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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 number of milliseconds that epoll_wait() will block.
Time is measured against the CLOCK_MONOTONIC clock. The call 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 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 description. The events field contains 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;
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().
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 (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.
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.
CONFORMING TO
epoll_wait() is Linux-specific.
NOTES
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.
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
sizeof(long) is 4 and the kernel HZ value is 1000, this means that timeouts greater than
35.79 minutes are treated as infinity.
C library/kernel differences
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)).
SEE ALSO
epoll_create(2), epoll_ctl(2), epoll(7)
COLOPHON
This page is part of release 5.05 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at https://www.kernel.org/doc/man-pages/.