#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <pthread.h>
#include <signal.h>
#include <fcntl.h>
#include <syslog.h>
#include <sys/resource.h>void daemonize(const char *cmd)
{int                 i,fd0,fd1,fd2;pid_t               pid;struct rlimit       r1;struct sigaction    sa;umask(0);//获取文件描述符最大值getrlimit(RLIMIT_NOFILE,&r1);//创建子进程if((pid = fork()) < 0){perror("fork() error");exit(0);}else if(pid > 0)  //使父进程退出exit(0);setsid();  //创建会话//创建子进程避免获取终端sa.sa_handler = SIG_IGN;sigemptyset(&sa.sa_mask);sa.sa_flags = 0;sigaction(SIGHUP,&sa,NULL);if((pid = fork()) < 0){perror("fork() error");exit(0);}else if(pid > 0)exit(0);//修改目录chdir("/");//关闭不需要的文件描述符if(r1.rlim_max == RLIM_INFINITY)r1.rlim_max = 1024;for(i=0;i<r1.rlim_max;++i)close(i);//打开文件描述符fd0 = open("/dev/null",O_RDWR);fd1 = dup(0);fd2 = dup(0);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);}
}int main()
{daemonize("ls");sleep(30);  //主进程休眠，以便查看守护进程状态exit(0);
}

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <signal.h>
#include <fcntl.h>
#include <syslog.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];//打开文件，不存在则创建fd = open(LOCKFILE,O-RDWR|O_CREAT,LOCKMODE);if(fd < 0){syslog(LOG_ERR,"can't open %s : %s",LOCKFILE,strerror(errno));exit(1);}//对文件加锁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); //将文件长度截短为0sprintf(buf,"%ld",(long)getpid());write(fd,buf,strlen(buf)+1);return 0;
}

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <signal.h>
#include <fcntl.h>
#include <syslog.h>
#include <sys/stat.h>
#include <sys/resource.h>#define LOCKFILE "/var/run/daemon.pid"
#define LOCKMODE (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH )sigset_t mask;
int lockfile(int fd)
{struct flock f1;f1.l_type = F_WRLCK;f1.l_start = 0;f1.l_whence = SEEK_SET;f1.l_len = 0;return fcntl(fd,F_SETLK,&f1);
}
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;
}void daemonize(const char *cmd)
{int                 i,fd0,fd1,fd2;pid_t               pid;struct rlimit       r1;struct sigaction    sa;umask(0);getrlimit(RLIMIT_NOFILE,&r1);if((pid = fork()) < 0){perror("fork() error");exit(0);}else if(pid > 0)exit(0);setsid();sa.sa_handler = SIG_IGN;sigemptyset(&sa.sa_mask);sa.sa_flags = 0;sigaction(SIGHUP,&sa,NULL);if((pid = fork()) < 0){perror("fork() error");exit(0);}else if(pid > 0)exit(0);chdir("/");if(r1.rlim_max == RLIM_INFINITY)r1.rlim_max = 1024;for(i=0;i<r1.rlim_max;++i)close(i);fd0 = open("/dev/null",O_RDWR);fd1 = dup(0);fd2 = dup(0);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);}
}void reread()
{printf("read daemon config file again.\n");
}
void * thread_func(void *arg)
{int err,signo;while(1){sigwait(&mask,&signo);switch(signo){case SIGHUP:syslog(LOG_INFO,"Re-reading configuration file.\n");reread();break;case SIGTERM:syslog(LOG_INFO,"got SIGTERM;exiting.\n");exit(0);default:syslog(LOG_INFO,"unexpected signal %d.\n",signo);}}return NULL;
}
int main(int argc,char *argv[])
{pthread_t           tid;char                *cmd;struct sigaction    sa;if((cmd = strrchr(argv[0],'/')) == NULL)cmd = argv[0];elsecmd++;daemonize(cmd);if(already_running()){syslog(LOG_ERR,"daemon already running.\n");exit(1);}sa.sa_handler =SIG_DFL;sigemptyset(&sa.sa_mask);sa.sa_flags = 0;sigaction(SIGHUP,&sa,NULL);sigfillset(&mask);pthread_sigmask(SIG_BLOCK,&mask,NULL);pthread_create(&tid,NULL,thread_func,0);sleep(90);exit(0);
}