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

Author

Nov 17, 2009 at 9:22 AM

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.

382

Nov 17, 2009 at 9:29 AM

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.

Author

Nov 17, 2009 at 10:45 AM

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!

216

Nov 17, 2009 at 10:48 AM

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

Nov 17, 2009 at 1:38 PM

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.

Author

Nov 17, 2009 at 2:06 PM

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 *'"

382

Nov 17, 2009 at 2:13 PM

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

199

Palidine

Nov 17, 2009 at 2:22 PM

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

Author

Nov 17, 2009 at 3:28 PM

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...
<click><load><wait>
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!

Author

Nov 17, 2009 at 3:30 PM

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...

216

Nov 17, 2009 at 4:07 PM

Quote:
Original post by FlyingSolo

I feel rather sheepish at the moment...

I wonder how the guy with bajillion years of C experience feels...

Author