include/uapi/linux/eventpoll.h

epoll_event

struct epoll_event {
	__poll_t events;
	__u64 data;
} EPOLL_PACKED;

The data member specifies the data that the kernel should save and return when epoll_wait is called.

Compare this struct with the definition in glibc:

struct epoll_event
{
  uint32_t events;	/* Epoll events */
  epoll_data_t data;	/* User data variable */
} __EPOLL_PACKED;

epoll_data_t is defined as

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

eventpoll.h defines the following epoll event masks:

`EPOLLIN`	The associated file is available for `read(2)` operations.	0x00000001
`EPOLLPRI`	There is an exceptional condition on the file descriptor.	0x00000002
`EPOLLOUT`	The associated file is available for `write(2)` operations.	0x00000004
`EPOLLERR`		0x00000008
`EPOLLHUP`	Hang up happened on the associated file descriptor.	0x00000010
`EPOLLNVAL`		0x00000020
`EPOLLRDNORM`		0x00000040
`EPOLLRDBAND`		0x00000080
`EPOLLWRNORM`		0x00000100
`EPOLLWRBAND`		0x00000200
`EPOLLMSG`		0x00000400
`EPOLLRDHUP`	… This flag is especially useful for writing simple code to detect peer shutdown when using edge-triggered monitoring.	0x00002000
`EPOLLET`	Requests edge-triggered notification for the associated file descriptor. (The default behavior for epoll is level-triggered.	`1U << 31`

These bits can be set in the events member of struct epoll_event.