Linux epoll Programming Tutorial362

epoll is a high-performance event notification mechanism in the Linux kernel. It is designed to efficiently handle a large number of file descriptors and monitor events such as incoming connections, data availability, and file system changes. epoll is widely used in network servers, web servers, and other applications that require scalability and low latency.

In this tutorial, we will provide a comprehensive guide to Linux epoll programming. We will cover the basics of epoll, including its data structures, system calls, and usage patterns. We will also present examples of how to use epoll in both C and C++ applications.

Overview of epoll

epoll is an event-driven I/O mechanism. This means that instead of polling for events on each file descriptor, epoll allows you to register file descriptors with an epoll instance and then wait for events to occur on those file descriptors. When an event occurs, epoll will notify the application, which can then take appropriate action.

epoll is based on the following data structures:
epoll instance: An epoll instance is a data structure that represents a collection of file descriptors. It is created using the `epoll_create()` system call.
epoll event: An epoll event is a data structure that represents an event that has occurred on a file descriptor. It is created using the `epoll_event()` system call.

To use epoll, you must follow these steps:1. Create an epoll instance using the `epoll_create()` system call.
2. Add file descriptors to the epoll instance using the `epoll_ctl()` system call.
3. Wait for events to occur on the file descriptors using the `epoll_wait()` system call.
4. Handle the events that have occurred.

epoll_create()

The `epoll_create()` system call creates an epoll instance. The syntax of `epoll_create()` is as follows:```c
int epoll_create(int size);
```

The `size` parameter specifies the number of file descriptors that the epoll instance can initially hold. If the size is too small, you can increase it using the `epoll_ctl()` system call.

epoll_ctl()

The `epoll_ctl()` system call adds or modifies file descriptors in an epoll instance. The syntax of `epoll_ctl()` is as follows:```c
int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);
```

The `epfd` parameter specifies the epoll instance to which the file descriptor should be added or modified.

The `op` parameter specifies the operation to perform. The following operations are supported:
EPOLL_CTL_ADD: Add a file descriptor to the epoll instance.
EPOLL_CTL_MOD: Modify the events that are monitored on a file descriptor.
EPOLL_CTL_DEL: Delete a file descriptor from the epoll instance.

The `fd` parameter specifies the file descriptor to add, modify, or delete.

The `event` parameter specifies the events that should be monitored on the file descriptor. The following event types are supported:
EPOLLIN: The file descriptor is ready for reading.
EPOLLOUT: The file descriptor is ready for writing.
EPOLLPRI: The file descriptor has received a high-priority signal.
EPOLLERR: The file descriptor has encountered an error.
EPOLLHUP: The file descriptor has been closed.

epoll_wait()

The `epoll_wait()` system call waits for events to occur on the file descriptors in an epoll instance. The syntax of `epoll_wait()` is as follows:```c
int epoll_wait(int epfd, struct epoll_event *events, int maxevents, int timeout);
```

The `epfd` parameter specifies the epoll instance to wait for events on.

The `events` parameter specifies an array of epoll events. The `maxevents` parameter specifies the maximum number of events that can be returned in the array.

The `timeout` parameter specifies the maximum amount of time to wait for events, in milliseconds. If the timeout is set to -1, `epoll_wait()` will block until an event occurs.

`epoll_wait()` will return the number of events that occurred on the file descriptors in the `events` array.

Example

The following code shows how to use epoll in a C application:```c
#include
#include
#include
int main() {
int epfd, nfds;
struct epoll_event events[10];
epfd = epoll_create(10);
if (epfd == -1) {
perror("epoll_create");
exit(1);
}
// Add a file descriptor to the epoll instance.
struct epoll_event event;
= EPOLLIN;
= 0; // STDIN
if (epoll_ctl(epfd, EPOLL_CTL_ADD, 0, &event) == -1) {
perror("epoll_ctl");
exit(1);
}
// Wait for events to occur.
while ((nfds = epoll_wait(epfd, events, 10, -1)) == -1) {
perror("epoll_wait");
exit(1);
}
// Handle the events.
for (int i = 0; i < nfds; i++) {
if (events[i].events & EPOLLIN) {
// Handle the event.
}
}
// Clean up.
close(epfd);
return 0;
}
```