在iscsid主函数中有： log_pid = log_init(program_name, DEFAULT_AREA_SIZE, daemonize ? log_do_log_daemon : log_do_log_std, NULL);
初始化日志系统，现在进入log_init函数，对open-iscsi的日志处理方式进行详细分析：
/*
日志初始化函数
program_name:程序名称
size:日志块大小
func:记录日志的函数(通过syslog或者直接输出到终端)
*/
int log_init(char *program_name, int size,
void (*func)(int prio, void *priv, const char *fmt, va_list ap),
void *priv)
{
logdbg(stderr,"enter log_init\n");
log_name = program_name;
log_func = func;
log_func_priv = priv;
/*通过syslog记录，需要做一些初始化工作*/
if (log_func == log_do_log_daemon) {
struct sigaction sa_old;
struct sigaction sa_new;
pid_t pid;
/*打开syslog*/
openlog(log_name, 0, LOG_DAEMON);
/*设置日志的优先级，这里设置的日志优先级为log_debug*/
setlogmask (LOG_UPTO (LOG_DEBUG));
/*日志守护进程和其他进程进行通信的共享内存区域分配*/
if (logarea_init(size)) {
syslog(LOG_ERR, "logarea init failed");
return -1;
}
pid = fork();/*创建一个子进程记录日志*/
if (pid < 0) {
syslog(LOG_ERR, "starting logger failed");
exit(1);
} else if (pid) { /*父进程返回*/
syslog(LOG_WARNING,
"iSCSI logger with pid=%d started!", pid);
return pid;
}
daemon_init(); /*守护进程初始化*/
/* flush on daemon's crash */
/*设置信号捕捉函数*/
sa_new.sa_handler = (void*)catch_signal;
sigemptyset(&sa_new.sa_mask);
sa_new.sa_flags = 0;
sigaction(SIGSEGV, &sa_new, &sa_old );
sigaction(SIGTERM, &sa_new, &sa_old );
/*日志守护进程定时输出日志*/
while(1) {
log_flush();
sleep(1);
/*关闭标志设置，则退出*/
if (log_stop_daemon)
break;
}
/*结尾工作*/
__log_close();
exit(0);
}
return 0;
}
其实open-iscsi建立了一个守护进程来专门输出日志，同时建立内存共享区作为其他进程和日志守护进程的通信。将内存共享区作为一个循环消息队列，其他进程将输出日志作为一个消息块插入这个队列中，而日志守护进程则从这个队列中取出消息输出到日志文件中，核心主要就是入队列和出队列两个函数：
/*增加一个消息到共享内粗块中*/
int log_enqueue (int prio, const char * fmt, va_list ap)
{
int len, fwd;
/*消息缓冲区*/
char buff[MAX_MSG_SIZE];
struct logmsg * msg;
struct logmsg * lastmsg;
/*最后一个消息的位置*/
lastmsg = (struct logmsg *)la->tail;
/*共享内存块不为空，则将消息尾的位置后移*/
if (!la->empty) {
fwd = sizeof(struct logmsg) +
strlen((char *)&lastmsg->str) * sizeof(char) + 1;
la->tail += fwd;
}
/*将日志记录格式化到缓冲区中*/
vsnprintf(buff, MAX_MSG_SIZE, fmt, ap);
len = strlen(buff) * sizeof(char) + 1;
/* not enough space on tail : rewind */
/*共享内存块的尾部没有足够的空间存放这个新消息，则将回到块的起始地址开始存放*/
if (la->head <= la->tail &&
(len + sizeof(struct logmsg)) > (la->end - la->tail)) {
logdbg(stderr, "enqueue: rewind tail to %p\n", la->tail);
la->tail = la->start;
}
/* not enough space on head : drop msg */
/*若写入消息会覆盖其他消息，则抛弃这个消息，不存放*/
if (la->head > la->tail &&
(len + sizeof(struct logmsg)) > (la->head - la->tail)) {
logdbg(stderr, "enqueue: log area overrun, drop msg\n");
if (!la->empty)
la->tail = lastmsg;
return 1;
}
/* ok, we can stage the msg in the area */
/*存入消息后，设置非空*/
la->empty = 0;
msg = (struct logmsg *)la->tail;
msg->prio = prio;
/*拷贝消息内容*/
memcpy((void *)&msg->str, buff, len);
/*设置消息循环队列*/
lastmsg->next = la->tail;
msg->next = la->head;
logdbg(stderr, "enqueue: %p, %p, %i, %s\n", (void *)msg, msg->next,
msg->prio, (char *)&msg->str);
#if LOGDBG
dump_logarea();
#endif
return 0;
}
/*从共享内存区中取日志*/
int log_dequeue (void * buff)
{
struct logmsg * src = (struct logmsg *)la->head;
struct logmsg * dst = (struct logmsg *)buff;
struct logmsg * lst = (struct logmsg *)la->tail;
int len;
/*日志为空*/
if (la->empty)
return 0;
/*第一个消息的长度*/
len = strlen((char *)&src->str) * sizeof(char) +
sizeof(struct logmsg) + 1;
dst->prio = src->prio;
/*取出第一个消息*/
memcpy(dst, src, len);
/*改变第一个块的指针*/
if (la->tail == la->head)
la->empty = 1; /* purge the last log msg */
else {
la->head = src->next;
lst->next = la->head;
}
logdbg(stderr, "dequeue: %p, %p, %i, %s\n",
(void *)src, src->next, src->prio, (char *)&src->str);
