$ gcc -std=gnu99 -lssl -lcrypto sha256.c 
kristianf@ares /tmp/sha256 $ echo -n "test" | openssl dgst -sha256 
(stdin)= 9f86d081884c7d659a2feaa0c55ad015a3bf4f1b2b0b822cd15d6c15b0f00a08
kristianf@ares /tmp/sha256 $ ./a.out "test"
9f86d081884c7d659a2feaa0c55ad015a3bf4f1b2b0b822cd15d6c15b0f00a08