1．编写规则

(1).调用umask码设为0 直接调用umask()函数

umask(0);

(2).将父进程退出

pid_t pid;
pid = fork();
if(pid < 0)
{   
    perror("fork()");
    return -1; 
}   
if(pid > 0)
    exit(0);

(3).调用setsid创建一个新的会话，调用进程成为新会话的首进程，新的进程组组长，并且没有控制终端

if (setsid() == -1) 
{
    perror("setsid()");
    return -1; 
}   
 // PID == PGID == SID no tty

chdir("/");

(5).关闭不需要的文件描述符

(6).将文件描述符0,1,2 指向 /dev/null

fd = open("/dev/null", O_RDWR);
if (fd == -1) 
{
    perror("open()");
    return -1; 
}
dup2(fd, 0); 
dup2(fd, 1); 
dup2(fd, 2);

2.守护进程库函数

daemon(3)

#include <unistd.h>
int daemon(int nochdir, int noclose);

If nochdir is zero, daemon() changes the process's current working directory to the root directory ("/"); otherwise, the current working directory is left unchanged.

当nochdir为0的时候，将工作目录改为“/”目录

If noclose is zero, daemon() redirects standard input, standard output and standard error to /dev/null; otherwise, no changes are made to these file descriptors.

当noclise为0的时候，将文件描述符为1,2,3的指向/dev/null

返回值：

成功返回0；失败返回-1并且设置errno

3.出错记录

#include <syslog.h>
void openlog(const char *ident, int option, int facility);
void syslog(int priority, const char *format, ...);
void closelog(void);

openlog()

打开与日志的连接(ubuntu下守护进程日志路径“/var/log/syslog”)。 ident指向的字符串前置于每条消息，通常设置为程序名称。如果ident为NULL，则使用程序名称。

option可选项

LOG_CONS Write directly to system console if there is an error while sending to system logger.

LOG_NDELAY Open the connection immediately (normally, the connection is opened when the first message is logged).

LOG_NOWAIT Don't wait for child processes that may have been created while logging the message. (The GNU C library does not create a child process,so this option has no effect on Linux.)

LOG_ODELAY The converse of LOG_NDELAY; opening of the connection is delayed until syslog() is called. (This is the default, and need not be specified.)

LOG_PERROR (Not in POSIX.1-2001 or POSIX.1-2008.) Print to stderr as well.

LOG_PID Include PID with each message.

facility

可选项查看man手册

具体用法

openlog(NULL, LOG_PID | LOG_PERROR, LOG_DAEMON);
fp = fopen(FLNAME, "w");
    if (NULL == fp) 
    {
        // perror("fopen()");
        syslog(LOG_ERR, "fopen():%s", strerror(errno));
        exit(1);
    }
    while (1) 
    {
        time(&tm);
        tmp = localtime(&tm);
        if (NULL == tmp) 
        {
            // fprintf(stderr, "localtime() failed\n");
            syslog(LOG_ERR, "localtime() failed");
            goto ERROR;
        }
        strftime(buf, BUFSIZE, "%Y-%m-%d %H:%M:%S\n", tmp);
        fputs(buf, fp);
        // 调试
        syslog(LOG_INFO, "%s write into the file %s", buf, FLNAME);
        fflush(NULL);
        sleep(1);
    }

    fclose(fp);

4.单实例守护进程

一个守护进程可以开启多个，如果加以限制，只能开启一个，就是单实例守护进程

static int single_instance(void)
{
    int fd;
    char buf[BUFSIZE] = {};

    fd = open(DAEMON_FILE, O_RDWR | O_CREAT, 0666);
    if (fd < 0) {
        syslog(LOG_ERR, "open():%s", strerror(errno));
        return -1;
    }

    if (lockf(fd, F_TLOCK, 0) < 0) {
        if (errno == EACCES || errno == EAGAIN) {
            // 已被占用
            close(fd);
            return -1;
        }
        syslog(LOG_ERR, "lockf():%s", strerror(errno));
        exit(1);
    }

    ftruncate(fd, 0);
    snprintf(buf, BUFSIZE, "%d", getpid());
    write(fd, buf, strlen(buf));
}

综合程序

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <syslog.h>
#include <errno.h>
#include <string.h>

#define BUFSIZE 1024
#define FLNAME  "/tmp/out"

#define DAEMON_FILE "/var/run/daemon.pid"

static int mydaemon(void);
static int single_instance(void);
int main(void)
{
    time_t tm;
    struct tm *tmp = NULL;
    char buf[BUFSIZE] = {};
    FILE *fp = NULL;

    // 守护进程
#if 0
    if (mydaemon() < 0) {
        fprintf(stderr, "daemon failed\n");
        exit(1);
    }
#endif

    // 提交日志
    openlog(NULL, LOG_PID | LOG_PERROR, LOG_DAEMON);

#if 1
    if (daemon(0, 0) == -1) {
        perror("daemon()");
        exit(1);
    }
#endif

    // 单实例
    if (single_instance() < 0) {
        syslog(LOG_ERR, "single_instance() failed");
        exit(1);
    }

    fp = fopen(FLNAME, "w");
    if (NULL == fp) {
        // perror("fopen()");
        syslog(LOG_ERR, "fopen():%s", strerror(errno));
        exit(1);
    }

    while (1) {
        time(&tm);
        tmp = localtime(&tm);
        if (NULL == tmp) {
            // fprintf(stderr, "localtime() failed\n");
            syslog(LOG_ERR, "localtime() failed");
            goto ERROR;
        }
        strftime(buf, BUFSIZE, "%Y-%m-%d %H:%M:%S\n", tmp);
        fputs(buf, fp);
        // 调试
        syslog(LOG_INFO, "%s write into the file %s", buf, FLNAME);
        fflush(NULL);   
        sleep(1);
    }

    fclose(fp);
    closelog();
    exit(0);
ERROR:
    closelog();
    fclose(fp);
    exit(1);
}

static int mydaemon(void)
{
    pid_t pid;
    int fd;

    pid = fork();
    if (pid < 0) {
        perror("fork()");
        return -1;
    }
    if (pid > 0) 
        exit(0);
    if (setsid() == -1) {
        perror("setsid()");
        return -1;
    }
    // PID == PGID == SID no tty
    
    fd = open("/dev/null", O_RDWR);
    if (fd == -1) {
        perror("open()");
        return -1;
    }
    dup2(fd, 0);
    dup2(fd, 1);
    dup2(fd, 2);

    if (fd > 2)
        close(fd);

    umask(0);
    chdir("/");

    return 0;   
}

// 单实例
static int single_instance(void)
{
    int fd;
    char buf[BUFSIZE] = {};

    fd = open(DAEMON_FILE, O_RDWR | O_CREAT, 0666);
    if (fd < 0) {
        syslog(LOG_ERR, "open():%s", strerror(errno));          
        return -1;
    }

    if (lockf(fd, F_TLOCK, 0) < 0) {
        if (errno == EACCES || errno == EAGAIN) {
            // 已被占用
            close(fd);
            return -1;
        }
        syslog(LOG_ERR, "lockf():%s", strerror(errno));
        exit(1);
    }

    ftruncate(fd, 0);   
    snprintf(buf, BUFSIZE, "%d", getpid());
    write(fd, buf, strlen(buf));    

    // close(fd);
}