// struct ngx_log_s
// 日志结构 {{{
struct ngx_log_s {
 ngx_uint_t   log_level;  // 日志等级
 ngx_open_file_t  *file;   // 日志文件信息
 ngx_atomic_uint_t connection; // 引用该日志对象的连接数
 ngx_log_handler_pt handler;  // 输出日志时需要调用的回调函数
 void    *data;   // 回调函数所需要的参数
 ngx_log_writer_pt writer;
 void    *wdata;
 /*
  * we declare "action" as "char *" because the actions are usually
  * the static strings and in the "u_char *" case we have to override
  * their types all the time
  */
 char    *action;  // 日志记录前，保存当前正在进行的动作
 ngx_log_t   *next;   // 指向下一日志对象
}; // }}}

#define NGX_LOG_STDERR   0
#define NGX_LOG_EMERG    1
#define NGX_LOG_ALERT    2
#define NGX_LOG_CRIT    3
#define NGX_LOG_ERR    4
#define NGX_LOG_WARN    5
#define NGX_LOG_NOTICE   6
#define NGX_LOG_INFO    7
#define NGX_LOG_DEBUG    8

// struct ngx_open_file_s
// 打开文件结构 {{{
struct ngx_open_file_s {
 ngx_fd_t    fd;
 ngx_str_t    name;
 void    (*flush)(ngx_open_file_t *file, ngx_log_t *log);
   // 文件刷新时调用的回调
 void     *data; // 回调函数使用的参数
}; // }}}

// ngx_log_t *ngx_log_init(u_char *prefix);
// 初始化 ngx_log 结构，创建 errlog 文件 {{{
ngx_log_t *
ngx_log_init(u_char *prefix)
{
 u_char *p, *name;
 size_t nlen, plen;

 ngx_log.file = &ngx_log_file;
 ngx_log.log_level = NGX_LOG_NOTICE;

 name = (u_char *) NGX_ERROR_LOG_PATH;

 /*
  * we use ngx_strlen() here since BCC warns about
  * condition is always false and unreachable code
  */

 nlen = ngx_strlen(name);

 if (nlen == 0) {
  ngx_log_file.fd = ngx_stderr;
  return &ngx_log;
 }

 p = NULL;

 // 判断是否是绝对路径
#if (NGX_WIN32)
 if (name[1] != ':') {
#else
 if (name[0] != '/') {
#endif

  if (prefix) {
   plen = ngx_strlen(prefix);

  } else {
#ifdef NGX_PREFIX
   prefix = (u_char *) NGX_PREFIX;
   plen = ngx_strlen(prefix);
#else
   plen = 0;
#endif
  }

  if (plen) {
   name = malloc(plen + nlen + 2);
   if (name == NULL) {
    return NULL;
   }

   p = ngx_cpymem(name, prefix, plen);

   if (!ngx_path_separator(*(p - 1))) {
    *p++ = '/';
   }

   ngx_cpystrn(p, (u_char *) NGX_ERROR_LOG_PATH, nlen + 1);

   p = name;
  }
 }

 ngx_log_file.fd = ngx_open_file(name, NGX_FILE_APPEND,
         NGX_FILE_CREATE_OR_OPEN,
         NGX_FILE_DEFAULT_ACCESS);

 if (ngx_log_file.fd == NGX_INVALID_FILE) {
  ngx_log_stderr(ngx_errno,
      "[alert] could not open error log file: "
      ngx_open_file_n " \"%s\" failed", name);
#if (NGX_WIN32)
  ngx_event_log(ngx_errno,
      "could not open error log file: "
      ngx_open_file_n " \"%s\" failed", name);
#endif

  ngx_log_file.fd = ngx_stderr;
 }

 if (p) {
  ngx_free(p);
 }

 return &ngx_log;
}
// }}}

{
 log_level = 6, 
 file = 0x80e3000 <ngx_log_file>, 
 connection = 0, 
 handler = 0x0, 
 data = 0x0, 
 writer = 0x0, 
 wdata = 0x0, 
 action = 0x0, 
 next = 0x0
}

{
 fd = 3, 
 name = {
 len = 0, 
 data = 0x0
 }, 
 flush = 0x0, 
 data = 0x0
}

./configure --add-module=/var/local/ngx_log_if-master

server {
 location / {
  access_log_bypass_if ($status = 404); #不记录404状态的所有日志信息
  access_log_bypass_if ($uri ~* 'images'); #不记录uri中所有images目录下文件的日志信息
  access_log_bypass_if ($uri = '/index.html'); #不记录uri为/index.html的日志信息
 access_log_bypass_if ($host ~* 'tonv.cc'); #不记录host为tonv.cc的所有日志信息
 }
}