Wie nutzt man die Kernel Hash Funktionen wie etwa SHA256?

#include <unistd.h>
#include <string.h> // memcpy
#include <sys/socket.h>
#include <sys/sendfile.h>
#include <linux/if_alg.h>
#include <linux/socket.h>

#include <array>
#include <memory>

using sha256_t = std::array<unsigned char, 32>;

sha256_t sha256sum(int in, size_t buffer_size = 2048)
{
	std::unique_ptr<char> buffer(new char[buffer_size]);

	sha256_t digest;

	struct sockaddr_alg sa;
	sa.salg_family = AF_ALG;
	memcpy(sa.salg_type, "hash", 5);
	memcpy(sa.salg_name, "sha256", 7);

	int i, sockfd, fd;

	sockfd = socket(AF_ALG, SOCK_SEQPACKET, 0);
	if (bind(sockfd, (struct sockaddr *)&sa, sizeof(sa)) < 0) {
		perror("bind");
		exit(EXIT_FAILURE);
	}

	fd = accept(sockfd, NULL, 0);
	if (fd < 0) {
		perror("accept");
		exit(EXIT_FAILURE);
	}

	// "pipe" the in fd to the hash algorithm
	int pos;
	int bytes_read;
	while ((bytes_read = read(in, buffer.get(), buffer_size)) > 0)
	{
		pos = 0;
		while (pos < bytes_read)
		{
			int sent = send(fd, buffer.get()+pos, bytes_read-pos, MSG_MORE);
			if (sent < 0)
			{
				perror("send");
				exit(EXIT_FAILURE);
			}
			pos += wrote;
		}
	}
	send(fd, nullptr, 0, 0); // "flush" the hash

	pos = 0;
	while (pos < sizeof(sha256_t))
	{
		int bytes = read(fd, digest.data()+pos, sizeof(sha256_t)-pos);
		if (bytes < 0) {
			perror("read");
			exit(EXIT_FAILURE);
		}
		pos += bytes;
	}
	close(fd);
	close(sockfd);

	return digest;
}

int main(void)
{
	sha256_t digest = sha256sum(STDIN_FILENO);

	for (size_t i = 0; i < sizeof(sha256_t); i++) {
		printf("%02x",digest[i]);
	}
	printf("\n");
	return 0;
}