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
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
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();
统，支持sysupgrade刷机、文件系统快照和回滚、重写挂载系统；初步支持musl C 标准库，等等。