Question

The situation: I am creating a server daemon in c that accepts numerous simultaneous connections, and the clients will be sending data to the server. I currently have each client connection being spawned into a new thread.

The problem: if a client sends numerous lines of content very quickly (eg, 10 lines of data in less than a second), the server will see the first two lines, but not the rest.

The question: How can I “queue” the data coming in from the clients (the recv command in c)? Is this something that select or poll would be needed for? Basically, I want to make sure any client can send large amounts of data very quickly without having to worry about any content being dropped. How can this be achieved?

Sample Code: (note: the below code has obviously been heavily modified, esp. by removing error checking. I tried to modify my code so as to make the problem/solution clear without getting bogged down in semantics of irrelevant parts. Please don’t get caught up with any non-standard or missing elements here)

//this function handles the threads
void *ThreadedFunction(void *arg) {
    // do some stuff, like: pull vars out of mystruct
    int nbytes;
    char buf[256];
    while(1) {
        if((nbytes=recv(conid, buf, sizeof buf, 0)) <= 0) {
            //handle break in connection
        } else {
            //for this example, just print out data from client to make my point
            buf[nbytes] = 0;
            printf("%s\n",buf);
        }
    }
}

//main just sets up the connections and creates threads
int main(int argc. char *argv[])
{
    // bind(), listen(), etc... blah blah blah

    while(1) {
        conid = accept(...); //get a connection
        // ... build mystruct to pass vars to threaded function ...
        pthread_t p;
        pthread_create(&p,NULL,ThreadedFunction,&mystruct); //create new thread
    }
}

Answer 1

You don’t need to “queue” the data coming in from the clients.
Because TCP do that for you. Flow control of TCP even slows down clients, if the server is too slow to make space to TCP receiving buffer.

So, probably there is bug in the code of server or client. Maybe client sends ‘\0’ in the end of each line. In that case, the following code would not print all lines:

if((nbytes=recv(conid, buf, sizeof buf, 0)) <= 0) {
    //handle break in connection
} else {
    //for this example, just print out data from client to make my point
    buf[nbytes] = 0;
    printf("%s\n",buf);
}

It is even expected that the 2nd line is the last line what you see, if client sends ‘\0’ at the end of each line.

For example:

If client sends the following lines:

"abc\n\0"
"def\n\0"
"ghi\n\0"

TCP will usually send those by using two packets, that contains following:

"abc\n\0"
"def\n\0ghi\n\0"

Server usually needs 2 recv calls to receive the incoming data.
So your server will use 2 print calls:

printf("%s\n", "abc\n\0\0");
printf("%s\n", "def\n\0ghi\n\0\0");

And the result output is:

abc
def