How to publish binary data

[samuelyhsu]

I am using vcpkg 7e21420f775f72ae938bdeb5e6068f722088f06a，windows10，msvc2022，amd64
The examples are all strings. When I try to publish binary data such as vector<uint8_t>, it prompts for non ASCII characters and crashes

[fpagliughi]

I'm not sure what you mean by, " it prompts for non ASCII characters". What is prompting, etc?

The async_client has a number of publish functions that publish binary data. The most basic takes a pointer to the buffer (const void*) and number of bytes, like:

vector<uint8_t> buf;
// fill the buf
cli.publish("some/topic", buf.data(), buf.size());

In C++, a string can also hold arbitrary binary data. So we typecast a string to a type called binary just to show intent. You can then fill and publish that. In a lot of cases, it's easier than using a vector since it's more functional.

You can do something like:

mqtt::binary buf { "\x00\x01\x02\x03\x04" };
cli.publish(buf);

[samuelyhsu]

I already tried this with v1.5.0, the program crashes when running to publish. v1.4.1 is ok.

vector<uint8_t> buf{0,1,2,255,254,253};
cli.publish("some/topic", buf.data(), buf.size());

[fpagliughi]

Ok. I’ll see if I can recreate it.

[dgabriels]

Just to possibly add more context to this, I believe I might be hitting this as well.

I'm also using vcpkg @ 1.5.0, and it had also been working @ 1.4.1. I'm also on Windows (Linux works fine). And I'm also sending binary data.

This is only an issue for me when in debug - release works fine.

I'm hitting a debug assertion within MQTTPacket_formatPayload (MQTTPacket.c) of the paho.mqtt.c package, line 863 of current version. Obviously, this explains why release works and debug doesn't.

The assertion is happening within isprint. For my particular case, the first character of the binary data was a negative signed char, but is seems isprint is asserting that the char be within the range [-1, 255].

Looking at the commits for paho.mqtt.c, it looks like they "Allowed buffer initialization to work on windows" (33e195a) on Dec 30, which directly impacts the data being fed into MQTTPacket_formatPayload.

All that being said, ultimately this may be a paho.mqtt.c issue and not a cpp one.

[fpagliughi]

Interesting... there's an issue up on the Paho C library about this problem:

eclipse-paho/paho.mqtt.c#1565

[fpagliughi]

I will push this out to the following (v1.6) release, hoping that it gets fixed in the upstream Paho C library by then.

[fpagliughi]

It looks like the the previos Paho C v1.3.15 contained a fix for this. I am currently testing this library over the subsequent v1.3.16, and will close this if things look good.