linux之popen、open、fopen

1，popen：每次fread都要重新做popen，否则值不刷新。

     //linux在应用层输出设备节点值日志
     
     char bat_vol_get[16] = {0};
     char bat_percentage_get[16] = {0};
     
     while(1)
     {
         sleep(3);
         
         FILE *fd_vol = popen("cat /sys/class/power_supply/battery/voltage_now", "r");
         FILE *fd_per = popen("cat /sys/class/power_supply/battery/capacity", "r");     
         
         fread(bat_vol_get, 1, 15, fd_vol);
         fread(bat_percentage_get, 1, 15, fd_per);
          
         printf("bat_vol_get=%s", bat_vol_get);   
         printf("bat_percentage_get=%s", bat_percentage_get);

         pclose(fd_vol);
         pclose(fd_per);
     }

2，open：open一次后每次read都能获取到刷新的值。

    int fd = 0;
    char result[2] = {0};	
    unsigned char value = 0;
    
	fd = open("/dev/ds18b20", O_RDONLY);
	if (fd < 0) 
	{
		SYS_INFO("open /dev/ds18b20 failed with %s!\n", strerror(errno));
		return;
	}
	
    read(fd, &result, sizeof(result));	//读到的是字符串
    value = atoi(result);               //将字符串转化为int类型
    
    close(fd);