How to detect SD card presence in C++ In Linux

Question

Good afternoon,

I'm running a simple loop for checking whether SD card has been inserted or removed, but it doesn't seem to work correctly all the time.

The problem is, when the program starts with SD card inserted, the if(f.good()) statement is True.

When SD card is removed this statement is False.

However, when the SD card is re-inserted, this statement is still False.

Is there a more reliable way in C++ to detect SD card presence? I'm Running Linux Yocto, based on OpenEmbedded. I would prefer to avoid using any external libraries, and use file IO or system calls if possible.

Thanks.

My loop is shown below,

    while (running)
    {
        ifstream f("/dev/mmcblk1");
        if (f.good()) {
            f.close();
            if (!mounted)
            {
                system("mount /dev/mmcblk1p1 /mnt/Storage");   
                mounted = true;
            }
            sdPresent = true;
        }
        else {
            f.close();
            sdPresent = false;
            mounted = false;
        }
        std::this_thread::sleep_for(std::chrono::milliseconds(500));
    }

Answer 1

The proper direct way to know if a removable device has a media inserted is reading the device size from sysfs.

In your case it would be somewhere like /sys/block/mmcblk1/size. That file should exist always, and contain a unsigned long long value. If it is 0 then no media, if it is >0 then you have a media of that size.

bool has_media()
{
    std::ifstream fsize("/sys/block/mmcblk1/size");
    unsigned long long size;
    return (fsize >> size) && (size > 0);
}

If, instead of polling and sleeping, you want proper notifications of the media insertion/removal, and you don't want / cannot use libudev, then you would need to use uevent and a netlink socket. See this excelent answer with the details.