Linux守护进程实现

Linux守护进程

redis版：

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版：

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);
}

lighttpd版：

#ifdef HAVE_FORK
static void daemonize(void) {
#ifdef SIGTTOU
	signal(SIGTTOU, SIG_IGN);
#endif
#ifdef SIGTTIN
	signal(SIGTTIN, SIG_IGN);
#endif
#ifdef SIGTSTP
	signal(SIGTSTP, SIG_IGN);
#endif
	if (0 != fork()) exit(0);

	if (-1 == setsid()) exit(0);

	signal(SIGHUP, SIG_IGN);

	if (0 != fork()) exit(0);

	if (0 != chdir("/")) exit(0);
}
#endif

nginx版：

ngx_int_t
ngx_daemon(ngx_log_t *log)
{
    int  fd;

    switch (fork()) {
    case -1:
        ngx_log_error(NGX_LOG_EMERG, log, ngx_errno, "fork() failed");
        return NGX_ERROR;

    case 0:
        break;

    default:
        exit(0);
    }

    ngx_pid = ngx_getpid();

    if (setsid() == -1) {
        ngx_log_error(NGX_LOG_EMERG, log, ngx_errno, "setsid() failed");
        return NGX_ERROR;
    }

    umask(0);

    fd = open("/dev/null", O_RDWR);
    if (fd == -1) {
        ngx_log_error(NGX_LOG_EMERG, log, ngx_errno,
                      "open(\"/dev/null\") failed");
        return NGX_ERROR;
    }

    if (dup2(fd, STDIN_FILENO) == -1) {
        ngx_log_error(NGX_LOG_EMERG, log, ngx_errno, "dup2(STDIN) failed");
        return NGX_ERROR;
    }

    if (dup2(fd, STDOUT_FILENO) == -1) {
        ngx_log_error(NGX_LOG_EMERG, log, ngx_errno, "dup2(STDOUT) failed");
        return NGX_ERROR;
    }

#if 0
    if (dup2(fd, STDERR_FILENO) == -1) {
        ngx_log_error(NGX_LOG_EMERG, log, ngx_errno, "dup2(STDERR) failed");
        return NGX_ERROR;
    }
#endif

    if (fd > STDERR_FILENO) {
        if (close(fd) == -1) {
            ngx_log_error(NGX_LOG_EMERG, log, ngx_errno, "close() failed");
            return NGX_ERROR;
        }
    }

    return NGX_OK;
}

注释版：

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>

int daemonize(void)
{
        int fd;
        switch(fork()) {
                case -1:
                        return (-1);
                case 0:
                        break;
                default:
                        //将父进程结束,让子进程变成真正的孤儿进程,并被init进程接管
                        exit(0);
        }
        //子进程成为新的会话组长和新的进程组长，并与原来的登录会话和进程组脱离
        setsid();

        if ((fd = open("daemon.log", O_CREAT|O_RDWR|O_APPEND, 0)) != -1) {
                //dup2(int oldhandle, int newhandle)复制文件句柄
                dup2(fd, STDIN_FILENO);
                dup2(fd, STDOUT_FILENO);
                dup2(fd, STDERR_FILENO);
                //0,1和2文件句柄分别与标准输入,标准输出,标准错误输出相关联
                //所以用户应用程序调用open函数打开文件时,默认都是以3索引为开始句柄
                //fd已经由新的句柄代替,关闭fd句柄
                if (fd > STDERR_FILENO) (void)close(fd);
        }
        return 0;
}

int main(int argc, char *argv[])
{
        daemonize();
        printf("%s\n","hello");
        return 0;
}

# cc daemonize.c -o daemonize

# ./daemonize

# ps aux | grep daemonize

参考：

http://blog.linuxphp.org/archives/1476/

http://www.cnblogs.com/xuxm2007/archive/2011/07/29/2121280.html

http://blog.csdn.net/yyyzlf/article/details/5267954

http://docs.linuxtone.org/ebooks/C&CPP/c/ch34s03.html