Understanding IO Multiplexing: select, poll, and epoll in Linux with Code Examples

The article begins with a brief introduction to JNI usage in Java, showing how native C functions can be called from Java code and demonstrating compilation and execution steps with

gcc -fPIC -I"$JAVA_HOME/include" -I"$JAVA_HOME/include/linux" -shared -o libsynchronizer.so DataSynchronizer.c

and java -Djava.library.path=. DataSynchronizer.

It then explains the classic select model, describing its bitmap‑based file‑descriptor set, the parameters of the

select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *errorfds, struct timeval *timeout)

call, and typical usage patterns, including a complete C example of a server that accepts a connection and uses FD_ZERO, FD_SET, and select to detect readable sockets.

Next, the poll model is introduced as an improvement over select. The article presents the poll(struct pollfd *fds, nfds_t nfds, int timeout) interface, explains the pollfd structure (fd, events, revents), and provides a comparable server example that replaces the select logic with poll calls.

The core of the article focuses on the epoll model, detailing how epoll_create, epoll_ctl, and epoll_wait work together, the internal use of an interest list, a ready list, and a red‑black tree for efficient descriptor management. It clarifies the difference between level‑triggered (LT) and edge‑triggered (ET) modes, illustrating with diagrams and concrete output examples of how buffers transition from empty to non‑empty and how notifications are generated.

A complete, well‑commented epoll server implementation in C is provided, showing functions such as SetNonblocking, AddFd, lt_process, and et_process, and the main loop that creates the listening socket, registers it with epoll, and processes events using either LT or ET mode.

Finally, the article discusses performance considerations, debunks the myth that epoll relies on mmap for speed, and emphasizes that its high performance stems from kernel‑level descriptor handling and optional edge‑triggered notifications, making it the preferred choice for high‑concurrency network services.