概述

在 NGINX 源码中搜索 error_log 可以发现有多处指令定义。比如在 ngx_http_core_module 中指令定义

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static ngx_command_t  ngx_http_core_commands[] = {
...
    { ngx_string("error_log"),
          NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_1MORE,
          ngx_http_core_error_log,
          NGX_HTTP_LOC_CONF_OFFSET,
          0,
          NULL },
...
};

static char *
ngx_http_core_error_log(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
{
    ngx_http_core_loc_conf_t *clcf = conf;

    return ngx_log_set_log(cf, &clcf->error_log);
}

观察 ngx_mail_core_module、ngx_errlog_module 等其他模块的 error_log 指令处理函数，可以发现最终都会调用到 ngx_log_set_log 函数。其定义如下：

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char *
ngx_log_set_log(ngx_conf_t *cf, ngx_log_t **head)
{
    ngx_log_t          *new_log;
    ngx_str_t          *value, name;
    ngx_syslog_peer_t  *peer;

    if (*head != NULL && (*head)->log_level == 0) {
        new_log = *head;

    } else {
        new_log = ngx_pcalloc(cf->pool, sizeof(ngx_log_t));
        if (new_log == NULL) {
            return NGX_CONF_ERROR;
        }

        if (*head == NULL) {
            *head = new_log;
        }
    }

    value = cf->args->elts;

    if (ngx_strcmp(value[1].data, "stderr") == 0) {
        ngx_str_null(&name);
        cf->cycle->log_use_stderr = 1;

        new_log->file = ngx_conf_open_file(cf->cycle, &name);
        if (new_log->file == NULL) {
            return NGX_CONF_ERROR;
        }
    } else if (ngx_strncmp(value[1].data, "memory:", 7) == 0) {
		// 内存调试日志，只有 NGX_DEBUG 模式支持
#if (NGX_DEBUG)
        size_t                 size, needed;
        ngx_pool_cleanup_t    *cln;
        ngx_log_memory_buf_t  *buf;

        value[1].len -= 7;
        value[1].data += 7;

        needed = sizeof("MEMLOG  :" NGX_LINEFEED) + cf->conf_file->file.name.len + NGX_SIZE_T_LEN + NGX_INT_T_LEN + NGX_MAX_ERROR_STR;
        size = ngx_parse_size(&value[1]);

        if (size == (size_t) NGX_ERROR || size < needed) {
            ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "invalid buffer size \"%V\"", &value[1]);
            return NGX_CONF_ERROR;
        }

        buf = ngx_pcalloc(cf->pool, sizeof(ngx_log_memory_buf_t));
        if (buf == NULL) {
            return NGX_CONF_ERROR;
        }

        buf->start = ngx_pnalloc(cf->pool, size);
        if (buf->start == NULL) {
            return NGX_CONF_ERROR;
        }

        buf->end = buf->start + size;

        buf->pos = ngx_slprintf(buf->start, buf->end, "MEMLOG %uz %V:%ui%N", size, &cf->conf_file->file.name, cf->conf_file->line);
        ngx_memset(buf->pos, ' ', buf->end - buf->pos);
        cln = ngx_pool_cleanup_add(cf->pool, 0);
        if (cln == NULL) {
            return NGX_CONF_ERROR;
        }

        cln->data = new_log;
        cln->handler = ngx_log_memory_cleanup;

        new_log->writer = ngx_log_memory_writer;
        new_log->wdata = buf;

#else
        ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "nginx was built without debug support");
        return NGX_CONF_ERROR;
#endif

    } else if (ngx_strncmp(value[1].data, "syslog:", 7) == 0) {
        peer = ngx_pcalloc(cf->pool, sizeof(ngx_syslog_peer_t));
        if (peer == NULL) {
            return NGX_CONF_ERROR;
        }

        if (ngx_syslog_process_conf(cf, peer) != NGX_CONF_OK) {
            return NGX_CONF_ERROR;
        }

        new_log->writer = ngx_syslog_writer;
        new_log->wdata = peer;
    } else {
        // 普通文件日志，关联一个文件对象。此时并未打开文件
        // 如果跟踪代码走进去，可以发先会将当前文件存入 cycle->open_files 动态数组中，文件的打开动作在 ngx_init_cycle 阶段将所有动态数组中文件打开
        new_log->file = ngx_conf_open_file(cf->cycle, &value[1]);
        if (new_log->file == NULL) {
            return NGX_CONF_ERROR;
        }
    }

