linux守护进程实现

 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

char * pid_file = "/var/run/daemon.pid";

int init_daemon()
{
 int pid ;
 FILE * lockfd = NULL;
 
 int pidno=0;
 int lockfile=0;
 int val=0;
 char buf[256];
 struct flock lock;

 lockfile=open(pid_file,O_RDWR | O_CREAT, S_IWUSR | S_IRUSR | S_IRGRP | S_IROTH);
 if(lockfile<0)
 {
  exit(1);
 }

 /* see if we can read the contents of the lockfile: Lockfile exists but cannot be read */
 if((val=read(lockfile,buf,(size_t)10))<0)
 {
  exit(1);
 }

 /* we read something - check the PID : Lockfile does not contain a valid PID*/
 if(val>0)
 {
  if((val=sscanf(buf,"%d",&pidno))<1)
  {
   exit(1);
  }
 }

 /* check for SIGHUP */
 if(val==1 && (pid=(pid_t)pidno)==getpid())
 {
  close(lockfile);
  return 0;
 } 

 /* parent process goes away.. */
 if (pid = fork())
 {
  exit(0);
 }
 else if ( pid < 0 )
 {
  exit(1);
 }
 
 /* child continues... */
 /* child becomes session leader... */
 setsid();  
 
 /* place a file lock on the lock file */
 lock.l_type=F_WRLCK;
 lock.l_start=0;
 lock.l_whence=SEEK_SET;
 lock.l_len=0;
 if(fcntl(lockfile,F_SETLK,&lock)<0)
 {
  if(errno==EACCES || errno==EAGAIN)
  {
   fcntl(lockfile,F_GETLK,&lock);
  }
  exit(1);
 }

 /* write PID to lockfile... */
 lseek(lockfile,0,SEEK_SET);
 ftruncate(lockfile,0);
 sprintf(buf,"%d\n",(int)getpid());
 write(lockfile,buf,strlen(buf));

 /* make sure lock file stays open while program is executing... */
 val=fcntl(lockfile,F_GETFD,0);
 val|=FD_CLOEXEC;
 fcntl(lockfile,F_SETFD,val);
 
 close(0);
 close(1);
 close(2);

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


int main()
{
 FILE *fp=NULL;
 time_t t;

 init_daemon();

 while(1)
 {
  sleep(5);
  if((fp=fopen("daemon.log","a"))!= NULL )
  {
   t=time(0); 
   fprintf(fp,"I'm here at %s.",asctime(localtime(&t)) ); 
   fclose(fp);
  }
 }
 return 0;
}