there.
I am trying to program a server.
The server receive some information from client
and it sends the information to the other server and it receive the response.
Do I need to use select() on this case?
or Pthread only is enough to do this.
my server has many clients connecting concurrently
please answer me kindly.
And if there is, please tell me the source code or site that I can refer to
One approach to implementing a server that handles multiple clients is to create a pthread for each client connection, so that you can read/write each connection in a dedicated thread. It sounds like what you're asking is "if I have a pthread for each client connection, do I still need to use select?"
In a very simple server you might be able to dispense with select: the connection thread looks something like:
do {
read(conn, cmd);
response = process(cmd);
write(conn, response);
} while (cmd != DONE)
But, even with a dedicated client thread, you may find that you still want to use select to check for available input before calling read on the client connection. For example, if you need to be able to implement an idle timeout on your connection, you won't want to just do a blocking read.
I'm not sure what you mean by using pthread to communicate between client and server - my experience of pthread is the POSIX Thread library which doesn't relate to communications....
Documentation and examples of how to use select are broadly available:
http://support.sas.com/documentation/onlinedoc/sasc/doc750/html/lr2/select.htm
http://www.lowtek.com/sockets/select.html
are examples of what you might find by searching "select c" using google....
Related
I have a program that needs to:
Handle 20 connections. My program will act as client in every connection, each client connecting to a different server.
Once connected my client should send a request to the server every second and wait for a response. If no request is sent within 9 seconds, the server will time out the client.
It is unacceptable for one connection to cause problems for the rest of the connections.
I do not have access to threads and I do not have access to non-blocking sockets. I have a single-threaded program with blocking sockets.
Edit: The reason I cannot use threads and non blocking sockets is that I am on a non-standard system. I have a single RTOS(Real-Time Operating System) task available.
To solve this, use of select is necessary but I am not sure if it is sufficient.
Initially I connect to all clients. But select can only be used to see if a read or write will block or not, not if a connect will.
So when I have connected to say 2 clients and they are all waiting to be served, what if the 3rd does not work, the connection will block causing the first 2 connections to time out as well.
Can this be solved?
I think the connection-issue can be solved by setting a timeout for the connect-operation, so that it will fail fast enough. Of course that will limit you if the network really is working, but you have a very long (slow) path to some of the server(s). That's bad design, but your requirements are pretty harsh.
See this answer for details on connection-timeouts.
It seems you need to isolate the connections. Well, if you cannot use threads you can always resort to good-old-processes.
Spawn each client by forking your server process and use traditional IPC mechanisms if communication between them is required.
If you can neither use a multiprocess approach I'm afraid you'll have a hard time doing that.
I am trying to perform multithreading on a socket in C in order to develop a connector between two different software applications. I would like it to work in the following manner. One piece of software will start running as the server, it will be performing a variety of functions including listening for a socket connection on a designated port. This software will function by it self and only use data from the connected network socket when it is established and receiving reliable data. So for this piece I would like to be able to listen to a connection, and when one is made fork a process and when data is received from this socket set some variable that will be used by some other update thread to notify it that it has these extra precision information that can be considered. On the other side of this equation I want to create a program that when it boots up will attempt to connect to the port of the other application, once this connects it will then simply call a function that will send out the information in non blocking fashion. My whole goal is to create a connector that will allow the programmers of the other two pieces of code to feel as tho they aren't dealing with a socket what so ever.
I have been able to get multi threaded socket communication going but I am now trying to modify this so it will be usable as I have described and I am confused as to how to avoid multiple access to that variable that will notify the system on the server side that the data has arrived as well as create the non-blocking interaction on the client side. Any help ill be appreciated.
-TJ
The question is not so clear to me, but if you need to make different pieces of software talking easily you can consider using a framework message library like ZeroMQ www.zeromq.org
It seems like you have a double producer-consumer problem here:
Client side Server
producer -> sender thread -> receiver thread -> consumer thread
In this case, the most useful data structure to use is a blocking queue on both sides, like intel TBB's concurrent_bounded_queue.
This allows you to post tasks from one thread and have another thread pull the data when it's available in a thread-safe manner.
First of all, the environment I'm working on is Windows 7 and Visual Studio 2010.
I already wrote a server that uses the select method to retrieve data from more than one client.
Also I wrote a client that connected to the server above, by running (client.exe localhost 4444 Peter). "Peter" is the username that this user wants to use.
Now let's say we have two users connected on the server. Each of them has the ability to run the command /help. This returns some other commands that the user can use. One command of these is /listusr that returns all the users on the server.
