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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 failed");			exit(1);		}        switch (signo)        {            case SIGHUP:            {                syslog(LOG_INFO, "Re-reading configuration file");                reread();                break;            }            case SIGTERM:            {                syslog(LOG_INFO, "got SIGTERM; exiting");                exit(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++;    /*    * Become a daemon.    */    daemonize(cmd);    /*    * Make sure only one copy of the daemon is running.    */    if (already_running())    {        syslog(LOG_ERR, "daemon already running");        exit(1);    }    /*    * Restore SIGHUP default and block all signals.    */    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");    }    /*    * Create a thread to handle SIGHUP and SIGTERM.    */    err = pthread_create(&tid, NULL, thr_fn, 0);    if (err != 0)    {        err_exit(err, "can’t create thread");    }    /*    * Proceed with the rest of the daemon.    */    /* ... */    exit(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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