When does recv() return?

Question

I have made a server and a client using sockets. My server is written in c and runs on my raspberry pi and my client is written in Swift and is running on my Mac/iPhone.

What i don't get is when my servers recv() function execute what i have sent from my client? If i send like 0-10 bytes it doesn't return from recv() function before i close the connection. But if I send 100+ bytes it executes immediately.

I would like my server to return from recv() every time i write a message from my client.

Here is my servers recv() function:

//Receive a message from client
while( (read_size = recv(sock , client_message , 2000 , 0)) > 0 )
{
    printf("%s", client_message);
}

Here is my Clients write method:

func SendMsg(msg:String) -> Int{
    return outputStream!.write(msg, maxLength: countElements(msg))
}

Everything works fine beside I'm not in control of when my server executes my messages.

Answer 1

Output to stdout is by default line buffered, so when using printf , make sure there is a newline character ( '\\n' ) at the end. If the message itself doesn't already contain a newline, then :

printf("%s\n", client_message);

As a side note : also make sure that client_message is properly null terminated before passing it to printf , or you might run into undefined behavior (ref. @JoachimPileborg's comment). If you can't ensure null termination, this would do the trick too :

printf("%.*s\n", read_size, client_message);

Answer 2

The server doesn't get the message because the client hasn't sent it.

You need to fflush the outputStream on the client side!