Generate SHA hash in C++ using OpenSSL library

From the command line, it's simply:

printf "compute sha1" | openssl sha1

You can invoke the library like this:

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

int main()
{
    unsigned char ibuf[] = "compute sha1";
    unsigned char obuf[20];

    SHA1(ibuf, strlen(ibuf), obuf);

    int i;
    for (i = 0; i < 20; i++) {
        printf("%02x ", obuf[i]);
    }
    printf("\n");

    return 0;
}

OpenSSL has a horrible documentation with no code examples, but here you are:

#include <openssl/sha.h>

bool simpleSHA256(void* input, unsigned long length, unsigned char* md)
{
    SHA256_CTX context;
    if(!SHA256_Init(&context))
        return false;

    if(!SHA256_Update(&context, (unsigned char*)input, length))
        return false;

    if(!SHA256_Final(md, &context))
        return false;

    return true;
}

Usage:

unsigned char md[SHA256_DIGEST_LENGTH]; // 32 bytes
if(!simpleSHA256(<data buffer>, <data length>, md))
{
    // handle error
}

Afterwards, md will contain the binary SHA-256 message digest. Similar code can be used for the other SHA family members, just replace "256" in the code.

If you have larger data, you of course should feed data chunks as they arrive (multiple SHA256_Update calls).

Adaptation of @AndiDog version for big file:

static const int K_READ_BUF_SIZE{ 1024 * 16 };

std::optional<std::string> CalcSha256(std::string filename)
{
    // Initialize openssl
    SHA256_CTX context;
    if(!SHA256_Init(&context))
    {
        return std::nullopt;
    }

    // Read file and update calculated SHA
    char buf[K_READ_BUF_SIZE];
    std::ifstream file(filename, std::ifstream::binary);
    while (file.good())
    {
        file.read(buf, sizeof(buf));
        if(!SHA256_Update(&context, buf, file.gcount()))
        {
            return std::nullopt;
        }
    }

    // Get Final SHA
    unsigned char result[SHA256_DIGEST_LENGTH];
    if(!SHA256_Final(result, &context))
    {
        return std::nullopt;
    }

    // Transform byte-array to string
    std::stringstream shastr;
    shastr << std::hex << std::setfill('0');
    for (const auto &byte: result)
    {
        shastr << std::setw(2) << (int)byte;
    }
    return shastr.str();
}

C version of @Nayfe code, generating SHA1 hash from file:

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

static const int K_READ_BUF_SIZE = { 1024 * 16 };
unsigned char* calculateSHA1(char *filename)
{
    if (!filename) {
        return NULL;
    }

    FILE *fp = fopen(filename, "rb");
    if (fp == NULL) {
        return NULL;
    }

    unsigned char* sha1_digest = malloc(sizeof(char)*SHA_DIGEST_LENGTH);
    SHA_CTX context;

    if(!SHA1_Init(&context))
        return NULL;

    unsigned char buf[K_READ_BUF_SIZE];
    while (!feof(fp))
    {
        size_t total_read = fread(buf, 1, sizeof(buf), fp);
        if(!SHA1_Update(&context, buf, total_read))
        {
            return NULL;
        }
    }
    fclose(fp);

    if(!SHA1_Final(sha1_digest, &context))
        return NULL;

    return sha1_digest;
}

It can be used as follows:

unsigned char *sha1digest = calculateSHA1("/tmp/file1");

The res variable contains the sha1 hash.

You can print it on the screen using the following for-loop:

char *sha1hash = (char *)malloc(sizeof(char) * 41);
sha1hash[40] = '\0';
int i;
for (i = 0; i < SHA_DIGEST_LENGTH; i++)
{
    sprintf(&sha1hash[i*2], "%02x", sha1digest[i]);
}
printf("SHA1 HASH: %s\n", sha1hash);