How Does Select () Work?

How does select system call works?

The select system call monitors three sets of independent file descriptors.

If none of the descriptors in the three sets become ready, select would return after the interval pointed by timeout.

If timeout is NULL, select would block till at least one descriptor in the three sets is ready..

What is select function?

The select function is used to determine the status of one or more sockets. For each socket, the caller can request information on read, write, or error status. The set of sockets for which a given status is requested is indicated by an fd_set structure.

What is select system call?

select is a system call and application programming interface (API) in Unix-like and POSIX-compliant operating systems for examining the status of file descriptors of open input/output channels. The select system call is similar to the poll facility introduced in UNIX System V and later operating systems.

Is select a block?

2 Answers. It can easily be tested, but I assure you select() will never block if there is data already available to read on one of the readfds. … When you return to select() it blocks, waiting for more data. However your peer on the other side of the connection is waiting for a response to the data already sent.

Is Select thread safe?

According to the Linux manual page, select is a thread safe function and a cancellation point. On Linux some operating systems, one thread will successfully enter select , while the other threads would be blocked (the body of select is a critical section).

Why Epoll is faster than select?

By contrast, with epoll , the epoll socket itself has a wait list. … By contrast, each call to select that blocks must add the process to every wait queue for every socket being monitored. Ironically, with select , the largest cost comes from checking if sockets that have had no activity have had any activity.