emit from another thread/function?

zwokaros

Hello

I'm using threads and I can finally have a smooth GUI

Anyway at some part I can't emit

emit packetEvent(QString("hello"));
    pcap_loop(fp, 0, packet_handler, NULL);

(it's a sniffer)

So I can emit before calling the pcap_loop function, but how do I emit from the packet_loop function?

Thanks

aha_1980

hi @zwokaros

Anyway at some part I can't emit
emit packetEvent(QString("hello"));
    pcap_loop(fp, 0, packet_handler, NULL);
(it's a sniffer)

So I can emit before calling the pcap_loop function, but how do I emit from the packet_loop function?

how about doing it from the packet_handler callback?

zwokaros

@aha_1980

I'm not sure I can do that

packet_handler will be called everytime I recieve a packet

void    packet_handler(u_char *state, const struct pcap_pkthdr *header, const u_char *pkt_data)

So I'm not sure how to do it

aha_1980

@zwokaros said in emit from another thread/function?:

@aha_1980

I'm not sure I can do that

packet_handler will be called everytime I recieve a packet

... and isn't that what you want to do, emit when a packet arrived?

zwokaros

@aha_1980
It is but I don't know how to implement it

kshegunov

pcap_loop(fp, 0, packet_handler, this); //< the current object goes as additional data.

void packet_handler(u_char * object, const struct pcap_pkthdr *header, const u_char *pkt_data)
{
   WhateverTheClassWas * emitter = reinterpret_cast<WhateverTheClassWas *>(object);
   emit emitter->packetEvent(QString("Whatever"));
   // ...
}

Read the documentation carefully, it says you can pass user data to the callback. User data means a pointer to some unspecified datastructure (an opaque pointer). It's your responsibility to manage the lifetime and to do the conversions to/from the correct type, however you can pass practically any data set to the callback.

SGaist

Hi,

Following @kshegunov's suggestion, I'd rather go with QMetaObject::invokeMethod. It will be clearer and cleaner. Emitting signal from another class is not something you should do. IIRC, they were even declared as protected in the past.

zwokaros

@kshegunov

Thanks a lot for your answer!

Allthough I couldn't pass this as an argument because of the pcap library, I made a global variable from which I emit in the packet_handler function!

aha_1980

@zwokaros said in emit from another thread/function?:

@kshegunov

Thanks a lot for your answer!

Allthough I couldn't pass this as an argument because of the pcap library,

why not? sure you can!

I made a global variable from which I emit in the packet_handler function!

that works in your case, but breaks when you have multiple callbacks.

kshegunov

@SGaist is correct as usual, QMetaObject::invokeMethod would indeed be preferred to emitting directly.

@zwokaros said in emit from another thread/function?:

I made a global variable from which I emit in the packet_handler function!

To add to @aha_1980, this also breaks reentrancy, so if you're working with multiple threads it'd hurt - you'd need to manually synchronize access. Just pass the object as additional data following the docs is my advice.