/*将取出消息的内存块清零*/
memset((void *)src, 0, len);
return len;
}
/*共享内存块的分配和释放*/
/*释放共享内存等*/
static void free_logarea (void)
{
int shmid;
if (!la)
return;
/*释放信号量集*/
if (la->semid != -1)
semctl(la->semid, 0, IPC_RMID, la->semarg);
/*释放消息缓冲区*/
if (la->buff) {
shmdt(la->buff);
shmctl(la->shmid_buff, IPC_RMID, NULL);
la->buff = NULL;
la->shmid_buff = -1;
}
/*释放消息循环队列的共享内存块*/
if (la->start) {
shmdt(la->start);
shmctl(la->shmid_msg, IPC_RMID, NULL);
la->start = NULL;
la->shmid_msg = -1;
}
/*释放la结构本身*/
shmid = la->shmid;
shmdt(la);
shmctl(shmid, IPC_RMID, NULL);
la = NULL;
}
/*创建共享内存作为其他进程和日志进程之间的通信*/
static int logarea_init (int size)
{
int shmid;
logdbg(stderr,"enter logarea_init\n");
/*创建一个共享内存区域来保存全局变量la，返回值是共享内存id*/
if ((shmid = shmget(IPC_PRIVATE, sizeof(struct logarea),
0644 | IPC_CREAT | IPC_EXCL)) == -1) {
syslog(LOG_ERR, "shmget logarea failed %d", errno);
return 1;
}
/*允许本进程访问这块共享内存区域*/
la = shmat(shmid, NULL, 0);
if (!la) {
syslog(LOG_ERR, "shmat logarea failed %d", errno);
shmctl(shmid, IPC_RMID, NULL);
return 1;
}
/*使用logearea结构的全局变量保存共享内存块的id，起始地址、缓冲区等信息*/
la->shmid = shmid;
la->start = NULL;
la->buff = NULL;
la->semid = -1;
/*创建消息通信的共享内存块大小必须大于最大消息的大小*/
if (size < MAX_MSG_SIZE)
size = DEFAULT_AREA_SIZE;
/*创建消息通信的共享内存块*/
if ((shmid = shmget(IPC_PRIVATE, size,
0644 | IPC_CREAT | IPC_EXCL)) == -1) {
syslog(LOG_ERR, "shmget msg failed %d", errno);
free_logarea();
return 1;
}
/*保存消息通信的共享内存id*/
la->shmid_msg = shmid;
/*保存消息通信的共享内存块起始地址*/
la->start = shmat(la->shmid_msg, NULL, 0);
if (!la->start) {
syslog(LOG_ERR, "shmat msg failed %d", errno);
free_logarea();
return 1;
}
/*将消息通信的共享内存块清零*/
memset(la->start, 0, size);
/*设置消息通信的共享内存块的状态和起始、结束地址*/
la->empty = 1;
la->end = la->start + size;
la->head = la->start;
la->tail = la->start;
/*创建一块共享内存块来作为消息的缓冲区*/
if ((shmid = shmget(IPC_PRIVATE, MAX_MSG_SIZE + sizeof(struct logmsg),
0644 | IPC_CREAT | IPC_EXCL)) == -1) {
syslog(LOG_ERR, "shmget logmsg failed %d", errno);
free_logarea();
return 1;
}
/*允许本进程访问这块共享内存，并返回块的起始地址*/
la->buff = shmat(shmid, NULL, 0);
if (!la->buff) {
syslog(LOG_ERR, "shmat logmsgfailed %d", errno);
free_logarea();
return 1;
}
/*创建一个新的信号量集，这里这个创建的信号量集只有一个信号量*/
if ((la->semid = semget(SEMKEY, 1, 0600 | IPC_CREAT)) < 0) {
syslog(LOG_ERR, "semget failed %d", errno);
free_logarea();
return 1;
}
/*设置创建的信号量集只指定信号量的值，这里设置这个唯一的信号量的值为0*/
la->semarg.val=1;
if (semctl(la->semid, 0, SETVAL, la->semarg) < 0) {
syslog(LOG_ERR, "semctl failed %d", errno);
free_logarea();
return 1;
}
/*设置消息缓冲区*/
la->shmid_buff = shmid;
la->ops[0].sem_num = 0;
la->ops[0].sem_flg = 0;
return 0;
}
还有关于日志处理的其他一些函数，在这里一起分析：
/*调用syslog输出日志*/
static void log_syslog (void * buff)
{
struct logmsg * msg = (struct logmsg *)buff;
syslog(msg->prio, "%s", (char *)&msg->str);
}
/*通过守护进程模式输出日志*/
void log_do_log_daemon(int prio, void *priv, const char *fmt, va_list ap)
{
struct sembuf ops[1];
ops[0].sem_num = la->ops[0].sem_num;
ops[0].sem_flg = la->ops[0].sem_flg;
ops[0].sem_op = -1;
if (semop(la->semid, ops, 1) < 0) {
syslog(LOG_ERR, "semop down failed %d", errno);
return;
}
log_enqueue(prio, fmt, ap);
ops[0].sem_op = 1;
if (semop(la->semid, ops, 1) < 0)
syslog(LOG_ERR, "semop up failed");
}
/*直接通过标准输出，输出日志*/
void log_do_log_std(int prio, void *priv, const char *fmt, va_list ap)
{
if (prio == LOG_INFO) {
vfprintf(stdout, fmt, ap);
fprintf(stdout, "\n");
} else {
fprintf(stderr, "%s: ", log_name);
vfprintf(stderr, fmt, ap);
fprintf(stderr, "\n");
fflush(stderr);
}
}
/*根据设置的日志处理函数，处理log_warn日志*/
void log_warning(const char *fmt, ...)
