NAME¶
epoll_wait, epoll_pwait - wait for an I/O event on an epoll file descriptor
SYNOPSIS¶
#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);
DESCRIPTION¶
The
epoll_wait() system call waits for events on the
epoll(7)
instance referred to by the file descriptor
epfd. 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(). The
maxevents argument must be greater than zero.
The
timeout argument specifies the minimum number of milliseconds that
epoll_wait() will block. (This interval will be rounded up to the
system clock granularity, and kernel scheduling delays mean that the blocking
interval may overrun by a small amount.) Specifying a
timeout of -1
causes
epoll_wait() to block indefinitely, while specifying a
timeout equal to zero cause
epoll_wait() to return immediately,
even if no events are available.
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 an
epoll_ctl(2) (
EPOLL_CTL_ADD,
EPOLL_CTL_MOD)
while the
events member will contain the returned event bit field.
epoll_pwait()¶
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);
The
sigmask argument may be specified as NULL, in which case
epoll_pwait() is equivalent to
epoll_wait().
RETURN VALUE¶
When successful,
epoll_wait() 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() returns -1 and
errno is set appropriately.
ERRORS¶
- 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 either
any of the requested events occurred or the timeout expired; see
signal(7).
- EINVAL
- epfd is not an epoll file descriptor, or
maxevents is less than or equal to zero.
VERSIONS¶
epoll_wait() was added to the kernel in version 2.6. Library support is
provided in glibc starting with version 2.3.2.
epoll_pwait() was added to Linux in kernel 2.6.19. Library support is
provided in glibc starting with version 2.6.
epoll_wait() is Linux-specific.
NOTES¶
While one thread is blocked in a call to
epoll_pwait(), it is possible
for another thread to add a file descriptor to the waited-upon
epoll
instance. If the new file descriptor becomes ready, it will cause the
epoll_wait() call to unblock.
For a discussion of what may happen if a file descriptor in an
epoll
instance being monitored by
epoll_wait() is closed in another thread,
see
select(2).
BUGS¶
In kernels before 2.6.37, a
timeout value larger than approximately
LONG_MAX / HZ milliseconds is treated as -1 (i.e., infinity). Thus, for
example, on a system where the
sizeof(long) is 4 and the kernel
HZ value is 1000, this means that timeouts greater than 35.79 minutes
are treated as infinity.
SEE ALSO¶
epoll_create(2),
epoll_ctl(2),
epoll(7)
COLOPHON¶
This page is part of release 3.44 of the Linux
man-pages project. A
description of the project, and information about reporting bugs, can be found
at
http://www.kernel.org/doc/man-pages/.