For https://www.linux.org.ru/forum/general/14218450
$ gcc create_index.c -o create_index
$ gcc get_line.c -o get_line
./create_index <data_file> <index_file>
./get_line <data_file> <index_file> <line_num>
$ time ./create_index bigbinfile indexfile
real 0m4.175s
user 0m3.974s
sys 0m0.201s
$ time ./get_line bigbinfile indexfile 2904776
o_2~R.96`ÑæÄkO<Óié2CýT
real 0m0.002s
user 0m0.001s
sys 0m0.001s
$ ls -sh bigbinfile
1.5G bigbinfile