#include <stdio.h>
#include <string.h>
#include <openssl/sha.h>

int main()
{
    unsigned char digest[SHA512_DIGEST_LENGTH];
    char string[] = "hello world";

    SHA512((unsigned char*)&string, strlen(string), (unsigned char*)&digest);

    char mdString[SHA512_DIGEST_LENGTH*2+1];

    for(int i = 0; i < SHA512_DIGEST_LENGTH; i++)
         sprintf(&mdString[i*2], "%02x", (unsigned int)digest[i]);

    printf("SHA512 digest: %sn", mdString);

    return 0;
}

gcc sha512_sample1.cpp -o sample1 -lcrypto

~$ ./sample1
SHA512 digest: 309ecc489c12d6eb4cc40f50c902f2b4d0ed77ee511a7c7a9bcd3ca86d4cd86f989dd35bc5ff499670da34255b45b0cfd830e81f605dcf7dc5542e93ae9cd76f