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epoll_wait() - Unix, Linux System Call

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epoll_wait - wait for an I/O event on an epoll file descriptor


#include <sys/epoll.h> 

int epoll_wait(int epfd, struct epoll_event * events, int maxevents, int timeout);


Wait for events on the epoll file descriptor epfd for a maximum time of timeout milliseconds. The memory area pointed to by events will contain the events that will be available for the caller. Up to maxevents are returned by epoll_wait(2).

The maxevents parameter must be greater than zero. Specifying a timeout of -1 makes epoll_wait(2) wait indefinitely, while specifying a timeout equal to zero makes epoll_wait(2) to return immediately even if no events are available (return code equal to zero).

The struct epoll_event is defined as :

typedef union epoll_data { void *ptr; int fd; __uint32_t u32; __uint64_t u64; } epoll_data_t;

struct epoll_event { __uint32_t events; /* Epoll events */ epoll_data_t data; /* User data variable */ };

The data of each returned structure will contain the same data the user set with a epoll_ctl(2) (EPOLL_CTL_ADD,EPOLL_CTL_MOD) while the events member will contain the returned event bit field.


When successful, epoll_wait(2) returns the number of file descriptors ready for the requested I/O, or zero if no file descriptor became ready during the requested timeout milliseconds. When an error occurs, epoll_wait(2) returns -1 and errno is set appropriately.


EBADF epfd is not a valid file descriptor.
EFAULT The memory area pointed to by events is not accessible with write permissions.
EINTR The call was interrupted by a signal handler before any of the requested events occurred or the timeout expired.
EINVAL epfd is not an epoll file descriptor, or maxevents is less than or equal to zero.


epoll_wait(2) is a new API introduced in Linux kernel 2.5.44. The interface should be finalized by Linux kernel 2.5.66.


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