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(7)
instance referred to by the file descriptor
epfd. The buffer pointed to
by
events is used to return information from the ready list about file
descriptors in the interest list that have some events available. Up to
maxevents are returned by
epoll_wait(). The
maxevents
argument must be greater than zero.
The
timeout argument specifies the number of milliseconds that
epoll_wait() will block. Time is measured against the
CLOCK_MONOTONIC clock.
A call to
epoll_wait() will block until either:
- •
- a file descriptor delivers an event;
- •
- the call is interrupted by a signal handler; or
- •
- the timeout expires.
Note that the
timeout 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 field of each returned
epoll_event structure contains the
same data as was specified in the most recent call to
epoll_ctl(2)
(
EPOLL_CTL_ADD,
EPOLL_CTL_MOD) for the corresponding open file
descriptor.
The
events field is a bit mask that indicates the events that have
occurred for the corresponding open file description. See
epoll_ctl(2)
for a list of the bits that may appear in this mask.
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;
pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
ready = epoll_wait(epfd, &events, maxevents, timeout);
pthread_sigmask(SIG_SETMASK, &origmask, NULL);
The
sigmask argument may be specified as NULL, in which case
epoll_pwait() is equivalent to
epoll_wait().
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.
- 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 (1) any of the
requested events occurred or (2) the timeout expired; see
signal(7).
- EINVAL
- epfd is not an epoll file descriptor, or maxevents is
less than or equal to zero.
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.
While one thread is blocked in a call to
epoll_wait(), 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.
If more than
maxevents file descriptors are ready when
epoll_wait() is called, then successive
epoll_wait() calls will
round robin through the set of ready file descriptors. This behavior helps
avoid starvation scenarios, where a process fails to notice that additional
file descriptors are ready because it focuses on a set of file descriptors
that are already known to be ready.
Note that it is possible to call
epoll_wait() on an
epoll instance
whose interest list is currently empty (or whose interest list becomes empty
because file descriptors are closed or removed from the interest in another
thread). The call will block until some file descriptor is later added to the
interest list (in another thread) and that file descriptor becomes ready.
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
sizeof(long) is 4 and the kernel
HZ
value is 1000, this means that timeouts greater than 35.79 minutes are treated
as infinity.
The raw
epoll_pwait() system call has a sixth argument,
size_t
sigsetsize, which specifies the size in bytes of the
sigmask
argument. The glibc
epoll_pwait() wrapper function specifies this
argument as a fixed value (equal to
sizeof(sigset_t)).
epoll_create(2),
epoll_ctl(2),
epoll(7)