    if (ngx_log_set_levels(cf, new_log) != NGX_CONF_OK) {
        return NGX_CONF_ERROR;
    }

    if (*head != new_log) {
        // log 链表是按 log_level 从大到小的顺序排列的，将一个新的 ngx_log_t 对象插入到一个已存在的 log 链表中。
        // 一般 log 链表的头是一个固定地址，因此需要插入到头部时需要更改头部指针指向
        ngx_log_insert(*head, new_log);
    }

    return NGX_CONF_OK;
}

ngx_log_error_core

作为 NGINX 的日志输出函数，通过 ngx_log_error、ngx_log_debug 能够非常方便打印调试信息（NGINX 提供了 HEX 转换函数也方便打印二进制信息），其实两个函数是宏定义，真正的实现者是 ngx_log_error_core。

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void
ngx_log_error_core(ngx_uint_t level, ngx_log_t *log, ngx_err_t err, const char *fmt, ...)
{
    u_char      *p, *last, *msg;
    ssize_t      n;
    ngx_uint_t   wrote_stderr, debug_connection;

    // 输出缓冲区，当输出信息过长时会导致信息截断，可以调整 NGX_MAX_ERROR_STR 来调整输出信息长度
    u_char       errstr[NGX_MAX_ERROR_STR];

    // 1. 进行输出信息拼接
    last = errstr + NGX_MAX_ERROR_STR;
    p = ngx_cpymem(errstr, ngx_cached_err_log_time.data, ngx_cached_err_log_time.len);
    p = ngx_slprintf(p, last, " [%V] ", &err_levels[level]);

    /* pid#tid */
    p = ngx_slprintf(p, last, "%P#" NGX_TID_T_FMT ": ", ngx_log_pid, ngx_log_tid);

    if (log->connection) {
        p = ngx_slprintf(p, last, "*%uA ", log->connection);
    }

    msg = p;

#if (NGX_HAVE_VARIADIC_MACROS)
    va_start(args, fmt);
    p = ngx_vslprintf(p, last, fmt, args);
    va_end(args);
#else
    p = ngx_vslprintf(p, last, fmt, args);
#endif

    if (err) {
        p = ngx_log_errno(p, last, err);
    }

    if (level != NGX_LOG_DEBUG && log->handler) {
        p = log->handler(log, p, last - p);
    }

    if (p > last - NGX_LINEFEED_SIZE) {
        p = last - NGX_LINEFEED_SIZE;
    }

    ngx_linefeed(p);

    wrote_stderr = 0;
    debug_connection = (log->log_level & NGX_LOG_DEBUG_CONNECTION) != 0;

    // 遍历 log 进行信息输出
    while (log) {
        if (log->log_level < level && !debug_connection) {
            break;
        }
		// 可以自定义 writer 行为
        if (log->writer) {
            log->writer(log, level, errstr, p - errstr);
            goto next;
        }

        if (ngx_time() == log->disk_full_time) {
            /*
             * on FreeBSD writing to a full filesystem with enabled softupdates
             * may block process for much longer time than writing to non-full
             * filesystem, so we skip writing to a log for one second
             */
            goto next;
        }

        // 输出信息写入文件
        n = ngx_write_fd(log->file->fd, errstr, p - errstr);
        if (n == -1 && ngx_errno == NGX_ENOSPC) {
            log->disk_full_time = ngx_time();
        }

        if (log->file->fd == ngx_stderr) {
            wrote_stderr = 1;
        }

    next:

        log = log->next;
    }

    // 输出到终端
    if (!ngx_use_stderr || level > NGX_LOG_WARN || wrote_stderr) {
        return;
    }

	// 输出信息调整
    msg -= (7 + err_levels[level].len + 3);
    (void) ngx_sprintf(msg, "nginx: [%V] ", &err_levels[level]);
    (void) ngx_write_console(ngx_stderr, msg, p - msg);
}

阅读上面的代码可以发现输出日志会遍历 log 链表中进行输出。