# 7.5.3 操作bufferevent中的数据 ##通过bufferevent得到evbuffer 如果只是通过网络读取或者写入数据 ,而不能观察操作过程,是没什么好处的。bufferevent 提供了下列函数用于观察要写入或者读取的数据。 ```cpp struct evbuffer *bufferevent_get_input(struct bufferevent *bufev); struct evbuffer *bufferevent_get_output(struct bufferevent *bufev); ``` 这两个函数提供了非常强大的基础 :它们分别返回输入和输出缓冲区 。关于可以对 evbuffer 类型进行的所有操作的完整信息,请看下一章。 如果写入操作因为数据量太少而停止(或者读取操作因为太多数据而停止 ),则向输出缓冲 区添加数据(或者从输入缓冲区移除数据)将自动重启操作。 ##向bufferevent的输出缓冲区添加数据 ```cpp int bufferevent_write(struct bufferevent *bufev, const void *data, size_t size); int bufferevent_write_buffer(struct bufferevent *bufev, struct evbuffer *buf); ``` 这些函数向 bufferevent 的输出缓冲区添加数据。 bufferevent_write()将内存中从 data 处开 始的 size 字节数据添加到输出缓冲区的末尾 。bufferevent_write_buffer()移除 buf 的所有内 容,将其放置到输出缓冲区的末尾。成功时这些函数都返回 0,发生错误时则返回-1。 ##从bufferevent的输入缓冲区移除数据 ```cpp size_t bufferevent_read(struct bufferevent *bufev, void *data, size_t size); int bufferevent_read_buffer(struct bufferevent *bufev, struct evbuffer *buf); ``` 这些函数从 bufferevent 的输入缓冲区移除数据。bufferevent_read()至多从输入缓冲区移除 size 字节的数据,将其存储到内存中 data 处。函数返回实际移除的字节数。 bufferevent_read_buffer()函数抽空输入缓冲区的所有内容,将其放置到 buf 中,成功时返 回0,失败时返回 -1。 注意,对于 bufferevent_read(),data 处的内存块必须有足够的空间容纳 size 字节数据。 ###示例 ```cpp #include <event2/bufferevent.h> #include <event2/buffer.h> #include <ctype.h> void read_callback_uppercase(struct bufferevent *bev, void *ctx) { /* This callback removes the data from bev's input buffer 128 bytes at a time, uppercases it, and starts sending it back. (Watch out! In practice, you shouldn't use toupper to implement a network protocol, unless you know for a fact that the current locale is the one you want to be using.) */ char tmp[128]; size_t n; int i; while (1) { n = bufferevent_read(bev, tmp, sizeof(tmp)); if (n <= 0) break; /* No more data. */ for (i=0; i<n; ++i) tmp[i] = toupper(tmp[i]); bufferevent_write(bev, tmp, n); } } struct proxy_info { struct bufferevent *other_bev; }; void read_callback_proxy(struct bufferevent *bev, void *ctx) { /* You might use a function like this if you're implementing a simple proxy: it will take data from one connection (on bev), and write it to another, copying as little as possible. */ struct proxy_info *inf = ctx; bufferevent_read_buffer(bev, bufferevent_get_output(inf->other_bev)); } struct count { unsigned long last_fib[2]; }; void write_callback_fibonacci(struct bufferevent *bev, void *ctx) { /* Here's a callback that adds some Fibonacci numbers to the output buffer of bev. It stops once we have added 1k of data; once this data is drained, we'll add more. */ struct count *c = ctx; struct evbuffer *tmp = evbuffer_new(); while (evbuffer_get_length(tmp) < 1024) { unsigned long next = c->last_fib[0] + c->last_fib[1]; c->last_fib[0] = c->last_fib[1]; c->last_fib[1] = next; evbuffer_add_printf(tmp, "%lu", next); } /* Now we add the whole contents of tmp to bev. */ bufferevent_write_buffer(bev, tmp); /* We don't need tmp any longer. */ evbuffer_free(tmp); } ```