epoll_wait, epoll_pwait — 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) ; |
int
epoll_pwait( |
int | epfd, |
struct epoll_event * | events, | |
int | maxevents, | |
int | timeout, | |
const sigset_t * | sigmask) ; |
The epoll_wait
() system call
waits 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.
The relationship between epoll_wait
() and epoll_pwait
() is analogous to the
relationship between select(2) and pselect(2): like pselect(2), epoll_pwait
() allows an application to
safely wait until either a file descriptor becomes ready or
until a signal is caught.
The following epoll_pwait
() call:
ready = epoll_pwait(epfd, &events, maxevents, timeout, &sigmask);
is equivalent to atomically
executing the
following calls:
sigset_t origmask; sigprocmask(SIG_SETMASK, &sigmask, &origmask); ready = epoll_wait(epfd, &events, maxevents, timeout); sigprocmask(SIG_SETMASK, &origmask, NULL);
epoll_pwait
() was added to
Linux in kernel 2.6.19.
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.
epfd
is not
a valid file descriptor.
The memory area pointed to by events
is not accessible
with write permissions.
The call was interrupted by a signal handler before
any of the requested events occurred or the timeout
expired.
epfd
is not
an epoll
file
descriptor, or maxevents
is less than or
equal to zero.
epoll_create(2), epoll_ctl(2), epoll(7)
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