OpenWrt 系统日志之logread

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_LOCKINGint i;int len_first_part;int len_total = len_total; /* for gcc */char *copy = copy; /* for gcc */#endifif (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_LOCKINGlen_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;#elsewhile (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_LOCKINGfor (i = 0; i < len_total; i += strlen(copy + i) + 1) {fputs(copy + i, stdout);}free(copy);#endiffflush_all();

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