epolldatat

Epolldatat refers to the epoll_data_t union used in the Linux epoll I/O event notification API. It is the data field associated with epoll_event that allows a program to attach user-specific context to each monitored file descriptor.

The epoll_data_t union provides several ways to store the event’s associated information. It typically includes members such as a file descriptor (fd), a generic pointer (ptr), and integer representations (u32 and u64). The exact definition is exposed in the system headers that implement epoll, such as the Linux kernel headers and the C library headers, and is designed to be large enough to hold either a pointer or small integers depending on the platform.

In practice, epoll_event structures carry an events mask and a data field of type epoll_data_t. When a file descriptor is added to the epoll instance with epoll_ctl, the programmer fills the event’s data with the desired context—often a pointer to a per-connection or per-item state, or an integer index. When epoll_wait returns one or more ready events, the corresponding event entries expose the same data field, allowing the program to recover the associated context and interpret it according to how it was stored.

Common usage patterns favor storing a pointer to a per-connection structure in data.ptr, especially on 64-bit systems, while data.fd can be used to carry a file descriptor when appropriate. Developers must ensure that any stored pointers remain valid for the lifetime of the event and handle safety when objects are destroyed.

See also: epoll, epoll_ctl, epoll_wait, epoll_event.