One other command is the /talk2 and here is where my problem-question begins. I want to let the user choose to which of the other users want to talk. E.g if you want to talk to Peter, you give /talk2 Peter.
How am I going to start something like this? How will the server send the message from me to Peter (I have to add here that when a new user connects, the server saves his/her username and his/her socket number in a struct)?
Do I need to create new threads for each conversation or new processes? Can someone give me some hint or advice to continue my project? I'm little confused on how to manage at this point.
Neither. Your server should maintain some kind of data structure that matches a user id to a client socket handle. When a request comes in with the /talk2 command, the server should look up the corresponding socket handle for that userid and should simply relay that message using send().
A scalable way would be your sever is just responsible to tell both clients the IP address of the other side, and then Peter and you establish the connection so you can talk.
If you really want to have the sever transfer the conversation, you need to consider the following to gain a better scalability:
Use UDP instead of TCP
Use thread instead of process
Spawning a new process would be an ordeal for the server if the number of users interacting at a time are high. But on the other hand it will be simpler to code.
Threads do provide scalability, but then you must be extra careful in your code not to do anything silly. (For example, sending wrong chats to the wrong guys.)
Use select/poll techniques (I am not sure how they perform in the Windows environment, but it works cool with Linux.)
UDP will reduce transmission time, but I am not too sure if it's a good idea. Since you said you already have a code, it would be great pain to switch to UDP.
Just sending the address of required client is also a feasible idea. It reduces a lot of effort from the server, but now you will require dedicated clients.
Try each of them and check which one works best for you. It's a design problem, so there can't be a hard and fast solution. It will depend on the usage of your application. You may also want to use (may be you are already using) the sendto and recvfrom functions.
I'm currently working on a distributed networking project for some networking practice and the idea is to send a file from my server to a few different clients (after breaking up the file) and the clients will find the frequency of a string and return it back.
The problem I'm running into is how to identify each client and send data to each one.
The solution I've been working on to identify each client by their port. The problem arises as to how I handle multiple connections and ports. I know I have to use send() to send the data to a port once I open a connection and etc. but I have no idea how to do this across multiple connections ( I can do this with a single client and server but not with multiple clients)
Does anyone have any suggestions from a high level standpoint? I got one suggestion from a friend who said:
Open a socket
Listen for connections
When a connection request is received, spawn a new thread to handle the connection.
The main process will go back to step 2 to listen for new connections, while the new thread
will handle all data flow with the associated client.
But I'm not really sure I understand this... I've also been referencing http://shoe.bocks.com/net/#socket
Thanks
Your friend is correct. Follow first three steps (mentioned by him) and then you need to:
After spawning thread, send data (read from file) to new socket.
Once entire file is finished, you should disconnect and exit thread. On client side, you should handle disconnect and probably exit.
NOTES:
Also, you can use sendfile() instead of send() if you wish. You can use select() if you wish to handle all connections without spawning threads.
Refer http://beej.us/guide/bgnet/ for details.
EDIT:
how to identify each client? Ans: This is classical port discovery problem but in your case its simple. Server should be listening on well known port (say 12345) and all the clients will connect to it. Once they are connected, server has all sockfds. You need to use these sockfds to send data and identify them.
If you check out networkComms.net, an open source network communication library, once you have created a connection with a client you can keep track of that specific client by looking at it's NetworkIdentifier tag, a guid unique to each client.
If you will be sending large files to all of your clients also check out the included DistributedFileSystem which is specifically designed for that purpose.
I'm learning about TCP/IP and am trying to use it to execute different commands on my server.
I thought i'd start small and build up. I've got a current example running which has a server and client connect, and then the server sends the current time to the client.
Now i want to make it such that the server can handle multiple clients.
How can I do this? I think i could use fork, but is there a way to do it without having multiple processes to worry about?
Are there any good primers on this sort of thing, or could you provide some instructions on how to modify my existing code?
Thanks,
glibc Manual has a nice example. The missing code bits can be found earlier in the chapter. The nice thing about the example is that you do not need multiple threads
I would recommend the use of threads:
One server thread has the sole purpose of listening at the server socket for incoming connections. Once a connection is received, it is passed off to a worker thread, while the server keeps listening.
One or more worker threads. These threads will do the majority of the work. You can choose to use one thread per socket, or you can use the select function to allow one thread to handle multiple sockets.
I don't know any primers off the top of my head, sorry.
Take a look at Erik's answer on this other question. You don't really need to do multithreading.