Manpages - epoll_wait.2

epoll_wait, epoll_pwait, epoll_pwait2 - 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);
  int epoll_pwait2(int epfd, struct epoll_event *events,
   int maxevents, const struct timespec *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.

On success, *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. On failure, *epoll_wait*() returns -1 and errno is set to indicate the error.

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_pwait2*() was added to Linux in kernel 5.11.

*epoll_wait*(), *epoll_pwait*(), and *epoll_pwait2*() are 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.

*epoll_create*(2), *epoll_ctl*(2), *epoll*(7)

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