What does features of Ethernet devices in ethtool mean? How to changes features in C program?

As we know we can use ethtool -k to get features of Ethernet devices, and we can use ethtool -K to change the featueres' status.

I want to change some features of Ethernet devices in my C program.
I want to use ioctl with ETHTOOL_SFEATURES to do this:

#define ETHTOOL_SFEATURES   0x0000003b /* Change device offload settings */

I think maybe I need to pass an argument of struct ethtool_sfeatures to ioctl:

/**
 * struct ethtool_set_features_block - block with request for 32 features
 * @valid: mask of features to be changed
 * @requested: values of features to be changed
 */
struct ethtool_set_features_block {
    __u32   valid;
    __u32   requested;
};

/**
 * struct ethtool_sfeatures - command to request change in device's features
 * @cmd: command number = %ETHTOOL_SFEATURES
 * @size: array size of the features[] array
 * @features: feature change masks
 */
struct ethtool_sfeatures {
    __u32   cmd;
    __u32   size;
    struct ethtool_set_features_block features[0];
};

But I don't know what the valid of ethtool_set_features_block should be and I don't know what does the mask mean in comments.
I tried to compile ethtool with -g and debug when I use ethtool -K to change a feature, and I can get a mask, but I want to know if the mask would be different in different computer or in different Ethernet devices?
Maybe I should try to get mask of features every time before changing it so that I won't use mask which may be wrong?