Parse json file from AWS S3 with C++ and Jsoncpp - c++

Question

I have this C++ function that downloads S3 files as istreams using the AWS SDK C++:
std::istream& s3read(std::string bucket, std::string key) {
Aws::Client::ClientConfiguration aws_conf;
aws_conf.region = Aws::Environment::GetEnv("AWS_REGION");
aws_conf.caFile = "/etc/pki/tls/certs/ca-bundle.crt";
Aws::S3::S3Client s3_client(aws_conf);
Aws::S3::Model::GetObjectRequest object_request;
object_request.WithBucket(bucket.c_str()).WithKey(key.c_str());
auto get_object_outcome = s3_client.GetObject(object_request);
if (get_object_outcome.IsSuccess()) {
std::istream& res = get_object_outcome.GetResult().GetBody();
return res;
} else {
...
};
};
I call it from main.cpp and try to parse it with Jsoncpp:
std::istream& stream = s3read(bucket, key);
Json::Value json;
Json::Reader reader;
reader.parse(stream, json);
However, I keep getting segmentation fault. Why?
I think that the problem is that reader.parse needs binary data and the istream isn't. But, if I'm right, how can I parse the stream as binary?

Answer 1

The issue you'r have is classical returning reference to temporary
You can re-design your code a little, to avoid this. For example:
static Json::Value parse_json(std::istream& src) {
Json::Value ret;
Json::Reader reader;
reader.parse(src, ret);
return ret;
}
// Aws::String is actually same thing to std::string except the allocator
// in case of Android, otherwise this is std::string as it is.
// You can use function like s3read("foo","bar");
Json::Value s3read_json(const Aws::String& bucket,const Aws::String& key) {
static constexpr const char *FILE_NAME = "/etc/pki/tls/certs/ca-bundle.crt";
Aws::Client::ClientConfiguration aws_conf;
aws_conf.region = Aws::Environment::GetEnv("AWS_REGION");
aws_conf.caFile = FILE_NAME;
Aws::S3::S3Client s3_client(aws_conf);
Aws::S3::Model::GetObjectRequest object_request;
object_request.WithBucket( bucket ).WithKey( key );
auto object_outcome = s3_client.GetObject(object_request);
if (object_outcome.IsSuccess()) {
auto result = object_outcome.GetResult();
// destructor of object_outcome is not yet called
return parse_json( result.GetBody() );
} else {
...
// throw std::runtime_error("S3 connection failed");
};
};