Select function in socket programming

select() is the low-tech way of polling sockets for new data to read or for an open TCP window to write. Unless there's some compelling reason not to, you're probably better off using poll(), or epoll_wait() if your platform has it, for better performance.

The select() function allows you to implement an event driven design pattern, when you have to deal with multiple event sources.

Let's say you want to write a program that responds to events coming from several event sources e.g. network (via sockets), user input (via stdin), other programs (via pipes), or any other event source that can be represented by an fd. You could start separate threads to handle each event source, but you would have to manage the threads and deal with concurrency issues. The other option would be to use a mechanism where you can aggregate all the fd into a single entity fdset, and then just call a function to wait on the fdset. This function would return whenever an event occurs on any of the fd. You could check which fd the event occurred on, read that fd, process the event, and respond to it. After you have done that, you would go back and sit in that wait function - till another event on some fd arrives.

select facility is such a mechanism, and the select() function is the wait function. You can find the details on how to use it in any number of books and online resources.

The select function allows you to check on several different sockets or pipes (or any file descriptors at all if you are not on Windows), and do something based on whichever one is ready first. More specifically, the arguments for the select function are split up into three groups:

• Reading: When any of the file descriptors in this category are ready for reading, select will return them to you.

• Writing: When any of the file descriptors in this category are ready for writing, select will return them to you.

• Exceptional: When any of the file descriptors in this category have an exceptional case -- that is, they close uncleanly, a connection breaks or they have some other error -- select will return them to you.

The power of select is that individual file/socket/pipe functions are often blocking. Select allows you to monitor the activity of several different file descriptors without having to have a dedicated thread of your program to each function call.

In order for you to get a more specific answer, you will probably have to mention what language you are programming in. I have tried to give as general an answer as possible on the conceptual level.