Programming

Hello,


I have a client C and server C, the client connects to the server and sends a message. At runtime I want to attach to the message sent, a couple (x, y)

Do you know a mechanism that allows me this?


Thank you.
Cordially.

By couple(x,y) do you mean a struct of int?

Please define explictly what you mean by couple.

The answer is yes, but you probably want to start using a messge headers, that way everybody can decode what the incoming message is.

example:

Code:

the header template is nnnnXX[data]
where nnnn is the length of the packet,  XX  is a pre-agreed upon text code to tell you what the [data] is.

0010DT[raw integer] 
This says "I sent you a raw integer":  the way it is stored memory (4 bytes) 
 because the header is 6 bytes long 6 + 4 = 10 which is the whole length of the entire packet.

0011TXAABfd
I sent you an eleven byte message, the last five are text characters.

you decide what to use in XX. AK is commonly used as "I got it ok", NK (NAK) means I did not get your message. You could make your header template nnnnXXX[data] if you want more descriptive mesage types.

yes a struct of int,

To be more precise:

I want at run time, redirect the socket and attach on it a couple(struct (1,2) for example) and at the reception I will save this information

According to my research, I have to use the library interposition mechanism, LD-preload


Have you an idea about this mechanism if it really allows me to attach the couple at runtime?

Thank you so much for help

Yes and no. LD_PRELOAD is a very ugly hack, and shouldn't be used if there's any other possible way. Why can't you just modify your original program to send headers?

LD_PRELOAD can replace entire system calls with your own code -- indiscriminately. If you override write(), you override every single write and any program probably calls write() for quite a few things other than that socket. It might even open sockets you're not aware of, like a UNIX domain socket for logging and that sort of thing.

So it's not as simple as "add header". You need to check if the file descriptor for your fake send() call is a socket or not, and if it is, send headers. Afterwards, call send() directly.

But, if it's not your program, how do you know it calls send() once for per data item? What if it packs ten into one send()? Then you'll have to actually process the data yourself in your fake send() call, inserting 10 headers.

What if it sends in indiscriminate blocks, or worse, one number at a time? Your fake send() call might get three and a half chunks, half a chunk, a single integer, and have to remember these jumbles for later somehow, making you have to worry about threading and all sorts of IPC junk.

Would it be possible to have this program just connect to a different socket instead? Have that program connect to the actual destination, and process data on the way through. This will be far more straightforward than LD_PRELOAD.

i have juste the executable so i can't modify my original program

i'd like to use for example a library linked to my program , if there is a send or a receive it will redirect it and attach the couple

i want to do like that (i found it in a research)

I've already explained in detail why that's a bad idea -- please read it.

1) If you did this with a proxy, you could just recv() as much as you wanted and send() as much as you want, but when intercepting system calls, there's no guarantee you get what you actually want. Just because you're reading data in x-size chunks doesn't mean it's being written in x-size chunks. Your fake send() call may need to be extremely complex.

2) Just because send() is called doesn't mean it's a send() call you actually wanted to intercept.

3) It's plagued with security problems.

4) It's far harder than it looks, to the point that it's turned out to be literally impossible for most other folks I've tried to help with it. "overloading gettimeofday()", for instance, turned out to mean "overloading gettimeofday(), 9 other time-related functions, select(), half of lpthread, and recompiling". Altering the way a program works without recompiling it can cause it work strangely -- or, more to the point -- not work. Writing 17 bytes when it actually asked to write 12 can have more side-effects than you would have suspected. Yes, you've made it clear. It's just not a good idea.

If you're hellbent on doing so, check out the source code to libkeepalive. It'll show you how to build a library you can LD_PRELOAD.