Skip to main content
GameDev.net gamedev.net
🔒 Locked

Using a struct as a buffer with recvfrom (UDP, C)

Started by FlyingSolo Nov 16, 2009 at 6:44 PM 12 replies 8.5k views
Original Post
FlyingSolo
FlyingSolo
This has driven me potty all day yesterday and today. I have an app which is squirting a gob of data at me via UDP. It's a fairly complex chunk of data which is organised as a struct. How can I use a struct as the receive buffer with recvfrom ? No matter what I try, it bitches at me. This is what I want to use... struct AIRSTAT { float : pitch; float : roll; float : yaw; etc... }; AIRSTAT mybuf; How would/could I refer to mybuf using recvfrom? Would a union work which would combine my struct with a char array? I'm stuck - and believe me I've tried!! Help!
Badgerr
Badgerr
Assuming at this point you can be sure that the contents of the buffer are in the format you need, does 

recvfrom(socket, (char*)&mybuf, sizeof(AIRSTAT), from, fromlen);


not work?
________________________________Blog...
FlyingSolo
FlyingSolo
Nup. I've tried that, and just about every combination c has to offer!

With time always short, I finally relented and with my structure packed(1) I can memcpy the input char buffer to the struct and access the data from the struct.

Works like a charm, just a shame that I've got a 2k memcpy command in there that I really don't think I should need.

I've used Delphi a lot over the last 10 years and that gives you the wonderfully useful "absolute" command which allows you to 'overlay' variables on top of each other. A bit like a union, just easier.

I'm surprised I was unable to find a real solution to this problem, it isn't uncommon to send large and/or complex data structures across the net and being able to TX/RX structures of data would make life wonderfully easy.

Cheers.
Evil Steve
Evil Steve
Original post by FlyingSolo
Nup. I've tried that, and just about every combination c has to offer!/quote]In what way does it not work? That's the usual way of sending structs via sockets.
FlyingSolo
FlyingSolo
The MS C compiler whined about being unable to convert from my struct type to a char type.

Just had a chat with a friend of mine who has been a c programmer for about 35 gazillion years - and he can't do it either!
Antheus
Antheus
Quote:
Original post by FlyingSolo
The MS C compiler whined about being unable to convert from my struct type to a char type.

Just had a chat with a friend of mine who has been a c programmer for about 35 gazillion years - and he can't do it either!


So... How about posting some code...

As well as the version of the compiler you are using. Microsoft doesn't have a C compiler, so this is suspect at best.

Also - post a hex dump of the data you are trying to decode (Wireshark or similar).

And - post the code that is actually sending the data, regardless of language.
Windryder
Windryder
Quote:
Original post by FlyingSolo
The MS C compiler whined about being unable to convert from my struct type to a char type.


Sounds to me like you're trying to cast the struct type itself (as opposed to a pointer to it) into a char*... For example:

AIRSTAT myStruct;recvfrom(sock, (char*)myStruct, sizeof(myStruct), &from, &fromLen); // notice the missing '&' in front of myStruct.


You can't cast a non-pointer type to a char* just like that.
FlyingSolo
FlyingSolo

Oooh, lemme see....

struct nr {
float altitude;
float Vind;
float Alpha;
etc...
};

nr netrec;
char inbuf[2000];

int rxbytes=recvfrom(mysocket,(char*)inbuf, maximum_packet_size,0,(sockaddr*)&from,&fromLength);

Compiler is VC Express 2008.

Since I've now used memcpy I know that the integrity of the incoming data is good. It's a big lump at about 2k and no, I can't post the sending code.

Trying any which way to convince recvfrom to accept 'netrec' as the target buffer failed. The usual error was: "cannot convert from 'nr' to 'char *'"
Evil Steve
Evil Steve
Quote:
Original post by FlyingSolo

Oooh, lemme see....

struct nr {
float altitude;
float Vind;
float Alpha;
etc...
};

nr netrec;
char inbuf[2000];

int rxbytes=recvfrom(mysocket,(char*)inbuf, maximum_packet_size,0,(sockaddr*)&from,&fromLength);

Compiler is VC Express 2008.

Since I've now used memcpy I know that the integrity of the incoming data is good. It's a big lump at about 2k and no, I can't post the sending code.

Trying any which way to convince recvfrom to accept 'netrec' as the target buffer failed. The usual error was: "cannot convert from 'nr' to 'char *'"
The original code by Badgerr will work fine:
int rxbytes=recvfrom(mysocket,(char*)&netrec, maximum_packet_size,0,(sockaddr*)&from,&fromLength);

So long as sizeof(netrec) <= maximum_packet_size. If you have variable sized packets, you'll need to do some manipulation.
Palidine
Palidine
Quote:
Original post by FlyingSolo
The usual error was: "cannot convert from 'nr' to 'char *'"


As pointed out you are doing this:
(char*)netrec


instead of:
(char*)&netrec


You need to cast a pointer to netrec to char*, not try to cast the struct itself to char*.

Methinks you should re-read the chapter on pointers.

-me
FlyingSolo
FlyingSolo
Ok, I'm horribly embarrassed here. The sucker just compiled using badgerr's typecasting - but I tried that yesterday.

Time to fire up the other app and see what the data looks like...

Well I'll be damned, it works now.

I don't know what to say here - I tried /exactly/ the same code yesterday and it refused point blank. This is confusing in the extreme.

Although I've added a lot of other code since yesterday, I can't see there is anything new that would/could have affected this. I'm so sorry guys - I really don't know what to say!
FlyingSolo
FlyingSolo
Quote:
Original post by Palidine
Methinks you should re-read the chapter on pointers.


Oh I did, believe me! I would never post a question without doing all the research I could. I spent all day and all evening tinkering with this. All my C books got dusted off and Google took a battering.

I feel rather sheepish at the moment...

Antheus
Antheus
Quote:
Original post by FlyingSolo

I feel rather sheepish at the moment...


I wonder how the guy with bajillion years of C experience feels...
FlyingSolo
FlyingSolo
Quote:
Original post by Antheus
I wonder how the guy with bajillion years of C experience feels...


He now claims he told me to use that method. ;-/

I'm just happy that it works.

I can feel another network issue coming on but I'm afraid to mention it now.
Don't worry, the net works - but it works too well!

Topic Locked

This topic has been locked by a moderator. New replies are not allowed.

Sign in to reply to this topic.