For https://www.linux.org.ru/forum/general/14218450

Compile

 $ gcc create_index.c -o create_index
 $ gcc get_line.c -o get_line

Usage

./create_index <data_file> <index_file>
./get_line <data_file> <index_file> <line_num>

Example

$ 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