$ gcc -s -O2 nk_conv.c -o nk_conv

# NatsumeSosuke_Botchan.txt 青空文庫の夏目漱石の「坊っちゃん」のtxt
$ time cat NatsumeSosuke_Botchan.txt | while read line; do echo $line | ./nk_conv >> out.txt; done

real	0m0.996s
user	0m0.469s
sys	0m0.801s