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Comment by SilasX | Hacker News Reader

The original question didn't refer to such issues of parsing, just checking for whether the full message was received, which, I would think, would be handled by the TCP library read() (or whatever) call. It sounded like the OP (outworlder) was delving into lower-level TCP details that should have been abstracted away -- my thinking is that the TCP caller shouldn't have to concern itself with details about checking whether the full message has been received, at least not as a separate step. That is, it just wouldn't return anything until the full message is received, or would include some data structure that indicates it's not complete. Does that make sense?

Edit: On second thought, I guess OP meant that all of the results were coming back "complete" which doesn't obviate the issue of needing to do a check that handles the "not done" case.

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> the TCP caller shouldn't have to concern itself with details about checking whether the full message has been received

The caller absolutely must concern itself with this crucial “detail”. If you do otherwise, your code is broken, full stop.

You can implement a higher-level protocol on top which handles this kind of thing internally and presents a higher-level interface (e.g. not passing any partial data along to its caller until a full message has been received), but if you are just working with TCP directly, what you get is just a stream of bytes. The guarantee you get is that the bytes will be in order and without any gaps.

If you e.g. send UTF-8 encoded text, you must be prepared on the read side to have your stream of bytes cut off arbitrarily in the middle of a character.

Let function A call TCP read() and pass back a struct that includes the bytes it's received and a flag that indicates whether it's read to the end of the message.

Let function B call TCP read() and never return anything until it's received all the bytes of the message.

Both of those seem (IMHO) like functions you could have in a TCP library. Neither seems (IMHO) like a higher level protocol.

That might be nice. But that's not how it works.

There is no "message" in TCP. So no - you can not have this in a library.

The reason that this function doesn't exist is that even if the sender sent a "message" in a single API call, there is no guarantee that the networking code will send it in one IP packet (or one layer 2 frame). We don't want to couple the message size with the lower network protocol, the type of network equipment and so on, and also want to allow merging of messages for more efficient use of the network (If we have a large window size it allows better throughput).

So the job of separating the stream into messages is left to the application layer. (unlike for example in UDP, but then you have to worry about dropped messages)

What is that responding to? Of course TCP can split a message across multiple packets of the lower level protocol; that has no bearing on the concepts TCP works in and whether it can indicate end-of-message.

If your point is just that TCP doesn't have a concept of a "message" (a bytestream with a clear beginning and ending), then that's fair, but, as I said elsewhere [1] the original comment took for granted that TCP does have a well defined notion of "you've reached the end of the message", or at least, "there is no further data to receive". No one seemed to have a problem with that there, and I was just working off that assumption.

As before, I haven't checked whether this is true (I can't quickly verify from descriptions of TCP).

And, interestingly enough, there's this comment [2], which says that what I described does exist, but isn't the default. So ... I'm at least not getting a consistent answer to my question, and people who think they know what they're talking about are inconsistent with each other.

[1] https://news.ycombinator.com/item?id=23200293

[2] https://news.ycombinator.com/item?id=23200906

> the original comment took for granted that TCP does have a well defined notion of "you've reached the end of the message"

No, the original comment was complaining about a coworker who didn’t understand (and refused to listen when told otherwise) that there is no such notion in TCP. It was a response to another comment complaining about people on the internet (e.g. Stack Overflow) too often making the same mistake.

You’re more or less playing the part of that coworker here. It’s unclear why.

> Both of those seem (IMHO) like functions you could have in a TCP library. Neither seems (IMHO) like a higher level protocol.

It's responding to the last part. It is a higher level of protocol. In the traditional TCP/IP model, it's in the application level. There are many libraries with an API like you asked, they are just in a higher level.

(and TCP_WAITALL is a partial solution, applicable only if you know in advance the exact size of the message you are about to receive)

Those are perfect examples of a higher level protocol, since there is no way you can do it only with TCP.

If it makes you feel better, calling read() on a TCP socket is basically the same as calling read() on a file in a file system. In both cases, you can always end up reading less than you expected, and you always must check how much you actually read. In practice, this means that calls to read() for both should always be in a loop.

> which, I would think, would be handled by the TCP library read()

You would be wrong.

Thanks for the explanation, but I don't see why a library utility function wouldn't do that.

Most TCP libraries simply do not come with a function that does "read, but only give me 0 bytes or n bytes, and if you get some other number please hang on to the leftovers until next time". I guess I could follow your advice, but the first step of doing that would be to write such a function again.

They do... with an option called MSG_WAITALL.

That naturally leads to the question of why it's not the default, which can be answered by understanding the history of TCP and computer networking; and more interestingly, how might things have been different if MSG_WAITALL was the default from the beginning.

I don't think this gets you nonblocking all-or-nothing recv.

But if the goal is to process or retrieve a string of bytes, how do you know when you've got them all? That is the root of the problem: tcp isn't built to exchange messages, it's just the stream transport layer. If you want messages you have to encode and decide that yourself.

Sorry, the rest of the conversation seemed to be assuming that "whether you have received the full message" is well-defined at the level of TCP, as suggested by the original comment where I joined[1]:

>I had to fight a coworker on this. I had quickly created some client code just to validate that the server was working. Due to some quirk, all the messages were arriving in full in every read call. He told me to ship it.

>I said no! "I need to check if there's more data and if so add a loop to read again" "But it is working, release it". That went on for a while, to no avail. Wouldn't look at documentation either.

I was just going along with that.

I didn't check up on TCP further to verify whether this was actually true; if you are saying that's not a well-defined concept, you might want to reply to that comment to say so.

[1] https://news.ycombinator.com/item?id=23195230

Those quotes don't claim messages are received in full, you are misinterpreting them.

Of course, but they do indicate that "there is still more of the message to receive" is a well-defined concept (or at least "there is more data to receive" is), which is all my point requires.

(Edit: also, it would help if you said what the correct interpretation would be, since it has the phrase "messages were arriving in full in every read call".)

>but they do indicate that "there is still more of the message to receive"

Not at the TCP level they don't.

They just give you some bytes. It's up to you to decide whether you have the full message or not and if you want to try to read more. The TCP read functions just give you the data they have. There is no concept of one write at the sender end translating to a complete message at the other end. It's just a stream of bytes.

That poster's coworker was implementing a message-oriented protocol and testing the client and server on the same machine. When running the software in this configuration, the coworker observed that each read returned abd entire message, even though this was not going to be true in other configurations where the client and server are different machines or the messages are larger.

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