How to encapsulate library handle in Perl XS - perl

Question

I wanted to send/receive MQTT messages from Perl. For various reasons (MQTT 5 support, TLS) I don't want to use existing Perl libraries. So I tried to create XS bindings to Paho MQTT C Library. I somehow adapted provided example to link Perl module to Paho library using relly basic Perl XS:
#define PERL_NO_GET_CONTEXT
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include "ppport.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <MQTTClient.h>
#define CLIENTID "ExampleClientPub"
#define QOS 1
#define TIMEOUT 10000L
MODULE = paho PACKAGE = paho
int
mqtt_connect_and_send (server_address, username, topic, payload)
char * server_address
char * username
char * topic
char * payload
CODE:
MQTTClient client;
MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
MQTTClient_message msg = MQTTClient_message_initializer;
MQTTClient_deliveryToken token;
int rc;
/* connect to server */
MQTTClient_create(&client, server_address, CLIENTID,
MQTTCLIENT_PERSISTENCE_NONE, NULL);
conn_opts.keepAliveInterval = 20;
conn_opts.cleansession = 1;
conn_opts.username = username;
if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS)
{
/* didn't connect */
die("Failed to connect, return code %d", rc);
}
/* fill in message data and send it */
msg.payload = payload;
msg.payloadlen = strlen(payload);
msg.qos = QOS;
msg.retained = 0;
MQTTClient_publishMessage(client, topic, &msg, &token);
rc = MQTTClient_waitForCompletion(client, token, TIMEOUT);
/* shutdown connection */
MQTTClient_disconnect(client, 10000);
MQTTClient_destroy(&client);
if (rc != MQTTCLIENT_SUCCESS) {
/* didn't send the message */
die("Failed to send message, return code %d", rc);
}
RETVAL = 1;
OUTPUT:
RETVAL
This is working OK. But now I want to split function mqtt_connect_and_send to 3 functions: mqtt_connect, mqtt_send_message, mqtt_disconnect. And my question is - how to do this? How to create a handle (client in my case) in XS in one function, return it to Perl to somehow store it in a scalar and use that handle in ahother XS function to be used to send more messages? I want to be able to do this in Perl:
my $client = paho::mqtt_connect($server_spec, $username, $password, $more_opts);
$success = paho::mqtt_send($client, $data, $message_opts);
# ... more of mqtt_send's
paho::mqtt_disconnect($server)
I tried to set RETVAL RETVAL = &client or mXPUSHu(&client) but that I didn't get anywhere with that. Can you point me to some example how to get client to Perl and then back to XS to be used again?
Thank you.

Answer 1

Here is an example of how you can return the client as a perl object:
#define PERL_NO_GET_CONTEXT
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include "ppport.h" // allow the module to be built using older versions of Perl
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <MQTTClient.h>
#define CLIENTID "ExampleClientPub"
#define QOS 1
#define TIMEOUT 10000L
UV get_hash_uv(HV *hash, const char *key) {
#define get_hash_uv(a,b) get_hash_uv(aTHX_ a,b)
SV * key_sv = newSVpv (key, strlen (key));
UV value;
if (hv_exists_ent (hash, key_sv, 0)) {
HE *he = hv_fetch_ent (hash, key_sv, 0, 0);
SV *val = HeVAL (he);
STRLEN val_length;
char * val_pv = SvPV (val, val_length);
if (SvIOK (val)) {
value = SvUV (val);
}
else {
croak("Value of hash key '%s' is not a number", key);
}
}
else {
croak("The hash key for '%s' doesn't exist", key);
}
return value;
}
MODULE = Paho PACKAGE = Paho
PROTOTYPES: DISABLE
SV *
mqtt_connect(server_address, username)
char *server_address
char *username
CODE:
int rc;
MQTTClient client; // void *
MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
MQTTClient_create(&client, server_address, CLIENTID,
MQTTCLIENT_PERSISTENCE_NONE, NULL);
conn_opts.keepAliveInterval = 20;
conn_opts.cleansession = 1;
conn_opts.username = username;
if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS)
{
MQTTClient_destroy(&client);
croak("Failed to connect, return code %d", rc);
}
HV *hash = newHV();
SV *self = newRV_noinc( (SV *)hash );
SV *sv = newSVuv(PTR2IV(client));
hv_store (hash, "client", strlen ("client"), sv, 0);
RETVAL = sv_bless(self, gv_stashpv( "Paho::Client", GV_ADD ) );
OUTPUT:
RETVAL
MODULE = Paho PACKAGE = Paho::Client
void
DESTROY(self)
SV *self
CODE:
MQTTClient client; // void *
HV *hv = (HV *) SvRV(self);
UV addr = get_hash_uv(hv, "client");
client = (MQTTClient ) INT2PTR(SV*, addr);
MQTTClient_destroy(&client);
printf("Paho::Client destroy\n");
I am not able to test this yet, because I do not have good values for the input parameters server_address and username. Please provide data that we can test with.

Answer 2

There's no point in building a hash unless you want the class to be extensible.[1] As such, Håkon Hægland's solution can be simplified by returning a scalar-based object. Doing so is quite common for XS-based classes.
#define PERL_NO_GET_CONTEXT
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include "ppport.h" // allow the module to be built using older versions of Perl
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <MQTTClient.h>
#define CLIENTID "ExampleClientPub"
#define QOS 1
#define TIMEOUT 10000L
MODULE = paho PACKAGE = paho
PROTOTYPES: DISABLE
SV *
mqtt_connect(server_address, username)
char *server_address
char *username
CODE:
int rc;
MQTTClient client; // void *
MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
MQTTClient_create(&client, server_address, CLIENTID,
MQTTCLIENT_PERSISTENCE_NONE, NULL);
conn_opts.keepAliveInterval = 20;
conn_opts.cleansession = 1;
conn_opts.username = username;
if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS)
{
MQTTClient_destroy(&client);
croak("Failed to connect, return code %d", rc);
}
SV *sv = newSVuv(PTR2IV(client));
SV *self = newRV_noinc(sv);
RETVAL = sv_bless(self, gv_stashpv("Paho::Client", GV_ADD));
OUTPUT:
RETVAL
void
DESTROY(self)
SV *self
CODE:
MQTTClient client; // void *
client = INT2PTR(MQTTClient, SvUV(SvRV(self)));
MQTTClient_destroy(&client);
printf("Paho::Client destroy\n");
It can still be extended using the inside-out object technique. And of course, it can still be wrapped.