服务程序后台运行代码

使程序脱离终端，完全放在后台运行代码：

void daemon(void) {                                  
  int fr=0;                                               
                                                          
  fr = fork();                                            
  if( fr < 0 ) {                                          
    fprintf(stderr, "fork() failed\n");                   
    exit(1);                                              
  }                                                       
  if ( fr > 0 ) {                                         
    exit(0);                                              
  }                                                       
                                                          
  if( setsid() < 0 ) {                                    
    fprintf(stderr, "setsid() failed\n");                 
    exit(1);                                              
  }                                                       
                                                          
  fr = fork();                                            
  if( fr < 0 ) {                                          
    fprintf(stderr, "fork() failed\n");                   
    exit(1);                                              
  }                                                       
  if ( fr > 0 ) {                                         
    fprintf(stderr, "forked to background (%d)\n", fr);   
    exit(0);                                              
  }                                                       
                                                          
  umask(0);                                               
                                                          
  chdir("/");                                             
  close(0);                                               
  close(1);                                               
  close(2);                                               
                                                          
  open("/dev/null", O_RDWR);                              
  dup(0);                                                 
  dup(0);                                                 
}