nginx ngx_buf_t
2015-01-25
先来看看相关的数据结构：
struct ngx_buf_s {
u_char *pos;
u_char *last;
off_t file_pos;
off_t file_last;
u_char *start; /* start of buffer */
u_char *end; /* end of buffer */
ngx_buf_tag_t tag;
ngx_file_t *file;
ngx_buf_t *shadow;
/* the buf's content could be changed */
unsigned temporary:1;
/*
* the buf's content is in a memory cache or in a read only memory
* and must not be changed
*/
unsigned memory:1;
/* the buf's content is mmap()ed and must not be changed */
unsigned mmap:1;
unsigned recycled:1;
unsigned in_file:1;
unsigned flush:1;
unsigned sync:1;
unsigned last_buf:1;
unsigned last_in_chain:1;
unsigned last_shadow:1;
unsigned temp_file:1;
/* STUB */ int num;
};
struct ngx_chain_s {
ngx_buf_t *buf;
ngx_chain_t *next;
};
内存布局图如下：
注意所有的内存分配都应该从 ngx_pool_t 中分配。分配后的 buf 的各个字段的内存含义如下图：
一般buffer结构体可以表示一块内存，内存的起始和结束地址分别用start和end表示，pos和last表示实际的内容。如果内容已经处理过了，pos的位置就可以往后移动。如果读取到新的内容，last的位置就会往后移动。所以buffer可以在多次调用过程中使用。如果last等于end，就说明这块内存已经用完了。如果pos等于last，说明内存已经处理完了。
