import hashlib


def sha256sum(filename):
    """Return the SHA256 string representation of file specified by filename."""
    CHUNK_SIZE = 65536
    sha256 = hashlib.sha256()
    with open(filename, 'rb') as file:
        while True:
            chunk = file.read(CHUNK_SIZE)
            if not chunk:
                break
            sha256.update(chunk)
    return sha256.hexdigest()