什么情况下用daemon进程
生产环境下,除了我们ssh登录上去,然后手动跑的那部分以外,其他都是自动运行的,这些大部分都应该是后台执行的。如何才能后台执行呢?
- nohup ./XXX &
- 由系统的其他daemon进程启动。这样的话,你的程序是它的子进程,跟终端没关系。退出终端也不会导致进程退出。如写在crontab里。
- 写成daemon程序,可以手动执行,退出终端时程序不退出。
如何选择呢?
(1)首先,清理过期日志这一类需求,可以写一个死循环一直运行,也可以写在crontab里,每次执行完就退出(如果每分钟一次可以满足的话);
(2)crontab的需要接受最多1分钟的时延,如果实时性要求更高一些,那么就需要考虑写个死循环了,这个程序可以由crontab来start和restart,只有在挂了重启时才会出现1分钟时延;
(3)服务不能中断的(nginx、redis、apache,所有在线服务),一般都是daemon程序。但理论上用(2)似乎也可以;当然这两者细节上有很多区别。
怎么用daemon进程
linux C/C++可以直接调用int daemon(int, int)函数,不需要自己重新实现。
示例代码:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <iostream>
int main(int argc, char *argv[]) {
if (argc < 3) {
printf("Usage: $0 no_ch_dir no_close_fd\n");
exit(0);
}
int no_ch_dir = atoi(argv[1]);
int no_close_fd = atoi(argv[2]);
std::cout << "main pid : " << getpid() << std::endl;
std::cout << "main parent pid : " << getppid() << std::endl;
std::cout << "main pwd : " << get_current_dir_name() << std::endl;
if (daemon(no_ch_dir, no_close_fd) != 0) {
// 一般都用daemon(0,0)
// 成功返回0,失败返回-1
// daemon(0,0):chdir到/,关闭0,1,2描述符。
std::cout << "stdout: daemon = -1" << std::endl;
std::cerr << "stderr: daemon = -1" << std::endl;
return 1;
}
std::cout << "stdout: daemon = 0" << std::endl;
std::cerr << "stderr: daemon = 0" << std::endl;
std::cout << "sub pid : " << getpid() << std::endl;
std::cout << "sub parent pid : " << getppid() << std::endl;
std::cout << "sub pwd : " << get_current_dir_name() << std::endl;
while (1);
return 0;
}```
编译运行:
[[email protected] ~]g++test1.cc?otest1[chenming@localhost ] ./test1
Usage: 0nochdirnoclosefd[chenming@localhost ] ./test1 0 0
main pid : 7896
main parent pid : 7573
main pwd : /home/chenming
[[email protected] ~]ps?ef|greptest194:chenming78647573516:09pts/000:00:16vimtest1.cc95:chenming789719316:14?00:00:18./test10097:chenming78997573716:15pts/000:00:00grep?inE–colortest1[chenming@localhost ] ll /proc/7897/fd
total 0
lrwx——. 1 chenming chenming 64 May 1 16:15 0 -> /dev/null
lrwx——. 1 chenming chenming 64 May 1 16:15 1 -> /dev/null
lrwx——. 1 chenming chenming 64 May 1 16:15 2 -> /dev/null
[[email protected] ~]$ lsof -p 7897
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
test1 7897 chenming cwd DIR 253,0 4096 2 /
test1 7897 chenming rtd DIR 253,0 4096 2 /
test1 7897 chenming txt REG 253,0 8355 142202 /home/chenming/test1
test1 7897 chenming mem REG 253,0 1906308 38865 /lib/libc-2.12.so
test1 7897 chenming mem REG 253,0 122232 52742 /lib/libgcc_s-4.4.7-20120601.so.1
test1 7897 chenming mem REG 253,0 142600 38788 /lib/ld-2.12.so
test1 7897 chenming mem REG 253,0 202040 47921 /lib/libm-2.12.so
test1 7897 chenming mem REG 253,0 942040 52866 /usr/lib/libstdc++.so.6.0.13
test1 7897 chenming 0u CHR 1,3 0t0 3903 /dev/null
test1 7897 chenming 1u CHR 1,3 0t0 3903 /dev/null
test1 7897 chenming 2u CHR 1,3 0t0 3903 /dev/null
man 3 daemon可以查看到函数签名:
include
也可以自己实现一个:
include
include
include
include
include
include
redis里的实现要简单一些(redis.c):
void daemonize(void) {
int fd;
if (fork() != 0) exit(0); /* parent exits */
setsid(); /* create a new session */
/* Every output goes to /dev/null. If Redis is daemonized but
* the ‘logfile‘ is set to ‘stdout‘ in the configuration file
* it will not log at all. */
if ((fd = open("/dev/null", O_RDWR, 0)) != -1) {
dup2(fd, STDIN_FILENO);
dup2(fd, STDOUT_FILENO);
dup2(fd, STDERR_FILENO);
if (fd > STDERR_FILENO) close(fd);
}
}
memcached里的实现(daemon.c):
include
include
include
include
include “memcached.h”
int daemonize(int nochdir, int noclose)
{
int fd;
switch (fork()) {
case -1:
return (-1);
case 0:
break;
default:
_exit(EXIT_SUCCESS);
}
if (setsid() == -1)
return (-1);
if (nochdir == 0) {
if(chdir("/") != 0) {
perror("chdir");
return (-1);
}
}
if (noclose == 0 && (fd = open("/dev/null", O_RDWR, 0)) != -1) {
if(dup2(fd, STDIN_FILENO) < 0) {
perror("dup2 stdin");
return (-1);
}
if(dup2(fd, STDOUT_FILENO) < 0) {
perror("dup2 stdout");
return (-1);
}
if(dup2(fd, STDERR_FILENO) < 0) {
perror("dup2 stderr");
return (-1);
}
if (fd > STDERR_FILENO) {
if(close(fd) < 0) {
perror("close");
return (-1);
}
}
}
return (0);
}
“`