{
va_list ap;
va_start(ap, fmt);
log_func(LOG_WARNING, log_func_priv, fmt, ap);
va_end(ap);
}
/*根据设置的日志处理函数，处理log_err日志*/
void log_error(const char *fmt, ...)
{
va_list ap;
va_start(ap, fmt);
log_func(LOG_ERR, log_func_priv, fmt, ap);
va_end(ap);
}
/*根据设置的日志处理函数，处理log_debug日志*/
void log_debug(int level, const char *fmt, ...)
{
if (log_level > level) {
va_list ap;
va_start(ap, fmt);
log_func(LOG_DEBUG, log_func_priv, fmt, ap);
va_end(ap);
}
}
/*根据设置的日志处理函数，处理log_info日志*/
void log_info(const char *fmt, ...)
{
va_list ap;
va_start(ap, fmt);
log_func(LOG_INFO, log_func_priv, fmt, ap);
va_end(ap);
}
#if 0 /* Unused */
static void __dump_line(int level, unsigned char *buf, int *cp)
{
char line[16*3+5], *lp = line;
int i, cnt;
cnt = *cp;
if (!cnt)
return;
for (i = 0; i < 16; i++) {
if (i < cnt)
lp += sprintf(lp, " %02x", buf[i]);
else
lp += sprintf(lp, " ");
if ((i % 4) == 3)
lp += sprintf(lp, " |");
if (i >= cnt || !isprint(buf[i]))
buf[i] = ' ';
}
log_debug(level, "%s %.16s |", line, buf);
*cp = 0;
}
static void __dump_char(int level, unsigned char *buf, int *cp, int ch)
{
int cnt = (*cp)++;
buf[cnt] = ch;
if (cnt == 15)
__dump_line(level, buf, cp);
}
#define dump_line() __dump_line(level, char_buf, &char_cnt)
#define dump_char(ch) __dump_char(level, char_buf, &char_cnt, ch)
#endif /* Unused */
/*将共享内存区的日志通过syslog输出到文件中*/
static void log_flush(void)
{
int msglen;
struct sembuf ops[1];
ops[0].sem_num = la->ops[0].sem_num;
ops[0].sem_flg = la->ops[0].sem_flg;
while (!la->empty) {
/*获取信号量*/
ops[0].sem_op = -1;
if (semop(la->semid, ops, 1) < 0) {
syslog(LOG_ERR, "semop down failed %d", errno);
exit(1);
}
/*从共享内存区获取一个新消息*/
msglen = log_dequeue(la->buff);
/*释放信号量*/
ops[0].sem_op = 1;
if (semop(la->semid, ops, 1) < 0) {
syslog(LOG_ERR, "semop up failed");
exit(1);
}
/*取到消息则输出*/
if (msglen)
log_syslog(la->buff);
}
}
/*消息捕捉函数，处理终止信号*/
static void catch_signal(int signo)
{
switch (signo) {
case SIGSEGV:
log_flush();
break;
case SIGTERM:
log_stop_daemon = 1;
break;
}
log_debug(1, "pid %d caught signal -%d", getpid(), signo);
}
/*关闭日志*/
static void __log_close(void)
{
if (log_func == log_do_log_daemon) {
log_flush();
closelog();
free_logarea();
}
}
/*关闭日志*/
void log_close(pid_t pid)
{
int status;
/*非守护进程模式，则输出日志，然后关闭日志*/
if (log_func != log_do_log_daemon || pid < 0) {
__log_close();
return;
}
/*守护进程模式，则发消息，让守护进程自己处理*/
if (pid > 0) {
kill(pid, SIGTERM);
waitpid(pid, &status, 0);
}
}
参考文献：
