epoll_data_t - Linux


epoll_data_t is a library function in Linux that provides a way to wait for events on a set of file descriptors. It is used in conjunction with epoll_create() and epoll_ctl() to create an epoll instance and add or modify file descriptors to be monitored.


struct epoll_event epoll_data_t;

The epoll_data_t structure is defined as follows:

typedef union epoll_data {
    void        *ptr;
    int          fd;
    uint32_t     u32;
    uint64_t     u64;
} epoll_data_t;

The data member of the epoll_event structure points to an epoll_data_t structure. This structure can be used to store any user-defined data associated with the file descriptor.


The events member of the epoll_event structure can be used to specify the events that the epoll instance should wait for. The following flags can be used:

  • EPOLLIN: The file descriptor is ready for reading.
  • EPOLLOUT: The file descriptor is ready for writing.
  • EPOLLPRI: The file descriptor has received out-of-band data.
  • EPOLLERR: The file descriptor has encountered an error.
  • EPOLLHUP: The file descriptor has been closed.
  • EPOLLET: The epoll instance should use edge-triggered notification instead of level-triggered notification.


The following example shows how to use epoll_data_t to store a pointer to a custom data structure associated with a file descriptor:

struct my_data {
    int fd;
    char *buf;
    size_t buflen;

int main() {
    int epfd = epoll_create1(0);
    struct epoll_event ev;

    struct my_data data;
    data.fd = 0;
    data.buf = NULL;
    data.buflen = 0;

    ev.events = EPOLLIN;
    ev.data.ptr = &data;

    epoll_ctl(epfd, EPOLL_CTL_ADD, 0, &ev);

    while (1) {
        int nfds = epoll_wait(epfd, &ev, 1, -1);
        if (nfds == -1) {

        if (ev.events & EPOLLIN) {
            // Read data from the file descriptor.
            ssize_t nread = read(ev.data.ptr->fd, ev.data.ptr->buf, ev.data.ptr->buflen);
            if (nread == -1) {

            // Process the data.

    return 0;

Common Issues

One common issue with epoll_data_t is that it is not type-safe. This means that it is possible to store a pointer to a data structure of the wrong type in the data member of the epoll_event structure. This can lead to undefined behavior when the epoll instance is used to wait for events on the file descriptor.

To avoid this issue, it is important to ensure that the data structure pointed to by the data member of the epoll_event structure is of the correct type.


epoll_data_t can be used in conjunction with other Linux commands and tools to perform a variety of tasks. For example, it can be used with the select() and poll() commands to create a more efficient event loop. It can also be used with the inotify command to monitor changes to files and directories.

Related Commands

  • epoll_create()
  • epoll_ctl()
  • epoll_wait()
  • select()
  • poll()
  • inotify