1.守护进程的编程规则
1）用umask将文件模式创建屏蔽字设置为0
2）调用fork,然后使父进程退出
3）调用setsid创建一个新会话
4）将当前工作目录更改为根目录
5）关闭不再需要的的文件描述符
6）某些守护进程打开/dev/null使其具有文件描述符0,1,2,
例：初始化一个守护进程
#include "apue.h"
#include <syslog.h>
#include <fcntl.h>
#include <sys/resource.h>
void
daemonize(const char *cmd)
{
int i, fd0, fd1, fd2;
pid_t pid;
struct rlimit rl;
struct sigaction sa;
/*
* Clear file creation mask.
*/
umask(0);
/*
* Get maximum number of file descriptors.
*/
if (getrlimit(RLIMIT_NOFILE, &rl) < 0)
err_quit("%s: can't get file limit", cmd);
/*
* Become a session leader to lose controlling TTY.
*/
if ((pid = fork()) < 0)
err_quit("%s: can't fork", cmd);
else if (pid != 0) /* parent */
exit(0);
setsid();
/*
* Ensure future opens won't allocate controlling TTYs.
*/
sa.sa_handler = SIG_IGN;
sigemptyset(&sa.sa_mask);
sa.sa_flags = 0;
if (sigaction(SIGHUP, &sa, NULL) < 0)
err_quit("%s: can't ignore SIGHUP");
if ((pid = fork()) < 0)
err_quit("%s: can't fork", cmd);
else if (pid != 0) /* parent */
exit(0);
/*
* Change the current working directory to the root so
* we won't prevent file systems from being unmounted.
*/
if (chdir("/") < 0)
err_quit("%s: can't change directory to /");
/*
* Close all open file descriptors.
*/
if (rl.rlim_max == RLIM_INFINITY)
rl.rlim_max = 1024;
for (i = 0; i < rl.rlim_max; i++)
close(i);
/*
* Attach file descriptors 0, 1, and 2 to /dev/null.
*/
fd0 = open("/dev/null", O_RDWR);
fd1 = dup(0);
fd2 = dup(0);
/*
* Initialize the log file.
*/
openlog(cmd, LOG_CONS, LOG_DAEMON);
if (fd0 != 0 || fd1 != 1 || fd2 != 2) {
syslog(LOG_ERR, "unexpected file descriptors %d %d %d",
fd0, fd1, fd2);
exit(1);
}
}
2.出错记录
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使用syslog设施产生日志
有三种方法产生日志信息：
1）内核调用log函数
2）调用syslog函数以产生日志消息
3）用网络程序发送到其他主机的514端口
syslogd守护进程读取三种格式的日志信息,此守护进程在启动时读一个配置文件，/etc/syslog.conf，该文件决定了不同种类的消息送向何处。
#include <syslog.h> void openlog(const char *ident, int option, int |
| Returns: previous log priority mask value |
3.单实例守护进程
使用文件和记录锁保证只运行守护进程的一个副本
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <syslog.h>
#include <string.h>
#include <errno.h>
#include <stdio.h>
#include <sys/stat.h>
#define LOCKFILE "/var/run/daemon.pid"
#define LOCKMODE (S_IRUSR|S_IWUSR|S_IRGRP|S_IROTH)
extern int lockfile(int);
int
already_running(void)
{
int fd;
char buf[16];
fd = open(LOCKFILE, O_RDWR|O_CREAT, LOCKMODE);
if (fd < 0) {
syslog(LOG_ERR, "can't open %s: %s", LOCKFILE, strerror(errno));
exit(1);
}
if (lockfile(fd) < 0) {
if (errno == EACCES || errno == EAGAIN) {
close(fd);
return(1);
}
syslog(LOG_ERR, "can't lock %s: %s", LOCKFILE, strerror(errno));
exit(1);
}
ftruncate(fd, 0);
sprintf(buf, "%ld", (long)getpid());
write(fd, buf, strlen(buf)+1);
return(0);
}
4.守护进程的惯例
例：守护进程重新读配置文件
#include "apue.h"
#include <pthread.h>
#include <syslog.h>
sigset_t mask;
extern int already_running(void);
void
reread(void)
{
/* ... */
}
void *
thr_fn(void *arg)
{
int err, signo;
for (;;) {
err = sigwait(&mask, &signo);
if (err != 0) {
syslog(LOG_ERR, "sigwait failed");
exit(1);
}
switch (signo) {
case SIGHUP:
syslog(LOG_INFO, "Re-reading configuration file");
reread();
break;
case SIGTERM:
syslog(LOG_INFO, "got SIGTERM; exiting");
exit(0);
default:
syslog(LOG_INFO, "unexpected signal %d\n", signo);
}
}
return(0);
}
int
main(int argc, char *argv[])
{
int err;
pthread_t tid;
char *cmd;
struct sigaction sa;
if ((cmd = strrchr(argv[0], '/')) == NULL)
cmd = argv[0];
else
cmd++;
/*
* Become a daemon.
*/
daemonize(cmd);
/*
* Make sure only one copy of the daemon is running.
*/
if (already_running()) {
syslog(LOG_ERR, "daemon already running");
exit(1);
}
/*
* Restore SIGHUP default and block all signals.
*/
sa.sa_handler = SIG_DFL;
sigemptyset(&sa.sa_mask);
sa.sa_flags = 0;
if (sigaction(SIGHUP, &sa, NULL) < 0)
err_quit("%s: can't restore SIGHUP default");
sigfillset(&mask);
if ((err = pthread_sigmask(SIG_BLOCK, &mask, NULL)) != 0)
err_exit(err, "SIG_BLOCK error");
/*
* Create a thread to handle SIGHUP and SIGTERM.
*/
err = pthread_create(&tid, NULL, thr_fn, 0);
if (err != 0)
err_exit(err, "can't create thread");
/*
* Proceed with the rest of the daemon.
*/
/* ... */
exit(0);
}
另外一种实现
#include "apue.h"
#include <syslog.h>
#include <errno.h>
extern int lockfile(int);
extern int already_running(void);
void
reread(void)
{
/* ... */
}
void
sigterm(int signo)
{
syslog(LOG_INFO, "got SIGTERM; exiting");
exit(0);
}
void
sighup(int signo)
{
syslog(LOG_INFO, "Re-reading configuration file");
reread();
}
int
main(int argc, char *argv[])
{
char *cmd;
struct sigaction sa;
if ((cmd = strrchr(argv[0], '/')) == NULL)
cmd = argv[0];
else
cmd++;
/*
* Become a daemon.
*/
daemonize(cmd);
/*
* Make sure only one copy of the daemon is running.
*/
if (already_running()) {
syslog(LOG_ERR, "daemon already running");
exit(1);
}
/*
* Handle signals of interest.
*/
sa.sa_handler = sigterm;
sigemptyset(&sa.sa_mask);
sigaddset(&sa.sa_mask, SIGHUP);
sa.sa_flags = 0;
if (sigaction(SIGTERM, &sa, NULL) < 0) {
syslog(LOG_ERR, "can't catch SIGTERM: %s", strerror(errno));
exit(1);
}
sa.sa_handler = sighup;
sigemptyset(&sa.sa_mask);
sigaddset(&sa.sa_mask, SIGTERM);
sa.sa_flags = 0;
if (sigaction(SIGHUP, &sa, NULL) < 0) {
syslog(LOG_ERR, "can't catch SIGHUP: %s", strerror(errno));
exit(1);
}
/*
* Proceed with the rest of the daemon.
*/
/* ... */
exit(0);
}