openSSL sha256 calculation

calc_sha256.c

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <openssl/buffer.h>
#include <openssl/evp.h>
#include <openssl/sha.h>

int main(void) {

    unsigned char *hash = (unsigned char*) malloc(SHA256_DIGEST_LENGTH);
    if (!hash) {
        printf("OOM");
        return 1;
    }

    EVP_MD_CTX *sha256 = EVP_MD_CTX_new();
    int is_success = EVP_DigestInit(sha256, EVP_sha256());
    if (!is_success) {
        printf("EVP_DigestInit failed");
        return 1;
    }

    uint8_t test[1] = {0};
    is_success = EVP_DigestUpdate(sha256, &test, 0);
    if (!is_success) {
        printf("EVP_DigestUpdate failed");
        return 1;
    }

    is_success = EVP_DigestFinal_ex(sha256, hash, NULL);
    if (!is_success) {
        printf("EVP_DigestFinal failed");
        return 1;
    }

    EVP_MD_CTX_free(sha256);
    unsigned i;
    for (i = 0; i < SHA256_DIGEST_LENGTH; i++) {
        printf("%02X", hash[i]);
    }
    printf("\n");

    return 0;
}

/*
 * Compile: gcc -g -o calc_sha256 calc_sha256.c -lssl -lcrypto
 */