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#include "apue.h"#include 
   
    #include 
    
     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 fialed");            return 1;        }        switch(signo)        {            case SIGHUP:                syslog(LOG_INFO,"Re-reading configuration file");                reread();                break;            case SIGTERM:                syslog(LOG_INFO,"got SIGTERM; exiting");                return 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++;    daemonzie(cmd);    if(already_running())    {        syslog(LOG_ERR,"deamon already running");        return 1;    }    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");    err=pthread_create(&tid,NULL,thr_fn,0);    if(err!=0)        err_exit(err,"can't create thread");        return 0;}
    
   

Daemon rereading configuration files

#include "apue.h"#include 
   
    #include 
    
     extern int lockfile(int);extern int already_running(void);void reread(void){}void sigterm(int signo){    syslog(LOG_INFO,"get SIGTERM; exiting");    return 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++;    daemonize(cmd);    if(already_running())    {        syslog(LOG_ERR,"daemon already running");        return 1;    }    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));        return 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));        return 1;    }    return 0;}
    
   

Alternative implementation of daemon rereading configuration files

Client–Server Model

A common use for a daemon process is as a server process. Indeed, in Figure 13.2, we can call the syslogd process a server that has messages sent to it by user processes (clients) using a UNIX domain datagram socket.

In general, a server is a process that waits for a client to contact it, requesting some type of service.

#include "apue.h"#include 
   
    int set_cloexec(int fd){    int val;    if((val=fcntl(fd, F_GETFD,0))<0)        return -1;        val|=FD_CLOEXEC;    return fcntl(fd,F_SETFD,val);}
   

Set close-on-exec flag

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