUFFER_PTR_SET);

    while (1) {
         p = evbuffer_search(buf, str, len, &p);
         if (p.pos < 0)
             break;
         total++;
         evbuffer_ptr_set(buf, &p, 1, EVBUFFER_PTR_ADD);
    }

    return total;
}

直接添加数据到一个evbuffer

int evbuffer_reserve_space(struct evbuffer *buf, ev_ssize_t size,
    struct evbuffer_iovec *vec, int n_vecs);
int evbuffer_commit_space(struct evbuffer *buf,
    struct evbuffer_iovec *vec, int n_vecs);

例子:

struct evbuffer_iovec v[2];
int n, i;
size_t n_to_add = 2048;

/* Reserve 2048 bytes.*/
n = evbuffer_reserve_space(buf, n_to_add, v, 2);
if (n<=0)
   return; /* Unable to reserve the space for some reason. */

for (i=0; i<n && n_to_add > 0; ++i) {
   size_t len = v[i].iov_len;
   if (len > n_to_add) /* Don't write more than n_to_add bytes. */
      len = n_to_add;
   if (generate_data(v[i].iov_base, len) < 0) {
      /* If there was a problem during data generation, we can just stop
         here; no data will be committed to the buffer. */
      return;
   }
   /* Set iov_len to the number of bytes we actually wrote, so we
      don't commit too much. */
   v[i].iov_len = len;
}

/* We commit the space here.  Note that we give it 'i' (the number of
   vectors we actually used) rather than 'n' (the number of vectors we
   had available. */
if (evbuffer_commit_space(buf, v, i) < 0)
   return; /* Error committing */

evbuffer的网络IO

int evbuffer_write(struct evbuffer *buffer, evutil_socket_t fd);
int evbuffer_write_atmost(struct evbuffer *buffer, evutil_socket_t fd,
        ev_ssize_t howmuch);
int evbuffer_read(struct evbuffer *buffer, evutil_socket_t fd, int howmuch);

Evbuffer和回调函数

struct evbuffer_cb_info {
        size_t orig_size;
        size_t n_added;
        size_t n_deleted;
};

typedef void (*evbuffer_cb_func)(struct evbuffer *buffer,
    const struct evbuffer_cb_info *info, void *arg);
struct evbuffer_cb_entry;
struct evbuffer_cb_entry *evbuffer_add_cb(struct evbuffer *buffer,
    evbuffer_cb_func cb, void *cbarg);
int evbuffer_remove_cb_entry(struct evbuffer *buffer,
    struct evbuffer_cb_entry *ent);
int evbuffer_remove_cb(struct evbuffer *buffer, evbuffer_cb_func cb,
    void *cbarg);

#define EVBUFFER_CB_ENABLED 1
int evbuffer_cb_set_flags(struct evbuffer *buffer,
                          struct evbuffer_cb_entry *cb,
                          ev_uint32_t flags);
int evbuffer_cb_clear_flags(struct evbuffer *buffer,
                          struct evbuffer_cb_entry *cb,
                          ev_uint32_t flags);

例子：

#include <event2/buffer.h>
#include <stdio.h>
#include <stdlib.h>

/* Here's a callback that remembers how many bytes we have drained in
   total from the buffer, and prints a dot every time we hit a
   megabyte. */
struct total_processed {
    size_t n;
};
void count_megabytes_cb(struct evbuffer *buffer,
    const struct evbuffer_cb_info *info, void *arg)
{
    struct total_processed *tp = arg;
    size_t old_n = tp->n;
    int megabytes, i;
    tp->n += info->n_deleted;
    megabytes = ((tp->n) >> 20) - (old_n >> 20);
    for (i=0; i<megabytes; ++i)
        putc('.', stdout);
}

void operation_with_counted_bytes(void)
{
    struct total_processed *tp = malloc(sizeof(*tp));
    struct evbuffer *buf = evbuffer_new();
    tp->n = 0;
    evbuffer_add_cb(buf, count_megabytes_cb, tp);

    /* Use the evbuffer for a while.  When we're done: */
    evbuffer_free(buf);
    free(tp);
}

避免evbufferIO中的数据复制

typedef void (*evbuffer_ref_cleanup_cb)(const void *data,
    size_t datalen, void *extra);

int evbuffer_add_reference(struct evbuffer *outbuf,
    const void *data, size_t datlen,
    evbuffer_ref_cleanup_cb cleanupfn, void *extra);

例子：

#include <event2/buffer.h>
#include <stdlib.h>
#include <string.h>

/* In this example, we have a bunch of evbuffers that we want to use to
   spool a one-megabyte resource out to the network.  We do this
   without keeping any more copies of the resource in memory than
   necessary. */

#define HUGE_RESOURCE_SIZE (1024*1024)
struct huge_resource {
    /* We keep a count of the references that exist to this structure,
       so that we know when we can free it. */
    int reference_count;
    char data[HUGE_RESOURCE_SIZE];
};

struct huge_resource *new_resource(void) {
    struct huge_resource *hr = malloc(sizeof(struct huge_resource));
    hr->reference_count = 1;
    /* Here we should fill hr->data with something.  In real life,
       we'd probably load something or do a complex calculation.
       Here, we'll just fill it with EEs. */
    memset(hr->data, 0xEE, sizeof(hr->data));
    return hr;
}

void free_resource(struct huge_resource *hr) {
    --hr->reference_count;
    if (hr->reference_count == 0)
        free(hr);
}

static void cleanup(const void *data, size_t len, void *arg) {
    free_resource(arg);
}

/* This is the function that actually adds the resource to the
   buffer. */
void spool_resource_to_evbuffer(struct evbuffer *buf,
    struct huge_resource *hr)
{
    ++hr->reference_count;
    evbuffer_add_reference(buf, hr->data, HUGE_RESOURCE_SIZE,
        cleanup, hr);
}

将文件添加到evbuffer中

int evbuffer_add_file(struct evbuffer *output, int fd, ev_off_t offset,
    size_t length);