There is actually potential problem. fread() can return value between 0 to sizeof(block) in case of error or eof, I guess it is ok to have short read in case of eof, but we probably want to be sure there was no error.

It may be better to use do {} while (!ferror() && !feof()) loop there.

len is already size_t, you can drop the type cast.

I don't see the problem. The next fread() would return zero which ends the loop, and ferror() after that will return non-zero and we will report the error.