CODEWITHSUNDEEP
c++sockets
user1869878
user1869878

Reputation: 83

C++ Socket recv not receiving correct number of bytes?

I'm very new to C++, but I'm trying to learn some basics of TCP socket coding. Anyway, I've been able to send and receive messages, but I want to prefix my packets with the length of the packet (like I did in C# apps I made in the past) so when my window gets the FD_READ command, I have the following code to read just the first two bytes of the packet to use as a short int.

char lengthBuffer[2];

int rec = recv(sck, lengthBuffer, sizeof(lengthBuffer), 0);

short unsigned int toRec = lengthBuffer[1] << 8 | lengthBuffer[0];

What's confusing me is that after a packet comes in the 'rec' variable, which says how many bytes were read is one, not two, and if I make the lengthBuffer three chars instead of two, it reads three bytes, but if it's four, it also reads three (only odd numbers). I can't tell if I'm making some really stupid mistake here, or fundamentally misunderstanding some part of the language or the API. I'm aware that recv doesn't guarantee any number of bytes will be read, but if it's just two, it shouldn't take multiple reads.

Upvotes: 1

Views: 6513

Answers (3)

Julian
Julian

Reputation: 1736

Because you cannot assume how much data will be available, you'll need to continuously read from the socket until you have the amount you want. Something like this should work:

ssize_t rec = 0;
do {
    int result = recv(sck, &lengthBuffer[rec], sizeof(lengthBuffer) - rec, 0);
    if (result == -1) {
        // Handle error ...
        break;
    }
    else if (result == 0) {
        // Handle disconnect ...
        break;
    }
    else {
        rec += result;
    }
}
while (rec < sizeof(lengthBuffer));

Upvotes: 5

Christophe
Christophe

Reputation: 73542

Streamed sockets:

The sockets are generally used in a streamed way: you'll receive all the data sent, but not necessarily all at once. You may as well receive pieces of data.

Your approach of sending the length is hence valid: once you've received the length, you cann then load a buffer, if needed accross successive reads, until you got everything that you expected. So you have to loop on receives, and define a strategy on how to ahandle extra bytes received.

Datagramme (packet oriented) sockets:

If your application is really packet oriented, you may consider to create a datagramme socket, by requesting linux or windows socket(), the SOCK_DGRAM, or better SOCK_SEQPACKET socket type.

Risk with your binary size data:

Be aware that the way you send and receive your size data appers to be assymetric. You have hence a major risk if the sending and receiving between machine with CPU/architectures that do not use the same endian-ness. You can find here some hints on how to ame your code platform/endian-independent.

Upvotes: 1

Slava
Slava

Reputation: 44268

TCP socket is a stream based, not packet (I assume you use TCP, as to send length of packet in data does not make any sense in UDP). Amount of bytes you receive at once does not have to much amount was sent. For example you may send 10 bytes, but receiver may receive 1 + 2 + 1 + 7 or whatever combination. Your code has to handle that, be able to receive data partially and react when you get enough data (that's why you send data packet length for example).

Upvotes: 0

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