get space which is used on an APFS volume

Hello,

I am trying to get space which is consumed by APFS volume. The call getattrlist() works fine on macOS 15 (Apple silicon). However, it returns EINVAL on macOS 11.7.10 (Intel) if ATTR_VOL_SPACEUSED is defined.

struct VolAttrBuf
{
    u_int32_t length;
    off_t spaceUsed;
} __attribute__((aligned(4), packed));


int64_t GetVolumeSpaceUsed(const std::string& mountPath)
{
    struct attrlist attrList;

    std::memset(&attrList, 0, sizeof(attrList));

    attrList.bitmapcount = ATTR_BIT_MAP_COUNT;
    attrList.volattr = ATTR_VOL_INFO | ATTR_VOL_SPACEUSED;

    VolAttrBuf attrBuf;

    if (getattrlist(mountPath.c_str(), &attrList, &attrBuf, sizeof(attrBuf), 0) ||
        attrBuf.length > sizeof(attrBuf))
    {
        std::cout << "getattrlist() failed with errno (" << errno << ")" << std::endl;

        return -1;
    }

    return attrBuf.spaceUsed;
}

Is it bug or ATTR_VOL_SPACEUSED is unsupported on macOS 11? Are there any other way to get space which is used on an APFS volume? (C++)

Thank you in advance, Pavel