前言

刚开始接触OpenWrt的时候，根本不知道如何调试各个进程，我之前从事IP Camera开发可能也局限了我的知识面，认为系统就改是那个样子。

其实不然，就像Linux发行版那样，他们都有各自都管理系统，同一个的消息通知系统，dbus和ubus这些。系统调试也是一样dmesg, 现在还接触到了logread。

初探

logread是在调试luci的时候用到的，极其方便，对于不太了解OpenWrt系统构成对人尤甚。

这个需要写进程对人对syslogd提供支持，否则说来知识惘然，我们需要做系统，需要做好对系统，就需要油完善对日志管理，精简无冗余对才是最有用的，这是我们使用其的目的。废话不多说，直接看卡logread的组成吧。

在busybox中实现了syslogd 和 logread.

syslogd用来记录log, logged则用来读取log.

logread的代码很简洁，主要实现过程是：连接共享内存－>信号量加锁－>读取共享内存中的信息并输出->信号量解锁。

连接共享内存

<span style="white-space:pre">	</span>log_shmid = shmget(KEY_ID, 0, 0);
	if (log_shmid == -1)
		bb_perror_msg_and_die("can't %s syslogd buffer", "find");

	/* Attach shared memory to our char* */
	shbuf = shmat(log_shmid, NULL, SHM_RDONLY);
	if (shbuf == NULL)
		bb_perror_msg_and_die("can't %s syslogd buffer", "access");

	log_semid = semget(KEY_ID, 0, 0);
	if (log_semid == -1)
		error_exit("can't get access to semaphores for syslogd buffer");

信号量加锁

<span style="white-space:pre">	</span>if (semop(log_semid, SMrdn, 2) == -1)
		error_exit("semop[SMrdn]");

读取共享内存中的信息并输出

	/* Suppose atomic memory read */
	/* Max possible value for tail is shbuf->size - 1 */
	cur = shbuf->tail;

	/* Loop for logread -f, one pass if there was no -f */
	do {
		unsigned shbuf_size;
		unsigned shbuf_tail;
		const char *shbuf_data;
#if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
		int i;
		int len_first_part;
		int len_total = len_total; /* for gcc */
		char *copy = copy; /* for gcc */
#endif
		if (semop(log_semid, SMrdn, 2) == -1)
			error_exit("semop[SMrdn]");

		/* Copy the info, helps gcc to realize that it doesn't change */
		shbuf_size = shbuf->size;
		shbuf_tail = shbuf->tail;
		shbuf_data = shbuf->data; /* pointer! */

		if (DEBUG)
			printf("cur:%u tail:%u size:%u\n",
					cur, shbuf_tail, shbuf_size);

		if (!follow) {
			/* advance to oldest complete message */
			/* find NUL */
			cur += strlen(shbuf_data + cur);
			if (cur >= shbuf_size) { /* last byte in buffer? */
				cur = strnlen(shbuf_data, shbuf_tail);
				if (cur == shbuf_tail)
					goto unlock; /* no complete messages */
			}
			/* advance to first byte of the message */
			cur++;
			if (cur >= shbuf_size) /* last byte in buffer? */
				cur = 0;
		} else { /* logread -f */
			if (cur == shbuf_tail) {
				sem_up(log_semid);
				fflush_all();
				sleep(1); /* TODO: replace me with a sleep_on */
				continue;
			}
		}

		/* Read from cur to tail */
#if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
		len_first_part = len_total = shbuf_tail - cur;
		if (len_total < 0) {
			/* message wraps: */
			/* [SECOND PART.........FIRST PART] */
			/*  ^data      ^tail    ^cur      ^size */
			len_total += shbuf_size;
		}
		copy = xmalloc(len_total + 1);
		if (len_first_part < 0) {
			/* message wraps (see above) */
			len_first_part = shbuf_size - cur;
			memcpy(copy + len_first_part, shbuf_data, shbuf_tail);
		}
		memcpy(copy, shbuf_data + cur, len_first_part);
		copy[len_total] = '\0';
		cur = shbuf_tail;
#else
		while (cur != shbuf_tail) {
			fputs(shbuf_data + cur, stdout);
			cur += strlen(shbuf_data + cur) + 1;
			if (cur >= shbuf_size)
				cur = 0;
		}
#endif

信号量解锁

 unlock:
		/* release the lock on the log chain */
		sem_up(log_semid);

#if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
		for (i = 0; i < len_total; i += strlen(copy + i) + 1) {
			fputs(copy + i, stdout);
		}
		free(copy);
#endif
		fflush_all();

http://man.cx/syslogd(8)

OpenWrt 系统日志之logread