epoll_wait(2) System Calls Manual epoll_wait(2)
NAME
epoll_wait, epoll_pwait, epoll_pwait2 - wait for an I/O event on an
epoll file descriptor
LIBRARY
Standard C library (libc, -lc)
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 *_Nullable sigmask);
int epoll_pwait2(int epfd, struct epoll_event *events,
int maxevents, const struct timespec *_Nullable timeout,
const sigset_t *_Nullable sigmask);
DESCRIPTION
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 de-
scriptors 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 inter-
val may overrun by a small amount. Specifying a timeout of -1 causes
epoll_wait() to block indefinitely, while specifying a timeout equal to
zero causes epoll_wait() to return immediately, even if no events are
available.
The struct epoll_event is described in epoll_event(3type).
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 descrip-
tor.
The events field is a bit mask that indicates the events that have oc-
curred for the corresponding open file description. See epoll_ctl(2)
for a list of the bits that may appear in this mask.
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().
epoll_pwait2()
The epoll_pwait2() system call is equivalent to epoll_pwait() except for
the timeout argument. It takes an argument of type timespec to be able
to specify nanosecond resolution timeout. This argument functions the
same as in pselect(2) and ppoll(2). If timeout is NULL, then
epoll_pwait2() can block indefinitely.
RETURN VALUE
On success, epoll_wait() returns the number of file descriptors ready
for the requested I/O operation, or zero if no file descriptor became
ready during the requested timeout milliseconds. On failure,
epoll_wait() returns -1 and errno is set to indicate the error.
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.
STANDARDS
Linux.
HISTORY
epoll_wait()
Linux 2.6, glibc 2.3.2.
epoll_pwait()
Linux 2.6.19, glibc 2.6.
epoll_pwait2()
Linux 5.11.
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 in-
stance. 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 descrip-
tors 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 an-
other 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.
C library/kernel differences
The raw epoll_pwait() and epoll_pwait2() system calls have a sixth argu-
ment, size_t sigsetsize, which specifies the size in bytes of the sig-
mask argument. The glibc epoll_pwait() wrapper function specifies this
argument as a fixed value (equal to sizeof(sigset_t)).
BUGS
Before Linux 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 in-
finity.
SEE ALSO
epoll_create(2), epoll_ctl(2), epoll(7)
Linux man-pages 6.9.1 2024-05-02 epoll_wait(2)
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